Antiference: sometimes I dispair
Bill Wright
their products through thick and thin. It's been a 'for better or worse'
experience, but in general it's been pretty good.
However the TC range was abolished after 30 years because Anti felt the need
to comply with the requirements of the CAI benchmarking scheme. That meant
that the Trumatch dipole had to go because it didn't have a balun. But
instead of redesigning the dipole so that a balun could be included Anti
simply used the fullwave dipole from the XG range. The result, a rather
unwieldy aerial that is easily damaged during installation and looks a fool.
I've always been of the opinion that the more simple the design of a yagi
the cleaner the response is likely to be. The size of the dipole means that
it is packed with but not on the aerial: another operation when building the
aerial. I do wish Anti had designed a simple halfwave folded dipole.
Then we were told that the metal (as opposed to plastic) coaxial line
connector was discontinued. This was a really good item, with a spring clip
around it to provide a firm grip. It was a perfect product and as such it
was taken for granted until it was no longer available. I must have used
many thousands of the things. The replacement was a brass thing, which would
have been all right I suppose, except that the inner receptacle expects a
pin that is 0.1mm smaller diameter than the actual size of the centre pin of
a coax plug. I bought 100 of these and I was astonished to find that it was
only possible to insert a coax plug about half way. I don't know whether it
was a batch fault or not. Maybe I was unlucky, maybe the other million were
OK, but I'll never find out because I've found an alternative product, which
although it is nowhere near as good as the old Anti product does have the
advantage that it will accept a coax plug without the inner brass receptacle
splaying out. I accept that Anti has to sell shite because that's the way of
the world these days, but I wish they could sell decent stuff alongside it.
Can't we have a choice of line connector? Of the 100 I bought 96 are now in
my metals for recycle bag. I have 100 from CPC and 100 from Grax and both
are OK, although I do wish I could have the old Anti product. If I'd known
they were going to discontinue it I would have bought a few thousand.
Next, let's take a look at the support arm on the TCX18B. It doesn't fit!
The length means that the two ends clash with the element clips. It needs to
be a few mm longer or shorter. I asked Anti and she said that that the same
size had to fit all groups. Why? Is it beyond the wit of man to either make
a support arm specially for the group B, or shave a bit off the shoulders of
the support arm clips, or even move the elements along the boom slightly?
This is the sort of 'sod the customer' instransigence that used to typify
British industry in the 1970s.
I know all big companies do strange things from time time, but Anti seems to
do more than her fair share. Last year we had the fiasco of the nuts on the
No 1 clamp. These suddenly became strange flattened things that stripped if
you looked at them. It didn't last long. But how many trips up and down the
ladder did it cause? The No. 1 clamp is Anti's very best product by the way,
so I confidently predict that it will be discontinued soon.
Bill
Brian Gaff
truematch dipole was both good, and an easy design to make. Why do we need a
balun anyway? Seems like just adding one more thing to go wrong. I also
despaired when the elements to some aerials were held on with plastic mounts
instead of metal ones. Result, a shower of elements when the plastic cracked
with the frost and sun.
Brian
--
Brian Gaff....Note, this account does not accept Bcc: email.
graphics are great, but the blind can't hear them
Email: bri...@blueyonder.co.uk
______________________________________________________________________________________________________________
"Bill Wright" <insertmybu...@f2s.com> wrote in message
news:o_adnR2X1sWzsTja...@pipex.net...
charles
Brian Gaff <Bri...@blueyonder.co.uk> wrote:
> I can only agree with you. I only used to do my own aerials, but the
> truematch dipole was both good, and an easy design to make. Why do we
> need a balun anyway?
The main reason is that an aerial is inherently a balanced device, co-axial
downlead is un-balanced. An aerial without a balun is likely to allow
pick-up in the downlead. This can either be impulsive interference, from
passing car, for instance) or, in the case of mixed polarized FM radio, a
cancelling signal.
Interestingly at an Antiference organised dealer meeting in Liverpool when
stereo radio arrived, someone queried the lack of a balun on their FM
aerials and was told that they weren't needed at those frequencies.
> Seems like just adding one more thing to go wrong. I also despaired when
> the elements to some aerials were held on with plastic mounts instead of
> metal ones.
That depends on what kind of plastic is used. My Fuba XC series aerial,
with plastic clips, is still in one piece after 30 years.
--
From KT24 - in "Leafy Surrey"
Using a RISC OS computer running v5.11
Usenet
Charles, Fuba XC range of aerials where excelent and going by bills
philisophy, is probably why they where discontinued...
Many an XC391 aerial I've installed to pull signal out of a thin air!
excuse the pun...
Glenn...
charles
Usenet <use...@glennmillar.plus.com> wrote:
> Charles, Fuba XC range of aerials where excelent and going by bills
> philisophy, is probably why they where discontinued...
On the basis of 'no call backs'?
But, I think Fuba have abandoned that market altogether.
tony sayer
>The length means that the two ends clash with the element clips. It needs to
>be a few mm longer or shorter. I asked Anti and she said that that the same
>size had to fit all groups. Why? Is it beyond the wit of man to either make
>a support arm specially for the group B, or shave a bit off the shoulders of
>the support arm clips, or even move the elements along the boom slightly?
>This is the sort of 'sod the customer' instransigence that used to typify
>British industry in the 1970s.
>
>I know all big companies do strange things from time time, but Anti seems to
>do more than her fair share. Last year we had the fiasco of the nuts on the
>No 1 clamp. These suddenly became strange flattened things that stripped if
>you looked at them. It didn't last long. But how many trips up and down the
>ladder did it cause? The No. 1 clamp is Anti's very best product by the way,
>so I confidently predict that it will be discontinued soon.
>
>Bill
>
>
Can't you get Andy Wades company to make them like you want?.
Or someone similar?..
--
Tony Sayer
tony sayer
<cha...@charleshope.demon.co.uk> scribeth thus
>In article <13qb7pm...@corp.supernews.com>,
> Usenet <use...@glennmillar.plus.com> wrote:
>
>> Charles, Fuba XC range of aerials where excelent and going by bills
>> philisophy, is probably why they where discontinued...
>
>On the basis of 'no call backs'?
>
>But, I think Fuba have abandoned that market altogether.
>
Prolly not needed these days..
--
Tony Sayer
Pete Smith
>their products through thick and thin. It's been a 'for better or worse'
>experience, but in general it's been pretty good.
>
Why don't you speak to the company directly? I used to do that with Kenwood
and Philips with some of their products. They were only too willing to
listen to complaints, suggestions and ideas. They sent a rep down to the
company to have a look at some returns and faults, then invited everyone to
see their design and manufacturing process. Some designs were changed and
updated to make them even better and cover problems encountered.
> However the TC range was abolished after 30 years because Anti felt the
> need to comply with the requirements of the CAI benchmarking scheme.
Which does leave a lot to be desired when you see some of the members of
CAI. The company can be CAI approved, but a subcontractor doesn't need to
be so there are a lot of cowboys.
CAI even think the Televes DAT75 is "good". Their measurements are
obviously never taken in real life situations.
>That meant that the Trumatch dipole had to go because it didn't have a
>balun.
Having a balun isn't always a requirement, especially if the aerial is
grouped. A balun introduces losses. Some of the printed PCB types don't
even work correctly.
>But instead of redesigning the dipole so that a balun could be included
>Anti simply used the fullwave dipole from the XG range. The result, a
>rather unwieldy aerial that is easily damaged during installation and looks
>a fool. I've always been of the opinion that the more simple the design of
>a yagi the cleaner the response is likely to be. The size of the dipole
>means that it is packed with but not on the aerial: another operation when
>building the aerial. I do wish Anti had designed a simple halfwave folded
>dipole.
>
They could have put a balun inside the connection housing OR designed one
with a sealed box containing the balun and F type connector. Screw
terminals shouldn't be used on aerials, not many seal the end of the coax or
the actual connection.
> Then we were told that the metal (as opposed to plastic) coaxial line
> connector was discontinued. This was a really good item, with a spring
> clip around it to provide a firm grip. It was a perfect product and as
> such it was taken for granted until it was no longer available. I must
> have used many thousands of the things. The replacement was a brass thing,
> which would have been all right I suppose, except that the inner
> receptacle expects a pin that is 0.1mm smaller diameter than the actual
> size of the centre pin of a coax plug. I bought 100 of these and I was
> astonished to find that it was only possible to insert a coax plug about
> half way. I don't know whether it was a batch fault or not. Maybe I was
> unlucky, maybe the other million were OK, but I'll never find out because
> I've found an alternative product, which although it is nowhere near as
> good as the old Anti product does have the advantage that it will accept a
> coax plug without the inner brass receptacle splaying out. I accept that
> Anti has to sell shite because that's the way of the world these days, but
> I wish they could sell decent stuff alongside it.
That's not a very good advert for your business suggesting you knowingly fit
s***e for customers!
Are you a member of CAI as they might like to see your post.
> Can't we have a choice of line connector? Of the 100 I bought 96 are now
> in my metals for recycle bag. I have 100 from CPC and 100 from Grax and
> both are OK, although I do wish I could have the old Anti product. If I'd
> known they were going to discontinue it I would have bought a few
> thousand.
>
> Next, let's take a look at the support arm on the TCX18B. It doesn't fit!
> The length means that the two ends clash with the element clips. It needs
> to be a few mm longer or shorter.
Buy a file.
> I asked Anti and she said that that the same size had to fit all groups.
> Why?
Mass production, cheaper to make.
>Is it beyond the wit of man to either make a support arm specially for the
>group B, or shave a bit off the shoulders of the support arm clips, or even
>move the elements along the boom slightly? This is the sort of 'sod the
>customer' instransigence that used to typify British industry in the 1970s.
>
> I know all big companies do strange things from time time, but Anti seems
> to do more than her fair share.
So why do you keep using their products if they are as bad as you describe.
You must be the only person that moans on and on about things. I don't see
anyone else doing it.
> Last year we had the fiasco of the nuts on the No 1 clamp. These suddenly
> became strange flattened things that stripped if you looked at them. It
> didn't last long. But how many trips up and down the ladder did it cause?
> The No. 1 clamp is Anti's very best product by the way, so I confidently
> predict that it will be discontinued soon.
>
> Bill
>
Maybe you should save your energy for the fictional series you write and
post in the wrong newsgroups? It's amazing how every other aerial fitter is
a cowboy, how you go to the rescue all the time ! Do you ever introduce a
fault to get work? Wouldn't it be funny if you had been filmed for BBC
House of Horrors.
Pete Smith
"charles" <cha...@charleshope.demon.co.uk> wrote in message
news:4f6b42b2...@charleshope.demon.co.uk...
> In article <Isfpj.774$XI...@text.news.virginmedia.com>,
> Brian Gaff <Bri...@blueyonder.co.uk> wrote:
>> I can only agree with you. I only used to do my own aerials, but the
>> truematch dipole was both good, and an easy design to make. Why do we
>> need a balun anyway?
>
> The main reason is that an aerial is inherently a balanced device,
> co-axial
> downlead is un-balanced.
Very easy to make a balun from a short piece of coax to connect to an aerial
without one.
> An aerial without a balun is likely to allow pick-up in the downlead.
What? the aerial balun has nothing to do with what a piece of coax picks up
after it.
> This can either be impulsive interference, from
> passing car, for instance) or, in the case of mixed polarized FM radio, a
> cancelling signal.
>
You haven't a clue what you're talking about. Do you know what mixed
polarisation is? Do you know the difference between VHF and UHF? Do you
not understand that the dipole on a TV aerial is not resonant in the
88-108MHz range? And what's all this rubbish about radio signals cancelling
out a TV signal? The only way that would happen is through RF desensing,
maybe living right under a VHF transmitter running a few thousand watts.
Impulse noise is picked up BY the coax, a balun at one or both ends will not
stop it. Running a coax too near a mains cable can cause breakup on a
freeview box if a circuit is switched on and off.
> Interestingly at an Antiference organised dealer meeting in Liverpool when
> stereo radio arrived, someone queried the lack of a balun on their FM
> aerials and was told that they weren't needed at those frequencies.
>
They are not really needed as it depends on the design of the aerial. It
depends on what impedance you are matching and also what type of feeder is
used. A folded dipole on a TV aerial will have a lower impedance than if
you took it away from the directors and reflector. It might be as low as
35ohms on the aerial and 300ohms away from it. If a straight dipole was
used on a TV aerial then it would need to be redesigned or some matching
arrangement added.
>
>
>> Seems like just adding one more thing to go wrong. I also despaired when
>> the elements to some aerials were held on with plastic mounts instead of
>> metal ones.
>
> That depends on what kind of plastic is used. My Fuba XC series aerial,
> with plastic clips, is still in one piece after 30 years.
>
Yes it does depend. Some manufacturers used plastic that was not resistant
to UV light, so it cracked after exposure to sunlight and elements would fly
off in the wind. It's the same as coax that doesn't resist UV light, it
cracks or goes hard. Remember the old brown coax that used to turn green.
Mark Carver
>> This can either be impulsive interference, from
>> passing car, for instance) or, in the case of mixed polarized FM radio, a
>> cancelling signal.
[snip]
> And what's all this rubbish about radio signals cancelling
> out a TV signal?
At no point in Charles' post was that stated. He was talking in general terms.
[snip]
--
Mark
Please replace invalid and invalid with gmx and net to reply.
Unknown
> Maybe you should save your energy for the fictional series you write and
> post in the wrong newsgroups?
>
> It's amazing how every other aerial fitter is a cowboy, how you go to
> the rescue all the time !
>
> Do you ever introduce a fault to get work?
> Wouldn't it be funny if you had been filmed for BBC House of Horrors.
Peter you should add yourself to the cowboy list, as you have sent out
two messages in this thread which contained flawed technical advice and
showed everybody that you do not understand antenna systems and there
design.
J
charles
Pete Smith <Peter...@techno-cellular.com> wrote:
> Having a balun isn't always a requirement, especially if the aerial is
> grouped. A balun introduces losses. Some of the printed PCB types don't
> even work correctly.
An incorrectly designed balun might do these things, and what has aerial
bandwidth got to do with the subject?
charles
Pete Smith <Peter...@techno-cellular.com> wrote:
> "charles" <cha...@charleshope.demon.co.uk> wrote in message
> news:4f6b42b2...@charleshope.demon.co.uk...
> > In article <Isfpj.774$XI...@text.news.virginmedia.com>,
> > Brian Gaff <Bri...@blueyonder.co.uk> wrote:
> >> I can only agree with you. I only used to do my own aerials, but the
> >> truematch dipole was both good, and an easy design to make. Why do we
> >> need a balun anyway?
> >
> > The main reason is that an aerial is inherently a balanced device,
> > co-axial downlead is un-balanced.
> Very easy to make a balun from a short piece of coax to connect to an
> aerial without one.
That is a 'stub balun' which provides a measure of impedance matching.
> > An aerial without a balun is likely to allow pick-up in the downlead.
> What? the aerial balun has nothing to do with what a piece of coax picks
> up after it.
Yes, it has - if it is transformer balun. Been there, seen it, bought the
T-shirt & the video.
> You haven't a clue what you're talking about. Do you know what mixed
> polarisation is? Do you know the difference between VHF and UHF? Do you
> not understand that the dipole on a TV aerial is not resonant in the
> 88-108MHz range? And what's all this rubbish about radio signals
> cancelling out a TV signal?
Read what I wrote, instead of imagining things. I never suggested and FM
signal would cancel out a tv one.
> The only way that would happen is through
> RF desensing, maybe living right under a VHF transmitter running a few
> thousand watts. Impulse noise is picked up BY the coax, a balun at one
> or both ends will not stop it.
It does. since the screen is properly acting as a screen.
> Running a coax too near a mains cable can cause breakup on a freeview box
> if a circuit is switched on and off.
Same problem.
> > Interestingly at an Antiference organised dealer meeting in Liverpool
> > when stereo radio arrived, someone queried the lack of a balun on their
> > FM aerials and was told that they weren't needed at those frequencies.
> >
> They are not really needed as it depends on the design of the aerial. It
> depends on what impedance you are matching and also what type of feeder
> is used. A folded dipole on a TV aerial will have a lower impedance
> than if you took it away from the directors and reflector. It might be
> as low as 35ohms on the aerial and 300ohms away from it. If a straight
> dipole was used on a TV aerial then it would need to be redesigned or
> some matching arrangement added.
A balun = BALanced to UNbalanced converter is not just for matching the
impedance as you seem to think.
> Remember the old brown coax that used to turn green.
was that the copper oxide leaching through?
Sandman
Bill Wright
"Pete Smith" <Peter...@techno-cellular.com> wrote in message
news:47a5ba74$1...@mk-nntp-2.news.uk.tiscali.com...
> You haven't a clue what you're talking about. Do you know what mixed
> polarisation is? Do you know the difference between VHF and UHF?
I'm staggered at your abuse of Charles. How dare you come on here with your
gauche and naive attitude towards those with years of practical experience
and accumulated knowledge? Charles has forgotten more about RF than you'll
ever know.
It's obvious from your other remarks that you have many misunderstandings
about RF.
Bill
Bill Wright
"tony sayer" <to...@bancom.co.uk> wrote in message
news:9lssHsHs...@bancom.co.uk...
As a matter of fact I already use Blake's log periodics, and yesterday I
obtained two Blake 10 element Gp B aerials just to see how I get on with
them.
Bill
Bill Wright
> >Do you ever introduce a
> fault to get work?
think the mere insinuation, in the context of a public forum, might be
enough for me to take action. Would you care to submit evidence here that I
have actually done this, before Monday at 9am? By the way I don't worry
about legal costs because my daughter and son-in-law are both lawyers. Or
are you man enough to apologise?
Bill
charles
<insertmybu...@f2s.com> wrote:
> "Pete Smith" <Peter...@techno-cellular.com> wrote in message
> news:47a5ba74$1...@mk-nntp-2.news.uk.tiscali.com...
> > You haven't a clue what you're talking about. Do you know what mixed
> > polarisation is? Do you know the difference between VHF and UHF?
> I'm staggered at your abuse of Charles. How dare you come on here with
> your gauche and naive attitude towards those with years of practical
> experience and accumulated knowledge? Charles has forgotten more about
> RF than you'll ever know.
Thank you, Bill.
> It's obvious from your other remarks that you have many misunderstandings
> about RF.
--
Brian Gaff
up on a cable, even though its unbalanced. Often its the match which is the
most important thing.
Brian
--
Brian Gaff - bri...@blueyonder.co.uk
Note:- In order to reduce spam, any email without 'Brian Gaff'
in the display name may be lost.
Blind user, so no pictures please!
"charles" <cha...@charleshope.demon.co.uk> wrote in message
news:4f6b42b2...@charleshope.demon.co.uk...
Brian Gaff
nothing I'd known no matter how hard you tried to waterproof the housing.
Brian
--
Brian Gaff - bri...@blueyonder.co.uk
Note:- In order to reduce spam, any email without 'Brian Gaff'
in the display name may be lost.
Blind user, so no pictures please!
"Usenet" <use...@glennmillar.plus.com> wrote in message
news:13qb7pm...@corp.supernews.com...
Brian Gaff
balanced input in tvs, and use balanced feeder? Reason, because where we run
cable affects it and thus its not practicle, so lets not get pedantic. Its
pretty pointless in most cases, as long as the matching is within reasonable
limits, it will work.
Brian
--
Brian Gaff - bri...@blueyonder.co.uk
Note:- In order to reduce spam, any email without 'Brian Gaff'
in the display name may be lost.
Blind user, so no pictures please!
"Pete Smith" <Peter...@techno-cellular.com> wrote in message
news:47a5ba74$1...@mk-nntp-2.news.uk.tiscali.com...
>
charles
Thee shouldn't be any significant losses in a transformer balun. There
will be plenty of loss due to an impedance mismatch.
> I've never seen much pick up on a cable, even though its unbalanced.
With a balanced aerial, the downlead will tend to be balanced. It's unbalanced cable, ie screened, that minimises pick up. Anyhow, you've previously mentioned pick up from an adjacent mains cable.
> Often its the match which is the most important thing.
Not in most domestic situations.
charles
Brian Gaff <bri...@blueyonder.co.uk> wrote:
> OK, then if its all that good to have a balanced line, then why not make
> a balanced input in tvs, and use balanced feeder? Reason, because where
> we run cable affects it and thus its not practicle, so lets not get
> pedantic. Its pretty pointless in most cases, as long as the matching is
> within reasonable limits, it will work.
are you sure you know what you are writing about. No-one has said its good
to have a balanced line, Tv sets have an UN-balanced input and the cable
that feeds them is UN-balanced. Its the aerial that is balanced. That's
way a BAL-UN is needed.
In the USA they tend to use Balanced feeder for Band II, so older tuners
sold over here had balanced inputs. Then you need Balun the other way
round to convert the UnBalance feeder to the Balanced input.
Mark Carver
> In article <7Kmpj.984$XI....@text.news.virginmedia.com>,
> Brian Gaff <bri...@blueyonder.co.uk> wrote:
>> Yes, but losses in the balun are also a problem.
>
> Thee shouldn't be any significant losses in a transformer balun. There
> will be plenty of loss due to an impedance mismatch.
>
>> I've never seen much pick up on a cable, even though its unbalanced.
>
> With a balanced aerial, the downlead will tend to be balanced. It's unbalanced cable, ie screened, that minimises pick up. Anyhow, you've previously mentioned pick up from an adjacent mains cable.
Of course the advantage of a balanced feeder is that any interference picked
up on it is, in theory, cancelled when it hits the input of the receiver.
*However* that will only be the case when the interference is induced into
both conductors equally. That's often not the case with the 300 ohm style
ribbon cable. The conductors are about 11-12mm apart. Hence the use of
*twisted* pair cable for balanced audio circuits, and telecoms/IT (including
good old CAT5 etc)
-<GB>-Carpy
"Bill Wright" <insertmybu...@f2s.com> wrote in message
news:o_adnR2X1sWzsTja...@pipex.net...
>I have been an Antiference customer since 1970. That's 37 years of using
>their products through thick and thin. It's been a 'for better or worse'
>experience, but in general it's been pretty good.
>
> Anti simply used the fullwave dipole from the XG range. The result, a
> rather unwieldy aerial that is easily damaged during installation and
> looks a fool. I've always been of the opinion that the more simple the
> design of a yagi the cleaner the response is likely to be. The size of the
> dipole means that it is packed with but not on the aerial: another
> operation when building the aerial. I do wish Anti had designed a simple
> halfwave folded dipole.
>
> connector was discontinued. This was a really good item, with a spring
> clip around it to provide a firm grip. It was a perfect product and as
> such it was taken for granted until it was no longer available. I must
> have used many thousands of the things. The replacement was a brass thing,
> which would have been all right I suppose, except that the inner
> receptacle expects a pin that is 0.1mm smaller diameter than the actual
> size of the centre pin of a coax plug. I bought 100 of these and I was
> astonished to find that it was only possible to insert a coax plug about
> half way. I don't know whether it was a batch fault or not. Maybe I was
> unlucky, maybe the other million were OK, but I'll never find out because
> I've found an alternative product, which although it is nowhere near as
> good as the old Anti product does have the advantage that it will accept a
> coax plug without the inner brass receptacle splaying out. I accept that
> Anti has to sell shite because that's the way of the world these days, but
> bag. I have 100 from CPC and 100 from Grax and both are OK, although I do
> wish I could have the old Anti product. If I'd known they were going to
> discontinue it I would have bought a few thousand.
>
> The length means that the two ends clash with the element clips. It needs
> to be a few mm longer or shorter. I asked Anti and she said that that the
> same size had to fit all groups. Why? Is it beyond the wit of man to
> either make a support arm specially for the group B, or shave a bit off
> the shoulders of the support arm clips, or even move the elements along
> the boom slightly? This is the sort of 'sod the customer' instransigence
> that used to typify British industry in the 1970s.
>
> I know all big companies do strange things from time time, but Anti seems
> to do more than her fair share. Last year we had the fiasco of the nuts on
> the No 1 clamp. These suddenly became strange flattened things that
> stripped if you looked at them. It didn't last long. But how many trips up
> and down the ladder did it cause? The No. 1 clamp is Anti's very best
> product by the way, so I confidently predict that it will be discontinued
> soon.
>
> Bill
>
>
I must admit to being disappointed with a lot of the stuff that comes from
Antiference now. The TCX aerials went through a stage of having the clip in
plastic reflectors which were flimsy & nasty, then I had a series of No1
clamps fail because of corrosion to the inner bend on the u-bolt (caused at
factory apparantly) which put an end to my clean sheet of genuine callbacks
(TCX18A was hanging by the cable).
Then we had some more changes which meant I had a few different batches of
aluminium coax plugs (much thinner metal & wouldn't screw up properly),
newer metal (wahey) reflector design which rattled in the wind (booo)
because of bad design. Even though it now seems the coax plugs and TCX
reflectors coming out from Antiference are ok-ish now, I still get
frustrated by sealed, boxed TCX aerials which have parts missing. I've had
to maike sure I always have an extra TCX on the van just incase I am a long
way from home and there's a bit missing.
The outlet plates I've tried have been awful, and don't get me started on
the DAB dipoles.
I must admit I do use their amplifiers quite frequently (F type connectors -
A240L Pro etc). They look good and work well, and even though they cost a
fair bit more than most rivals I still use them.
I must admit to feeling like I have lost faith in Antiference as a company.
I really would like to switch to another company but the fact remains the
stuff I use from Anti isn't available elsewhere so I'm forced to still use
some Anti products grudgingly. I dislike the way they underestimate the
intelligence of their customers when they try tricks like putting 300m of
benchmarked cable in a "new, innovative" cardboard box dispenser (with lots
of fancy stickers on it) and try charging a lot more than an equivalent
cable such as WF100. Of course I'd understand if this was for sale in Argos
but it's 300m of cable & only available in trade outlets.
It's also quite frustrating having to go to tens of different companies to
be able to stock up on what I like, but I don't have a choice.
charles
Bob Latham <b...@sick-of-spam.invalid> wrote:
>
[Snip]
> The bit I don't understand is why you say the aerial is balanced and that
> a balun will fix this. I'm sure I'm wrong but is the aerial not really a
> loop connected across the two output terminals? If neither side is earthed
> and there is balanced cable then its a balanced aerial. But, if one side
> is earthed then its not balanced surely?
Think what happens when the supporting metalwork is also earthed
> What's the difference between a balun and a transformer? I understand that
> impedances are another issue and the balun/transformer can correct this.
A decent balun is a transformer.
charles
-<GB>-Carpy <ca...@NOSPIDERSgamebashers.net> wrote:
> I must admit to feeling like I have lost faith in Antiference as a
> company.
Presumably this is "Antiference - THE curtain rail manufacturer" - as some
of their adverts used to say ?
Steve Thackery
> obviously never taken in real life situations.
Do tell me more! I also think the DAT75 is good - at least, it's the best
aerial I've used for picking up Freeview in Felixstowe (wideband
transmitter, well outside the official coverage area, lots of crap from
Holland hitting the back of the aerial).
I'm really keen to find out more from someone who's got a wider experience
than me.
SteveT
Bill Wright
"Mark Carver" <mark....@invalid.invalid> wrote in message
news:60mg73F...@mid.individual.net...
In the days when many Jap tuners meant for the US market (don't mention
pre-emphasis) landed in the UK, we did have a brief period of using 300ohm
balanced feeder. The aerial would be a horizontal folded dipole, with or
without a reflector. The feeder was a nightmare. It had to be kept well away
from metal gutters and fallpipes, and when there was heavy rain or snow it
became lossy. It was best to put a twist in it to equalise the effects of
interfering feeds, so it looked an absolute mess. Some people used to fix it
to the wall using spacers so it stood off about 3". Believe me, coax is
better! I was very pleased when Antiference brought out their 75/300
impedence converter and balun. Absurdly, I still carry one, just in case.
It's become like the 6" Father Christmas that I was given by Boots in 1977
as an incentive to have more films developed -- a sort of holy relic that
cannot be dispensed with.
I noticed a bit of 300 ohm ribbon feeder hanging from a chimney the other
day.
Bill
widgitt
The ones I bought recently to try out were already sloppy and stripped
completely as soon as I tightened them by hand. If they hadn't failed
at this stage it would have been a certain call back.
The actual aerial was good, though.
Bill Wright
"-<GB>-Carpy" <ca...@NOSPIDERSgamebashers.net> wrote in message
news:PEqpj.1181$XI....@text.news.virginmedia.com...
> I must admit to being disappointed with a lot of the stuff that comes from
> Antiference now. The TCX aerials went through a stage of having the clip
> in plastic reflectors which were flimsy & nasty, then I had a series of
> No1 clamps fail because of corrosion to the inner bend on the u-bolt
> (caused at factory apparantly) which put an end to my clean sheet of
> genuine callbacks (TCX18A was hanging by the cable).
bolt.
>
> Then we had some more changes which meant I had a few different batches of
> aluminium coax plugs (much thinner metal & wouldn't screw up properly),
> newer metal (wahey) reflector design which rattled in the wind (booo)
> because of bad design. Even though it now seems the coax plugs and TCX
> reflectors coming out from Antiference are ok-ish now, I still get
> frustrated by sealed, boxed TCX aerials which have parts missing. I've had
> to maike sure I always have an extra TCX on the van just incase I am a
> long way from home and there's a bit missing.
Yes I keep a box of spare bits!
> I must admit to feeling like I have lost faith in Antiference as a
> company. I really would like to switch to another company but the fact
> remains the stuff I use from Anti isn't available elsewhere so I'm forced
> to still use some Anti products grudgingly. I dislike the way they
> underestimate the intelligence of their customers when they try tricks
> like putting 300m of benchmarked cable in a "new, innovative" cardboard
> box dispenser (with lots of fancy stickers on it) and try charging a lot
> more than an equivalent cable such as WF100. Of course I'd understand if
> this was for sale in Argos but it's 300m of cable & only available in
> trade outlets.
>
> It's also quite frustrating having to go to tens of different companies to
> be able to stock up on what I like, but I don't have a choice.
If it helps anyone, here's my current line up.
Yagi aerials: Anti
Logs: Blake
Multiswitches: Anti
Remote eyes: Vision
Domestic dist amps with remote eye return amp: Global
Domestic dist amps without remote eye return amp: Taylor
Bypass kits: Global
Galvanised brackets: Blake
Dishes: Hirschmann
Cable: Cavel
Fixings: mostly Screwfix
Masts: Sat Solutions
Outlets: Triax modular
Earth bonding: Antiference
One in one out 16dB amps: Labgear
Distribution amps (high output) Taylor
Channel filters/levellers: Taylor
Notch filters: Taylor
Equalisers: Fuba, Alcad
Tap-off units terr: Taylor
Tap-off units sat: Taylor
Splitters terr: Taylor
Splitters sat: Taylor
Patresses, blank plates, etc: Contactum
Crimp 'f' plugs: Webro
Screw-on 'f' plugs: Anti
'f' line conns: CPC
Belling line conns: Grax
cable clips: Unifix
Coax plugs: Vision
Masthead amps and PSUs: Proception
'f' DC blocks: CPC
Steel enclosures: RS
Custom brackets, etc: Rossington Light Engineering
BNC crimp plugs: CPC
phonos scarts, HDMIs: CPC
DSB modulators: Vision
VSB modulators: Alcad
FTA boxes: Fortec
Bill
Bill Wright
> In article <PEqpj.1181$XI....@text.news.virginmedia.com>,
> -<GB>-Carpy <ca...@NOSPIDERSgamebashers.net> wrote:
>
>> I must admit to feeling like I have lost faith in Antiference as a
>> company.
>
>
> Presumably this is "Antiference - THE curtain rail manufacturer" - as some
> of their adverts used to say ?
Kirsch wasn't it?
Bill
tony sayer
>OK, then if its all that good to have a balanced line, then why not make a
>balanced input in tvs, and use balanced feeder? Reason, because where we run
>cable affects it and thus its not practicle, so lets not get pedantic. Its
>pretty pointless in most cases, as long as the matching is within reasonable
>limits, it will work.
Yes it will "work"..
Some bits of metal arranged in a -different- way will work -differently-
from others;!...
--
Tony Sayer
tony sayer
>to UV light, so it cracked after exposure to sunlight and elements would fly
>off in the wind. It's the same as coax that doesn't resist UV light, it
>cracks or goes hard. Remember the old brown coax that used to turn green.
>
>> From KT24 - in "Leafy Surrey"
>>
>> Using a RISC OS computer running v5.11
>>
>
>
Got to larf eh at some of the things you read on news net for someone
who has -cellular- in the addy!..
Poor old Charles, whatever were you doing all those years at the BBC in
the engineering department eh?.
Shame on you for putting all these half trooths around;!. And so
thankful you must be for our knowledgeable friend to come and correct
you;).....
--
Tony Sayer
tony sayer
Bet you wish that lot was under the one roof;!....
Must take up a lot of admin time?..
--
Tony Sayer
tony sayer
>I really would like to switch to another company but the fact remains the
>stuff I use from Anti isn't available elsewhere so I'm forced to still use
>some Anti products grudgingly. I dislike the way they underestimate the
>intelligence of their customers when they try tricks like putting 300m of
>benchmarked cable in a "new, innovative" cardboard box dispenser (with lots
>of fancy stickers on it) and try charging a lot more than an equivalent
>cable such as WF100. Of course I'd understand if this was for sale in Argos
>but it's 300m of cable & only available in trade outlets.
>
>It's also quite frustrating having to go to tens of different companies to
>be able to stock up on what I like, but I don't have a choice.
>
>
Have any of you contacted them directly, and if so what was the
result?..
Two well know end user aerial riggers complaining long and loud
something must be seriously wrong with them?..
--
Tony Sayer
tony sayer
>remember a fixing bolt through the middle of the 'loop' (for want of a
>better expression) holding it to the boom of the aerial. Is it this which
>is earthing the centre and therefore making the output balanced which
>needs a transformer to unbalance it. Is that correct? I take it that at
>600MHz the metal work of the aerial is effectively earthed even if there
>is no copper ground connection.
>
Haven't got all day but thats the electrically "quiet" part of the
aerial. Relative to that each end of the dipole is "hot" with respect to
the centre "cold" point. It doesn't have to be earthed as such thats
useful for lightning static protection etc, though a good whack of
lightning will see most all domestic installs off!.
Connecting a feeder that has one side earthed as it were is not the way
to transfer maximum power from this "balanced about a common point"
device so some means of doing this balanced to unbalanced operation is
needed so a simple BALUN transformer is usually used. This also converts
the higher impedance of the dipole to the lower impedance of the cable
so as as it ought be and max power transfer takes place without any
spurious pick up of the feeder..
This might help..
http://en.wikipedia.org/wiki/Dipole_antenna
>This prompts another long term question I've never got my head around.
>Should the aerial boom and support metal work be earthed? Could this help
>protect the down lead and TV from adjacent lighting?
>
Yes it can but depends on the way its done and the size of the
conductors, the usual CT 100 wouldn't be much use in a direct or nearby
hit.
Thats why something like inch by an eighth copper, nowadays ally strip
is used on telecom and broadcast installs!..
It can be of use if its close it depended on the magnitude of the
current of the strike which is a bit of a how longs a bit of string!..
No doubt w-tom the lightning expert who trawls usenet will be along with
chapter and verse;!...
--
Tony Sayer
charles
Bob Latham <b...@sick-of-spam.invalid> wrote:
> Thinking back to the last time I closely looked at a UHF aerial, I seem to
> remember a fixing bolt through the middle of the 'loop' (for want of a
> better expression) holding it to the boom of the aerial. Is it this which
> is earthing the centre and therefore making the output balanced which
> needs a transformer to unbalance it. Is that correct? I take it that at
> 600MHz the metal work of the aerial is effectively earthed even if there
> is no copper ground connection.
To the best of my knowledge.
> This prompts another long term question I've never got my head around.
> Should the aerial boom and support metal work be earthed? Could this help
> protect the down lead and TV from adjacent lighting?
I believe it is a requirement for communal systems, so is probably a good
idea for domestic ones. If it's for lighting protection you will need a
very substantial conductor.
Dave Farrance
>Haven't got all day but thats the electrically "quiet" part of the
>aerial. Relative to that each end of the dipole is "hot" with respect to
>the centre "cold" point. It doesn't have to be earthed as such thats
>useful for lightning static protection etc, though a good whack of
>lightning will see most all domestic installs off!.
>
>Connecting a feeder that has one side earthed as it were is not the way
>to transfer maximum power from this "balanced about a common point"
>device so some means of doing this balanced to unbalanced operation is
>needed so a simple BALUN transformer is usually used. This also converts
>the higher impedance of the dipole to the lower impedance of the cable
>so as as it ought be and max power transfer takes place without any
>spurious pick up of the feeder..
This is of course correct. It just seems that connecting a balanced
dipole to an unbalanced feed works better in practice than you might
infer from the theory.
I wonder if it's because a coax is not a theoretically perfect unbalanced
transmission line. In practice you have some inductance in the screen as
well. I suspect that if the coax is long enough to noticeably attenuate
the signal, it will also be doing the job of balanced-to-unbalanced
signal conversion by itself.
--
Dave Farrance
Bill Wright
"tony sayer" <to...@bancom.co.uk> wrote in message
news:5TvN6oMg...@bancom.co.uk...
No, it isn't too bad actually. That list was off the top of my head. It's
about half of the total I think.
Bill
Mark Carver
> Believe me, coax is
> better! I was very pleased when Antiference brought out their 75/300
> impedence converter and balun. Absurdly, I still carry one, just in case.
> It's become like the 6" Father Christmas that I was given by Boots in 1977
> as an incentive to have more films developed -- a sort of holy relic that
> cannot be dispensed with.
>
> I noticed a bit of 300 ohm ribbon feeder hanging from a chimney the other
> day.
When I was a student I had a Philips portable TV with a 75/300
converter/balun. It was fitted to the back of the case, and had nice
screw treminals to accept the loop aerial. Out of the other end was a
length of co-ax that plugged into the Belling-Lee RF input socket of
the TV itself.
In one of the bedsits I ended up in, the only decent reception was on
top of a wardrobe. I purchased 2 metres of 300 ohm ribbon cable,
attached one end to the screw terminals on the balun, fixed the other
end under some washers with the loop aerial, and screwed the whole lot
to the top of the wardrobe. Result: Four noise and ghost free TV
channels :-)
Ian Jackson
<DaveFa...@OMiTTHiSyahooANDTHiS.co.uk> writes
to a coax feeder, the outer of the coax becomes live with RF. As there
is usually a pretty good safety earth at the transmitter end, a lot of
RF current flows on the OUTSIDE of the coax outer, whereas this should
be RF-dead. In effect, the coax becomes part of the radiating system.
This may - or may not cause you problems, but there is a much greater
chance of causing interference to electronic equipment which gets hit by
the signal radiated by the coax.
The same problem occurs when receiving. Interference emanating from such
electronic equipment (switch-mode PSUs, timebase harmonics, switching
transients etc) gets picked up on the coax, and transferred to the
aerial, and the coax then feeds this interference down to the receiver.
If you have a balun at the aerial feedpoint, this problem doesn't
happen. The coax keeps the RF in and also keeps it out. The only
reception and radiation of the RF signal is via the actual aerial.
--
Ian
Carpy
Very helpful nice one. Quite a few interesting things there. Here's mine
with a few comments alongside & one question at the bottom.
Yagi aerials: Anti & Wolsey QR range (Triax)
FM aerials: ABC aerials Ltd (FM rod dipoles & multi elements)
DAB aerials - Anti rod dipole / Vision multi element Log
Logs: Fracarro / Antiference. Have tried the blakes & they are much more
robust. Might switch to them fully soon. Have one on "test" at the moment
outside!
Remote eyes: Global "Pro" range with F conn
Domestic dist amps with remote eye return amp: Global F for 9v & 16way /
Antiference "D" range for 230V.
Domestic dist amps without remote eye return amp: Antiference F (& some old
Labgear F type amps still left on the shelf)
Galvanised brackets: Blake but not direct. More expensive to order direct
even in large quantity! Same for masts etc.
Cable: Webro - Not tried the Cavel - Nowhere seems to stock it for anywhere
near the same price as Webro WF100, even when ordering 5000m quantity.
Fixings: UK fixings (cheaper than screwfix for same product)
Masts: Alltrade (cheaper than Sat Sol for same product)
Outlets: Triax modular & non modular
Distribution amps (high output) Taylor - Use loads of the TSC3054 amps.
Channel filters/levellers: Taylor
Crimp 'f' plugs: Always used Webro but found Vision now do an identical
product for much cheaper. Tried them and they are really good. Came in a
plastic tub with resealable lid. Not a flimsy cardboard box like Webro.
Would also like to test out some "waterproof" snap n seal (T&B) connectors
for LNBs. I get annoyed trying to fiddle around using amalgamating tape for
all 4 F connectors with numb hands.
Coax plugs: Antiference but increasingly unhappy with them. Very interesting
hearing you use Vision plugs. Not heard about them. Will hunt them down.
Masthead amps and PSUs: Vision
Custom brackets, etc: Rossington Light Engineering - Presumably they have a
few standard (custom!) sizes they make for you regularly? Do they give you
any kind of model / reference for these? I might give them a ring and order
a few to test them out. I've never liked the larger T&K brackets but no
other non-custom options out there.
Andy Burns
> Coax plugs: Antiference but increasingly unhappy with them. Very interesting
> hearing you use Vision plugs. Not heard about them. Will hunt them down.
http://www.vanjak.com/product_details.php?category_id=312&item_id=1405
Jim Lesurf
<b...@sick-of-spam.invalid> wrote:
> The bit I don't understand is why you say the aerial is balanced and
> that a balun will fix this. I'm sure I'm wrong but is the aerial not
> really a loop connected across the two output terminals? If neither side
> is earthed and there is balanced cable then its a balanced aerial. But,
> if one side is earthed then its not balanced surely?
The problem is that you wish to *avoid* connecting one 'side' of the
antenna to ground. cf below.
> A bit like the secondary of a transformer if neither side is earthed or
> its earthed in the centre then its balanced but earth one side and its
> not.
> In balanced circuits interference is induced equally into both legs and
> as the signal is the difference in the two legs common signals are
> rejected. In my mind an earth on one leg will mean it is no longer
> balanced at all. So doesn't the question of 'if a loop on an aerial is
> balanced or not' depend entirely on what you connect to it?
> What's the difference between a balun and a transformer? I understand
> that impedances are another issue and the balun/transformer can correct
> this.
Note to the knowledgeable. The following is obviously somewhat simplified
for the sake of clarity. :-)
For clarity, take the simple example of the antenna being just a center fed
half-wave rod dipole. In effect, two quarter-wave lengths of metal,
arranged along the same axis with a small gap between their closest ends
Put the antenna in free space where it is exposed to an RF field of the
appropriate wavelength, etc.
The field will tend to induce potential differences between the ends of
each piece of metal. So producing a potential difference between their
close ends. If the metal has no net electrical charge then this means that
the potentials at the close end vary so that when one is +V wrt ground the
other will be -V wrt ground. As a signal source this will present itself as
'balanced'.
Now consider a coaxial line down to a receiver (TV) that has an unbalanced
input. Unbalanced means that the potential on the outer screen is
everywhere essentially zero - although currents may flow along the screen.
In an ideal situation, a coax which has the same characteristic impedance
as the receiver it has the effect of making it seem as if the receiver were
actually moved to the end of the coax at the antenna! This means when you
connect the coax to the antenna it tries to 'ground' the piece of metal
you've connected to the screen. This means that the potential at that end
of that bit of metal can't now vary - or at least the coax tries to stop
this.
In practice various things happen when you do the above. When isolated the
total charge on each quarter-wave bit of metal remains constant. But when
you connect the coax this can change as currents flow on and off the bits
as the antenna drives signal power down the coax. But with an unbalanced
coax one of the quarter-wave bits has to have its charge varied like this
*despite* having one end linked nominally to ground. Whist the other does
not have this handicap.
The result is that the field around the antenna, and the variations in
potential and charge movements are not what they would have been. This
upsets the ability of the antenna to function as designed.
The consequences include the sensitivity and directional pattern of the
antenna being altered (Sod's Law being that this is for the worse! :-) )
and the antenna does tend to become sensitive to any currents or potential
on the *outside* of the screen. In effect, the screen of the coax now
becomes a part of the antenna and its interaction with the external world.
Since the receiver and antenna work on the basis of assuming the screen is
at ground, this can upset results.
(A balanced cable like twin feeder essentially arranges for *both*
conductors to be similarly exposed to interactions with the external
world. If this is 'balanced' then the effects may tend to cancel out.
The snag has been pointed out by someone else. Such cables are then
more prone than coax to things like signal loss due to interactions
with their external environment. Yer pays yer money... ;-> )
So it is a bad idea in principle to not have some way of avoiding a 'clash'
between the cable being 'unbalanced' and the antenna being 'balanced'.
How much this all matters depends entirely on the circumstances. If you are
lucky you won't notice that the antenna/cable isn't working as well it
could. In other cases there may be obvious problems which a decent antenna
balun arrangement might prevent.
Alas, the term 'balun' tends to be used for various things, so that may be
one reason for your confusion. A 'balun' may or may not look like a
'transformer' (i.e. some coils of wire looking like a conventional
transformer). Also the 'balun' may also provide impedance matching, which
is a different function to balanced-unbalanced coupling, but may also be
needed for efficient operation.
In addition, 'balanced' does not have to mean 'floating' (i.e. totally
isolated from ground potential). As with 'balun' there is sometimes some
confusion here as people sometimes assume these go together. They may or
may not. So the above isn't a question of if a TV has a 'live' chassis,
etc.
Slainte,
Jim
--
Electronics http://www.st-and.ac.uk/~www_pa/Scots_Guide/intro/electron.htm
Audio Misc http://www.audiomisc.co.uk/index.html
Armstrong Audio http://www.audiomisc.co.uk/Armstrong/armstrong.html
Carpy
Much as I'd like to think that mine & Bills custom is important to
Antiference, I doubt our combined spend on Anti products would come anywhere
near a fraction of the savings they are making with these cost cutting
exercises. I think this is the grim reality we have to face. I emailed them
/ sent feedback about 4 or 5 times through the old website but heard nothing
in reply. I even called the number on the old website after a while but it
was dead. All this was around the time the old site went into
disrepair.......... Must admit that I haven't bothered trying the contact
details on the new website but it's a bit late now as I'm finding better
replacement products all the time.
I always have a good whinge at my wholesaler when they bring in cheaper yet
inferior products in the place of superior more expensive products. It's now
happened with NPRMs. I ordered one the other day as per usual, and found
that half the base has been cut away so it's nothing like as stable or as
heavy as it used to be. I didn't even notice the £7 or so reduction in
price. I rang them and asked what happened to the decent ones, and they told
me they had managed to source them from a new supplier at a lower cost so
could pass on the saving. They didn't seem to realise this "fantastic" £7
saving means the product is now utterly shite.
I get quite annoyed that virtually every company now seems to think I want
cheaper prices at any cost. I don't mind paying more for a decent product
but increasingly I find that decision is being taken away from me. I find
myself having to replace supplied clamps with different ones, adding washers
and making all manner of adjustments just to improve the manufacturers
product.
Marky P
I've noticed some portable tellys with a loop aerial use a balanced
feeder conected to a balun.
Marky P.
Marky P
This is fun :-) Here's what I use for the few installations I've
done:
Logs: Fracarro (got some cheap ones off ebay for £6 and very pleased
with performance).
Yagi: Although I have used Televes X43's in a couple of installs, I
plan to use Blake in the future.
Brackets: Blake
Cable: Philex PF100
Belling Lees: CPC unbranded (and a pile of crap! PF100 doesn't fit
easily).
F-Plugs : CPC unbranded
F-Barrels: Unbranded off Ebay.
Belling couplers: Unbranded off Ebay (and excellent may I add)
Dist. amps with magic eye: Triax (the eyes are only £4 off ebay and
work very well)
Other dist amps: Whatever's going cheap on Ebay.
Marky P.
Mark Carver
> Note to the knowledgeable. The following is obviously somewhat simplified
> for the sake of clarity. :-)
It's excellent Jim, bookmarked from Google for future reference, and use.
--
Mark
Please replace invalid and invalid with gmx and net to reply.
tony sayer
>done:
>
>Logs: Fracarro (got some cheap ones off ebay for £6 and very pleased
>with performance).
>Yagi: Although I have used Televes X43's in a couple of installs, I
>plan to use Blake in the future.
>Brackets: Blake
>Cable: Philex PF100
>Belling Lees: CPC unbranded (and a pile of crap! PF100 doesn't fit
>easily).
>F-Plugs : CPC unbranded
>F-Barrels: Unbranded off Ebay.
>Belling couplers: Unbranded off Ebay (and excellent may I add)
>Dist. amps with magic eye: Triax (the eyes are only £4 off ebay and
>work very well)
>Other dist amps: Whatever's going cheap on Ebay.
>
>Marky P.
>
--
Tony Sayer
Graham.
"tony sayer" <to...@bancom.co.uk> wrote in message
news:+zQMCTMe...@bancom.co.uk...
I blame it on the multiple-choice RAE. I bet you're glad you don't live next
door.
--
Graham
%Profound_observation%
Marky P
wrote:
Nah! Pound shop in town :-)
Marky P.
tony sayer
<m...@privacy.com> scribeth thus
;).....
--
Tony Sayer
tony sayer
<DaveFa...@OMiTTHiSyahooANDTHiS.co.uk> scribeth thus
>tony sayer <to...@bancom.co.uk> wrote:
>
>>Haven't got all day but thats the electrically "quiet" part of the
>>aerial. Relative to that each end of the dipole is "hot" with respect to
>>the centre "cold" point. It doesn't have to be earthed as such thats
>>useful for lightning static protection etc, though a good whack of
>>lightning will see most all domestic installs off!.
>>
>>Connecting a feeder that has one side earthed as it were is not the way
>>to transfer maximum power from this "balanced about a common point"
>>device so some means of doing this balanced to unbalanced operation is
>>needed so a simple BALUN transformer is usually used. This also converts
>>the higher impedance of the dipole to the lower impedance of the cable
>>so as as it ought be and max power transfer takes place without any
>>spurious pick up of the feeder..
>
>This is of course correct. It just seems that connecting a balanced
>dipole to an unbalanced feed works better in practice than you might
>infer from the theory.
Well its not quite as bad as people may perceive it to be, indeed you
can make -OK- connections to a balanced feed from a bit of pro audio
gear and vice versa but with RF its a bit less forgiving especially
transmission...
>
>I wonder if it's because a coax is not a theoretically perfect unbalanced
>transmission line. In practice you have some inductance in the screen as
>well.
>I suspect that if the coax is long enough to noticeably attenuate
>the signal, it will also be doing the job of balanced-to-unbalanced
>signal conversion by itself.
Eh?...where did you get that idea from?..
>
--
Tony Sayer
Paul D.Smith
> I noticed a bit of 300 ohm ribbon feeder hanging from a chimney the other
> day.
>
I think this stuff is still in use in the US. My in-laws have a monster
VHF/UHF aerial in their attic which I need to hook up the next time I'm over
(to replace the rabbit ears they currently use). Both TV and aerial seem
to have 300ohm type connectors although I think there may be a balum on the
TV into an F-plug so I might be able to use co-ax and then a balum at the
aerial instead.
And yes, the rabbit ears give lousy reception but then they rarely watch TV
so they don't care!
Paul DS.
Dave Farrance
>In article <n63eq3178cqpujuba...@4ax.com>, Dave Farrance
><DaveFa...@OMiTTHiSyahooANDTHiS.co.uk> scribeth thus
>>I wonder if it's because a coax is not a theoretically perfect unbalanced
>>transmission line. In practice you have some inductance in the screen as
>>well.
>
>
>>I suspect that if the coax is long enough to noticeably attenuate
>>the signal, it will also be doing the job of balanced-to-unbalanced
>>signal conversion by itself.
>
>Eh?...where did you get that idea from?..
Uh... Well, see above. You inserted the gap in my paragraph.
--
Dave Farrance
Jim Lesurf
<b...@sick-of-spam.invalid> wrote:
> In article <4f6b8834...@charleshope.demon.co.uk>, charles
> <cha...@charleshope.demon.co.uk> wrote:
> > In article <4f6b79...@sick-of-spam.invalid>, Bob Latham
> > <b...@sick-of-spam.invalid> wrote:
> > >
> > [Snip]
> > > The bit I don't understand is why you say the aerial is balanced and
> > > that a balun will fix this. I'm sure I'm wrong but is the aerial not
> > > really a loop connected across the two output terminals? If neither
> > > side is earthed and there is balanced cable then its a balanced
> > > aerial. But, if one side is earthed then its not balanced surely?
> > Think what happens when the supporting metalwork is also earthed
> Ah, I see, I think? Aerials are not my subject really and this thread
> showed me the limits of my understanding.
> Thinking back to the last time I closely looked at a UHF aerial, I seem
> to remember a fixing bolt through the middle of the 'loop' (for want of
> a better expression) holding it to the boom of the aerial. Is it this
> which is earthing the centre and therefore making the output balanced
> which needs a transformer to unbalance it. Is that correct?
No. It is more likely to be t'other way about. i.e. you can ground that
point since symmetry means it will be happy to be grounded as the antenna
is balanced and symmetric wrt it. But in principle there is no need to
ground any point of a balanced antenna for RF purposes. Although there may
be some safefty or other concerns.
> I take it that at 600MHz the metal work of the aerial is effectively
> earthed even if there is no copper ground connection.
If there is no conducting link between the antenna and ground, then the
antenna is not 'earthed'. This may or may not matter, depending on
circumstances and requirements.
Jim Lesurf
> >Haven't got all day but thats the electrically "quiet" part of the
> >aerial. Relative to that each end of the dipole is "hot" with respect
> >to the centre "cold" point. It doesn't have to be earthed as such thats
> >useful for lightning static protection etc, though a good whack of
> >lightning will see most all domestic installs off!.
> >
> >Connecting a feeder that has one side earthed as it were is not the way
> >to transfer maximum power from this "balanced about a common point"
> >device so some means of doing this balanced to unbalanced operation is
> >needed so a simple BALUN transformer is usually used. This also
> >converts the higher impedance of the dipole to the lower impedance of
> >the cable so as as it ought be and max power transfer takes place
> >without any spurious pick up of the feeder..
> This is of course correct. It just seems that connecting a balanced
> dipole to an unbalanced feed works better in practice than you might
> infer from the theory.
Perhaps because the change in performance may be at the few dB level or
less in terms of sensitivity, etc. Given the wide range of powers (and
interference levels) many people experience this may not matter much for
most.
> I wonder if it's because a coax is not a theoretically perfect
> unbalanced transmission line. In practice you have some inductance in
> the screen as well. I suspect that if the coax is long enough to
> noticeably attenuate the signal, it will also be doing the job of
> balanced-to-unbalanced signal conversion by itself.
I suspect it may be more likely that the receiver simply isn't normally
either well matched, or perfectly unbalanced with its 'ground' genuinely at
an RF zero potential. At UHF frequencies I'd expect the external fields on
the coax to be well isolated from the internal ones - although maybe some
of the coaxes people use sometimes are poor?
Jim Lesurf
<ianREMOVET...@g3ohx.demon.co.uk> wrote:
> If you transmit on a dipole (folded or not) which is connected directly
> to a coax feeder, the outer of the coax becomes live with RF. As there
> is usually a pretty good safety earth at the transmitter end, a lot of
> RF current flows on the OUTSIDE of the coax outer, whereas this should
> be RF-dead. In effect, the coax becomes part of the radiating system.
> This may - or may not cause you problems, but there is a much greater
> chance of causing interference to electronic equipment which gets hit by
> the signal radiated by the coax.
> The same problem occurs when receiving. Interference emanating from such
> electronic equipment (switch-mode PSUs, timebase harmonics, switching
> transients etc) gets picked up on the coax, and transferred to the
> aerial, and the coax then feeds this interference down to the receiver.
In lectures, I'd tend to use the magic word 'reciprocity' at this point.
;->
Ian Jackson
<DaveFa...@OMiTTHiSyahooANDTHiS.co.uk> writes
The coax probably does indeed (to some extent) act as its own balun.
Assuming that the coax is a perfect screen, in order for any unwanted
signal/interference (which is picked up on the outside of the coax
screen) to be able to reach the receiver, it will have to make its way
up the outside of the coax screen until it reaches the aerial end. It
will then come back down in the correct 'push-pull' mode between the
coax inner and the inside of the screen. How much unwanted
signal/interference makes it up to the aerial will depend on the
distance from where the pick-up occurs and the aerial. The attenuation
could be considerable.
--
Ian
Bill Wright
"Jim Lesurf" <jc...@st-and.demon.co.uk> wrote in message
news:4f6bc96...@st-and.demon.co.uk...
> In article <4f6b79...@sick-of-spam.invalid>, Bob Latham
> <b...@sick-of-spam.invalid> wrote:
> Note to the knowledgeable. The following is obviously somewhat simplified
> for the sake of clarity. :-)
'unread' and went to sleep. I knew I was going to have to use my brain.
Having woken refreshed I have now read it and I must say it clarifies things
for me which I thought were already clear! So I guess you've helped me pass
the exam, if there is one!
In the days when FM tuners had two aerial terminals marked '300ohm' it was
sometimes necessary to use a 75/300 matcher/balun to connect the coax.
However, some tuners had the terminals but they were also marked up to show
that 75ohm cable (presumably unbalanced) could be connected instead. One
terminal was marked with a ground symbol. Some tuners had three terminals,
marked
75
300 ground
300.
and some had three terminals marked
300/75
ground
300
Now I've often wondered. Could the two 300 terminals connect to an
inductance (the primary of a transformer) which had a centre (or
intermediate tap)? In the first instance above the 300 ohm input would be
grounded at one end and thus unbalanced. In the second instance the centre
tap of the inductance would be grounded, so the 300ohm input would be
balanced, but a coax connected between the 300/75 end and the ground
terminal would be unbalanced. All this is pure hypothesis on my part of
course. What's Jim's opinion?
Incidentally, when the customer was after a weak signal (usually it was some
poor soul who had been made to move from somewhere nice to here, who wanted
to hear his local radio) it was always worth trying all permutations. If the
tuner presented any 75 ohm option this usually worked better than using a
balun and connecting to the 300ohm terminals.
On one occasion 30 years ago the chap had a portable radio with no aerial
socket. I tried connecting the coax to the base of the whip aerial but this
didn't work very well. In the end I fed the signal through a 20dB VHF
amplifier and into a wire halfwave, which I tacked to his door frame. This
worked fine and allowed him to move the radio between the kitchen and
lounge! No bandpass filter or anything!
Bill
Bill Wright
"Ian Jackson" <ianREMOVET...@g3ohx.demon.co.uk> wrote in message
news:SY$I5iCXj...@g3ohx.demon.co.uk...
(snipped)
> If you have a balun at the aerial feedpoint, this problem doesn't happen.
> The coax keeps the RF in and also keeps it out. The only reception and
> radiation of the RF signal is via the actual aerial.
A good clear explanation that anyone should be able to understand. It's good
to have a discussion of the fundamentals for a change.
Bill
Bill Wright
"Mark Carver" <markc...@onetel.com> wrote in message
news:f2a6abff-2136-4dfc...@c4g2000hsg.googlegroups.com...
> When I was a student I had a Philips portable TV with a 75/300
> converter/balun. It was fitted to the back of the case, and had nice
> screw treminals to accept the loop aerial. Out of the other end was a
> length of co-ax that plugged into the Belling-Lee RF input socket of
> the TV itself.
>
> In one of the bedsits I ended up in, the only decent reception was on
> top of a wardrobe. I purchased 2 metres of 300 ohm ribbon cable,
> attached one end to the screw terminals on the balun, fixed the other
> end under some washers with the loop aerial, and screwed the whole lot
> to the top of the wardrobe. Result: Four noise and ghost free TV
> channels :-)
An uncommonly civilised student room if it had a wardrobe! From what I've
seen the custom is to use ones clothes to keep the carpet clean.
Bill
Bill Wright
"Carpy" <ca...@nothanks.com> wrote in message
news:MhFpj.41291$801....@fe1.news.blueyonder.co.uk...
> Very helpful nice one. Quite a few interesting things there. Here's mine
> with a few comments alongside & one question at the bottom.
>
> Yagi aerials: Anti & Wolsey QR range (Triax)
> FM aerials: ABC aerials Ltd (FM rod dipoles & multi elements)
cut them down for DAB use. No baluns, but where there's loads of signal and
the aerial will be connected into the head end via an attenuator it doesn't
seem to do an7y harm.
> DAB aerials - Anti rod dipole / Vision multi element Log
I haven't tried these. I've taken to using the Euopean B III TV aerials
because they have less bandwidth and therefore more gain.
> Logs: Fracarro / Antiference. Have tried the blakes & they are much more
> robust. Might switch to them fully soon. Have one on "test" at the moment
> outside!
The achilles heel is that only one boom is fixerd to the mast. Had one
failure only though, and it was in a gain at Tan Hill (look it up!).
> Remote eyes: Global "Pro" range with F conn
Haven't seen these.
> Domestic dist amps with remote eye return amp: Global F for 9v & 16way /
> Antiference "D" range for 230V.
> Domestic dist amps without remote eye return amp: Antiference F (& some
> old Labgear F type amps still left on the shelf)
> Galvanised brackets: Blake but not direct. More expensive to order direct
> even in large quantity! Same for masts etc.
No, I get them from Sat Sol!
> Cable: Webro - Not tried the Cavel - Nowhere seems to stock it for
> anywhere near the same price as Webro WF100, even when ordering 5000m
> quantity.
> Fixings: UK fixings (cheaper than screwfix for same product)
I got annoyed with them because they sold me some horrible coax (it was like
a big spring when it came off the drum) .
> Masts: Alltrade (cheaper than Sat Sol for same product)
Interesting. Very interesting.
> Outlets: Triax modular & non modular
> Distribution amps (high output) Taylor - Use loads of the TSC3054 amps.
> Channel filters/levellers: Taylor
> Crimp 'f' plugs: Always used Webro but found Vision now do an identical
> product for much cheaper. Tried them and they are really good. Came in a
> plastic tub with resealable lid. Not a flimsy cardboard box like Webro.
I'll have a look at the product when I visit Sat Sol. I prefer the plugs
with the loose pin because you can be sure that the thing has gone on
properly with them.
> Would also like to test out some "waterproof" snap n seal (T&B) connectors
> for LNBs. I get annoyed trying to fiddle around using amalgamating tape
> for all 4 F connectors with numb hands.
It's a bind but it's what I do and I suppose I've just come to accept it.
I've never found any other method that works.
> Coax plugs: Antiference but increasingly unhappy with them. Very
> interesting hearing you use Vision plugs. Not heard about them. Will hunt
> them down.
They're good. The man who uses them here to make huge numbers of special
flyleads for HM Posh Hotels says they are much better to solder. The plastic
doesn't melt.
> Masthead amps and PSUs: Vision
>
> Custom brackets, etc: Rossington Light Engineering - Presumably they have
> a few standard (custom!) sizes they make for you regularly?
Not really. They're all for oddball jobs. We mostly do oddball jobs!
All in all this is very interesting. We should do it more often!
Bill
Bill Wright
> I get quite annoyed that virtually every company now seems to think I want
> cheaper prices at any cost. I don't mind paying more for a decent product
> but increasingly I find that decision is being taken away from me. I find
> myself having to replace supplied clamps with different ones, adding
> washers and making all manner of adjustments just to improve the
> manufacturers product.
It's very annoying. It always seems to be the good products that are
discontinued. If you try to work to decent standards yo feel as if your're
swimming against the tide.
Bill
Bill Wright
"Mark Carver" <mark....@invalid.invalid> wrote in message
> Jim Lesurf wrote:
>
>> Note to the knowledgeable. The following is obviously somewhat simplified
>> for the sake of clarity. :-)
>
> It's excellent Jim, bookmarked from Google for future reference, and use.
I have to admit I've pinched it for future plagiarism.
Bill
Bill Wright
> In message <p8sgq3hpkk33oi8ga...@4ax.com>, Dave Farrance
> <DaveFa...@OMiTTHiSyahooANDTHiS.co.uk> writes
> Assuming that the coax is a perfect screen, in order for any unwanted
> signal/interference (which is picked up on the outside of the coax screen)
> to be able to reach the receiver, it will have to make its way up the
> outside of the coax screen until it reaches the aerial end. It will then
> come back down in the correct 'push-pull' mode between the coax inner and
> the inside of the screen. How much unwanted signal/interference makes it
> up to the aerial will depend on the distance from where the pick-up occurs
> and the aerial. The attenuation could be considerable.
Yes but the ratio between the wanted signal (picked up by the dipole) and
the unwanted signal (picked up by the top end of the feeder) will not change
as the two travel down the coax together. So the length of the cable is
immaterial.
Bill
tony sayer
<DaveFa...@OMiTTHiSyahooANDTHiS.co.uk> scribeth thus
>tony sayer <to...@bancom.co.uk> wrote:
>
>>In article <n63eq3178cqpujuba...@4ax.com>, Dave Farrance
>><DaveFa...@OMiTTHiSyahooANDTHiS.co.uk> scribeth thus
>
>>>I wonder if it's because a coax is not a theoretically perfect unbalanced
>>>transmission line. In practice you have some inductance in the screen as
>>>well.
>>
>>
>>>I suspect that if the coax is long enough to noticeably attenuate
>>>the signal, it will also be doing the job of balanced-to-unbalanced
>>>signal conversion by itself.
>>
>>Eh?...where did you get that idea from?..
>
>Uh... Well, see above. You inserted the gap in my paragraph.
>
I'll have a look at that later..seem to be spammed something wicked this
am;!...
--
Tony Sayer
charles
<insertmybu...@f2s.com> wrote:
not quite. If there are straight sections in the downlead they can become
'tuned lengths' for particular frequencies.
Bill Wright
"charles" <cha...@charleshope.demon.co.uk> wrote in message
news:4f6cc9a5...@charleshope.demon.co.uk...
Only if the bends are of such small radius that they introduce impendence
'bumps', which would be bad installation practice.
Bill
Ian Jackson
<insertmybu...@f2s.com> writes
(which is likely to be TV signals). But if there's a lot of stuff
getting picked up near the bottom end (which is more likely to be
interference), it will have to get all the way to the top (on the
outside of the braid) before it can come down to the receiver in the
push-pull mode.
Assuming that the screening of the coax is perfect, at a given point on
the coax, anything picked up on the outside of the outer conductor will
also be taken up by the inner (in phase, same voltage). There is no
differential. Provided the receiver does not respond to these in-phase
signals, and only responds to the differential (push-pull) signals, it
does not matter how much unwanted signal gets picked up on the outer of
the coax. It's only when some of the in-phase gets converted into p-p
that the trouble (potentially) starts.
As well as the potential for interference, another effect of pick-up of
TV signals on the coax will be to skew the polar diagram for the aerial.
Obviously, in practice, as most TV aerials have no baluns, it's a case
of getting away with it adequately, for most of the time. I'm sure that
there must be some reports and learned papers around. I shall do a bit
of Googling.
--
Ian
charles
<insertmybu...@f2s.com> wrote:
> "charles" <cha...@charleshope.demon.co.uk> wrote in message
> news:4f6cc9a5...@charleshope.demon.co.uk...
> > In article <oI6dnYVTi6cFkTTa...@pipex.net>, Bill Wright
> > <insertmybu...@f2s.com> wrote:
> >
[Snip]
> >> Yes but the ratio between the wanted signal (picked up by the dipole)
> >> and the unwanted signal (picked up by the top end of the feeder) will
> >> not change as the two travel down the coax together. So the length of
> >> the cable is immaterial.
> >
> > not quite. If there are straight sections in the downlead they can
> > become 'tuned lengths' for particular frequencies.
> Only if the bends are of such small radius that they introduce impendence
> 'bumps', which would be bad installation practice.
true, but it happens. It make the installation look "neater".
Jim Lesurf
<insertmybu...@f2s.com> wrote:
LOL. It isn't plagiarism if you say that. :-)
Jim Lesurf
<insertmybu...@f2s.com> wrote:
> "Jim Lesurf" <jc...@st-and.demon.co.uk> wrote in message
> news:4f6bc96...@st-and.demon.co.uk...
> > In article <4f6b79...@sick-of-spam.invalid>, Bob Latham
> > <b...@sick-of-spam.invalid> wrote: Note to the knowledgeable. The
> > following is obviously somewhat simplified for the sake of clarity.
> > :-)
> In the days when FM tuners had two aerial terminals marked '300ohm' it
> was sometimes necessary to use a 75/300 matcher/balun to connect the
> coax. However, some tuners had the terminals but they were also marked
> up to show that 75ohm cable (presumably unbalanced) could be connected
> instead. One terminal was marked with a ground symbol. Some tuners had
> three terminals, marked 75 300 ground 300. and some had three terminals
> marked 300/75 ground 300
> Now I've often wondered. Could the two 300 terminals connect to an
> inductance (the primary of a transformer) which had a centre (or
> intermediate tap)? In the first instance above the 300 ohm input would
> be grounded at one end and thus unbalanced. In the second instance the
> centre tap of the inductance would be grounded, so the 300ohm input
> would be balanced, but a coax connected between the 300/75 end and the
> ground terminal would be unbalanced. All this is pure hypothesis on my
> part of course. What's Jim's opinion?
The answer is 'probably, yes', to the above, but it is hard to be sure as
I've seen various weird and wonderful 'fudges' used for such purposes back
in days of yore. Not always a recognisable inductive transformer or
auto-transformer (one coil with a tap as you describe) but people also
using small capacitors as well. Hard to say how well many of these worked
for two reasons. [1]
1) I have my doubts that many people who made FM tuners actuallly measured
their input impedance or degree of balance/unbalance. So they probably
fiddled about until it seemed OK. Engineering is making things work, even
if you are not clear why. ;->
2) The concept of 'ground' can be an elusive one in these circumstances as
I imagine you already know! :-)
Problem here is that the outer of the coax socket on the tuner may be wired
back via the ground of the set so the *set* behaves as 'unbalanced', but
this does not mean the actual socket outer is clamped to zero potential at
VHF/UHF. Almost certainly not. Ideally, this won't matter if the set,
cable and antenna are all matched, but I wonder how likely that is. ;->
Ideally, the coax outer would act as a screen (hence the name). But this
assumes its links to the tuner and the antenna are appropriate. It would
not matter if the tuner's 'ground' waggles at RF so long as the tuner
isolates this from the way it references screen to inner as seen by
the coax. Then any problems will be on the outer surfaces of the screen.
But if not, then problems can arise in a similar way to those when the
antenna has no balun.
> Incidentally, when the customer was after a weak signal (usually it was
> some poor soul who had been made to move from somewhere nice to here,
> who wanted to hear his local radio) it was always worth trying all
> permutations. If the tuner presented any 75 ohm option this usually
> worked better than using a balun and connecting to the 300ohm terminals.
Some of the sets I've seen in the past had dubious looking 'baluns'. My
suspicion is that these were included for the sake of having '300 Ohm'
terminals on the back for countries that needed this, but the set was
tested with 75 Ohm coax, not 300 Ohm feed.
> On one occasion 30 years ago the chap had a portable radio with no
> aerial socket. I tried connecting the coax to the base of the whip
> aerial but this didn't work very well.
Again, at the base of a whip of a portable the impedance could be almost
anything. Perhaps also varies as you move the radio from place to place.
Undergrad texts evade trying to analyse real-world situations like
this for reasons you can guess. :-) Instead, deal with simplified
examples by making convenient assumptions.
As you found, situations like this have to be dealt with on a
purely case-by-case basis.
> In the end I fed the signal through a 20dB VHF amplifier and into a wire
> halfwave, which I tacked to his door frame. This worked fine and allowed
> him to move the radio between the kitchen and lounge! No bandpass filter
> or anything!
Nowdays someone might call that a portable net radio. :-)
Slainte,
Jim
[1] I've not looked any many modern tuners, though, so things may have
improved. However my experience with commercial domestic audio/radio items
is that makers often cut corners whever an aspect is overlooked in
'reviews'.
Marky P
I remember my parent's hi-fi system (70's Goodmans receiver) had a 75
Ohm & a 300 Ohm socket. The 75 Ohm was a two pin socket and the 300
Ohm was screw terminals. I found that connecting the aerial downlead
to the 300 Ohm socket produced a higher signal level on the level
meter built into the receiver. Whether it was an accurate reading I
don't know, but I was only about 12 at the time and it looked good. I
didn't know what the hell 300 Ohm meant anyway.
Marky P.
Bill Wright
> the coax, anything picked up on the outside of the outer conductor will
> also be taken up by the inner (in phase, same voltage). There is no
> differential. Provided the receiver does not respond to these in-phase
> signals, and only responds to the differential (push-pull) signals, it
> does not matter how much unwanted signal gets picked up on the outer of
> the coax. It's only when some of the in-phase gets converted into p-p that
> the trouble (potentially) starts.
OK. OK . . . .
Hmm . . .
What about badly screened coax?
Bill
Ian Jackson
>
>"Ian Jackson" <ianREMOVET...@g3ohx.demon.co.uk> wrote in message
>news:4+ZiQXCD...@g3ohx.demon.co.uk...
>> In message <oI6dnYVTi6cFkTTa...@pipex.net>, Bill Wright
>> <insertmybu...@f2s.com> writes
>> Assuming that the screening of the coax is perfect, at a given point on
>> the coax, anything picked up on the outside of the outer conductor will
>> also be taken up by the inner (in phase, same voltage). There is no
>> differential. Provided the receiver does not respond to these in-phase
>> signals, and only responds to the differential (push-pull) signals, it
>> does not matter how much unwanted signal gets picked up on the outer of
>> the coax. It's only when some of the in-phase gets converted into p-p that
>> the trouble (potentially) starts.
>
>OK. OK . . . .
>
>Hmm . . .
>
>What about badly screened coax?
>
>Bill
>
comes back down!
--
Ian.
Jim Lesurf
<ianREMOVET...@g3ohx.demon.co.uk> wrote:
> In message <Beadnchtmdsqljfa...@pipex.net>, Bill Wright
> <insertmybu...@f2s.com> writes
> >
> >What about badly screened coax?
> >
> >Bill
> >
> The interference doesn't have to crawl all the way to the top before it
> comes back down!
:-)
I wonder... You can make directional couplers by having a linear
arrangement of 'leaks' from one guide to another. So the effect of a leaky
screen may differ for external fields which start going 'up' the coax from
those coming 'down'. Question of the relative phase delays of the leakage
at each point and how these add when propagation times and phases are taken
into account.
Slainte,
Jim
Ian Jackson
<1ic01xb.22zfo4y6emloN%Spa...@pembers.freeserve.co.uk.invalid>, Alan
Pemberton <Spa...@pembers.freeserve.co.uk.invalid> writes
>Ian Jackson <ianREMOVET...@g3ohx.demon.co.uk> wrote:
>
>> Assuming that the screening of the coax is perfect, at a given point on
>> the coax, anything picked up on the outside of the outer conductor will
>> also be taken up by the inner (in phase, same voltage).
>
>situation as a Faraday cage? ie no voltage on the outside can affect any
>conductor within.
>
Probably.
From what I remember, the Faraday cage works because the electricity is
in the form of free electrons. As all have the same negative charge,
they repel each other, and try to get away from each other as far as
possible. Apart from the Tardis, most things are larger on the outside.
As there's more elbow-room there, that's where the electrons head for.
Sorry for being technical.
--
Ian
Dave J.
I think you're getting mixed up between the way the grid in a thermionic
valve regulates current (electron) flow between cathode and anode and the
way a faraday screen bars electromagnetic waves.
There are scarcely any free electrons in the atmosphere, they would attach
to something PDQ. The only way electrons can travel through atmosphere
without being 'captured' is if they have enough energy (are moving fast
enough) and happen not to hit anything.
They would then be known as beta radiation, which only ever has quite a
short range in atmosphere.
A faraday screen blocks electromagnetic radiation, if the holes in the
screen are smaller than the wavelength (or is it some fraction of the
wavelength?) then the EM radiation won't penetrate.
Dave J.
Ian Jackson
writes
Don't forget that Mr Faraday invented his screen (again, if I remember
correctly, it started with an ice bucket) long before radiation was much
known about. However, as you say, the screening effect of coax uses a
different mechanism.
As I understand things, for coax, the inner is well-screened from
outside influences only when the interfering signal stays on the outside
of the outer. The depth of penetration through the outer decreases with
frequency. This 'skin effect' is caused by the inherent inductance of
outer conductor (and also occurs in ordinary wires etc. Because of this,
even well-screened coax is relatively poorly screened at low
frequencies, but, at the higher frequencies, a few wisps of braiding can
provide a reasonably good screen (but not perfect).
As you say, if the wavelength is small enough, it is increasingly able
to squeeze though holes in a screen (a bit like the old advert for
Homepride flour). The larger grains can't get through. [Apologies if
this is getting a bit like 'One song to the tune of another'.]
--
Ian
Bill Wright
"Alan Pemberton" <Spa...@pembers.freeserve.co.uk.invalid> wrote in message
news:1ic01xb.22zfo4y6emloN%Spa...@pembers.freeserve.co.uk.invalid...
> Ian Jackson <ianREMOVET...@g3ohx.demon.co.uk> wrote:
>
>> Assuming that the screening of the coax is perfect, at a given point on
>> the coax, anything picked up on the outside of the outer conductor will
>> also be taken up by the inner (in phase, same voltage).
>
It's a long time since you did anything physical . . .
Bill
Dickie mint
> It's very annoying. It always seems to be the good products that are
> discontinued. If you try to work to decent standards yo feel as if your're
> swimming against the tide.
>
> Bill
>
>
Couldn't resist this one!!!
Just the same in Broadcast Engineering!
Richard