Balun Required for Receive Only?
GrtPmpkin32
You might want to use 50 ohm coax instead of the 75 ohm you mentioned. It would
present a better match (within reason) to the antenna and receiver.
Also, a 1:1 balun can be of help in a receive situation, if only because it can
sometimes reduce the amount of noise to your radio. The only way I could see it
improving reception is by lessening some noise overall, which CAN be of great
help.
Linus
Jake Brodsky
>Is a balun required (helpful) for a receive only situation? I plan to put up
>a Barker Williamson ASW60C SWL trap dipole and feed it with a RG6 75-ohm
>cable. Total cable run will be about 50-feet (16 meters). Do I need a balun
>between the two? Would the use of a balun at the antenna reduce RFI pickup
>and/or improve overall signal reception?
To answer your primary question: NO, a balun is not absolutely
necessary.
Here is where it helps: If your house is filled with all sorts of
nasty noise sources (touch-lamps, light dimmers, TV sets, VCRs,
computers, etc...) that you can't or don't want to shut off, then you
will need to isolate your antenna from the house and get it as far
away from the house as possible. In that case, a balun at the antenna
feed point may be very useful at keeping the antenna isolated from the
coaxial feed line.
If you didn't have the balun, your coaxial shield would be able to
pick up some of the hash your house-hold electrical system was
radiating. How much depends on how far unbalanced the antenna was,
what band you were on, and how badly the stuff in your house is
radiating.
>I plan on burying about 15-feet (5 meters) of the cable. I read that coiling
>(creating a choke loop) and burying the cable could help to eliminate RFI
>problems. Any thoughts on this?
A coiled choke loop at the antenna can be an effective balun. If you
can support the feedpoint of the antenna well enough, then it may be
all you need. You can also use common mode ferrite chokes (often
found on computer peripherals these days). However, a commercial
balun is often lower loss and it has the advantage of being safe for
transmitting as well.
I don't think burying the coax will do much toward reducing RFI,
though obviously if it were picking up a noise source nearby, then
burying will help somewhat...
>Thanks for any input.
The bottom line: I use a plain old YB-400 to wake up to the BBC every
morning and for casual listening at night before I fall asleep. My
antenna is nothing more than the reel of wire that came with the
radio. Yet, I don't pick up much hash from my household appliances.
My ham shack is actually in an office one side of my barn. It's well
away from most of the noise sources I might get from the house.
Bottom line: I don't use baluns for receiving because I don't really
need them. And in many cases the limiting factors are really
atmospheric noise rather than terrestrial noise. So don't
automatically assume that you need one.
73,
Jake Brodsky, AB3A mailto:fru...@erols.com
"Beware of the massive impossible!"
Jake Brodsky
>You might want to use 50 ohm coax instead of the 75 ohm you mentioned. It would
>present a better match (within reason) to the antenna and receiver.
Ummm, I don't think so. A resonant dipole with 1/4 wave legs has an
impedance of roughly 72 ohms. I'd rather match the feedline to the
antenna and adjust for any difference in the shack than go the other
way...
In any case, the mis-match you're talking about is a mere 2:1 VSWR. I
can't think of any situation, particularly in SWL applications, where
you would notice enough loss to make a change worthwhile.
Snot
You are going through the effort to put up a nice antenna but then ruin the effort with cheap, high loss 75Ohm cable?
Tom Sevart
"Jake Brodsky" <fru...@erols.com> wrote in message
news:72360tgspvbqs7mu8...@4ax.com...
>
> In any case, the mis-match you're talking about is a mere 2:1 VSWR. I
> can't think of any situation, particularly in SWL applications, where
> you would notice enough loss to make a change worthwhile.
Let alone the fact that most of the time you'll be tuning outside the
resonant frequency range of the dipole anyhow, so it's not feasable to try
to keep matching the impedance. There won't be much noticeable change
anyhow.
--
Regards,
Tom Sevart
Amateur N2UHC
Registered Monitor KKS0CE
Al
a Barker Williamson ASW60C SWL trap dipole and feed it with a RG6 75-ohm
cable. Total cable run will be about 50-feet (16 meters). Do I need a balun
between the two? Would the use of a balun at the antenna reduce RFI pickup
and/or improve overall signal reception?
I plan on burying about 15-feet (5 meters) of the cable. I read that coiling
(creating a choke loop) and burying the cable could help to eliminate RFI
problems. Any thoughts on this?
Thanks for any input.
Al
Peter Maus
Al wrote:
>
> Is a balun required (helpful) for a receive only situation? I plan to put up
> a Barker Williamson ASW60C SWL trap dipole and feed it with a RG6 75-ohm
> cable. Total cable run will be about 50-feet (16 meters). Do I need a balun
> between the two? Would the use of a balun at the antenna reduce RFI pickup
> and/or improve overall signal reception?
Since a dipole is generally about 50 ohms anyway, why not use 50
ohm coax?
Reduce losses the length of your run especially at the the top
bands.
J Rob
or some kind of matching xformer built in. There is a (looks like pvc) 1-1/2 X
3 inch (aprox) tube at the feed point where you connect the coax.
Rob Mills ~~~
Dave
RG6 is excellent cable for use at HF frequencies. Losses are minimal
(as I recall, less than 50 Ohm cable for the same diameter). I have
built many folded dipoles and used 4:1 ununs to match them to RG6
cable, with spectacular results.
On Fri, 03 Nov 2000 19:54:31 GMT, "Snot" <sn...@nose.org> wrote:
>Sigh!
>You are going through the effort to put up a nice antenna but then ruin the effort with cheap, high loss 75Ohm cable?
>
Doug VL
http://www.bwantennas.com/asw90ins.htm says -
===== begin quote ====
The antenna uses a patented design that presents a uniform impedance to the
coaxial cable feedline. The SWR is less than 2:1 across the entire operating
range of the antenna.
The ASW-90 antenna is fabricated with #14 stranded copperweld wire. It also
features a high impact housing for the balun and matching network. The
feedline impedance is 50 ohms and comes equipped with an SO-239 (Female)
coax cable connector. Usage of RG-58 cable is recommended.
==== end quote ====
Al wrote in message ...
CW
with an unbalanced to unbalanced transformer?
--
CW
KC7NOD
Web Page http://www.cw.arjika.com
"Dave" <ric...@knac.com> wrote in message
news:3a0433fa...@news-server.socal.rr.com...
J Rob
features a high impact housing for the balun and matching network<
I have a ASW-60c (still in the box) and it has a pretty good sized housing
where the coax connector attaches. I suspect that it has a balun in it. I can't
believe they would have used this big of a housing to just attach a coax
connector.
Rob Mills ~~~
Tom Sevart
> Balun not unun, thanks.
Is there any difference between a balun and an unun, other than how they're
used?
Dave
On Sat, 4 Nov 2000 11:29:27 -0800, "CW" <cma...@sprynet.com> wrote:
>Question: How do you connect an unbalanced feedline to a balanced antenna
>with an unbalanced to unbalanced transformer?
>--
>CW
> KC7NOD
>Web Page http://www.cw.arjika.com
>
>"Dave" <ric...@knac.com> wrote in message
Bruce Jensen
"Doug VL" <swor...@netonecom.net> wrote:
> The ASW-90 antenna is fabricated with #14 stranded copperweld wire.
It also features a high impact housing for the balun and matching
network. The feedline impedance is 50 ohms and comes equipped with an
SO-239 (Female) coax cable connector. Usage of RG-58 cable is
recommended.<
This looks like an excellent antenna. How is it's noise response? How
much does it cost? (Couldn't find a price on the B&W pages...)
Thanks,
Bruce Jensen
Sent via Deja.com http://www.deja.com/
Before you buy.
J Rob
Go here, www.bwantennas.com/bwcat.htm it's $169.00 and 90ft long.
Cheers,
Rob Mills ~~~
Dave
I hope so, as this insures all parts of the system are at DC ground
and therefore continuously draining off the static charges.
On Sat, 4 Nov 2000 21:58:46 -0600, "Tom Sevart" <n2...@yahoo.com>
wrote:
>
>"Dave" <ric...@knac.com> wrote in message
Rich Griffiths
Unless you're going to use a high-quality cable that has an outer covering
specifically designed for direct burial, you shouldn't bury coax. If you don't
have this type of cable, but you need to get the cable underground, you should
run it through a pipe or tube (e.g., PVC) to avoid direct contact with the soil.
Otherwise, you've already heard the rest of my 2-cents worth:
- a balun would probably help, but is not required
- 50-ohm cable might be better than 75, but not enough to worry about
- a balun may reduce RFI, but how much depends on many factors
- a choke loop is a kludgey replacement for a balun (so don't do both)
Personally, I like to use baluns because (1) they're reasonably cheap and easy
to install, (2) I transmit using mine, and they help abate TVI, and (3) a proper
balun places both legs of the dipole at dc ground potential (but a choke balun
doesn't).
... Rich
Jake Brodsky
wrote:
>Is there any difference between a balun and an unun, other than how they're
>used?
Yes, but to describe them would require lots of drawings. This medium
isn't well suited for that...
I recommend Jerry Sevick's Book on Baluns and UnUns...
chuck
between the two...
so in my opinion i would go with the 50 ohm stuff !!!!
just my thought on the topic
cheers
chuck
Rich Griffiths wrote:
> I'm surprised that no one said "Don't bury the cable."
>
> Unless you're going to use a high-quality cable that has an outer covering
> specifically designed for direct burial, you shouldn't bury coax. If you don't
> have this type of cable, but you need to get the cable underground, you should
> run it through a pipe or tube (e.g., PVC) to avoid direct contact with the soil.
>
> Otherwise, you've already heard the rest of my 2-cents worth:
> - a balun would probably help, but is not required
> - 50-ohm cable might be better than 75, but not enough to worry about
> - a balun may reduce RFI, but how much depends on many factors
> - a choke loop is a kludgey replacement for a balun (so don't do both)
>
> Personally, I like to use baluns because (1) they're reasonably cheap and easy
> to install, (2) I transmit using mine, and they help abate TVI, and (3) a proper
> balun places both legs of the dipole at dc ground potential (but a choke balun
> doesn't).
>
> ... Rich
>
> Al wrote:
>
CW
system. The change in the match between 50 and 75 ohm coax would be so
slight that hearing a difference would be very difficult. More likely that
the 75 ohm cable had some problem (poor quality, high loss, bad connectors,
ect).
"chuck" <c.fo...@aci.on.ca> wrote in message
news:3A070CAB...@aci.on.ca...
Telamon
<cma...@sprynet.com> wrote:
Using the right coax is important for two reasons:
1. Not reflecting the desired signal from the radio input.
Using a 75 ohm cable would give you a 50% reflection.
2. Maintaining the coax's ability to function as a transmission
line in rejecting common mode noise.
The two reasons add up to a potentially large difference in signal
to noise.
In the case of a long wire that has a 400 to 600 ohm impedance
a 3 to 1 turns ratio transformer at the antenna end provides
approximately the proper source termination at one end of the
transmission line and the radio input provides the right load
termination at the other end.
The source / cable / load impedance needs to be the same to gain
the maximum signal to noise.
In the environment that most people find themselves in (electrically
noisy) they should be trying to cause the coax cable to behave as a
medium to transfer the desired signal from the antenna to the radio
and not have the coax behave as part of the antenna system.
You can't make a perfect transformer at the antenna end but every
reasonable effort will yield improvements.
--
Telamon
Remove the spamshield and the obvious to Email me but I would prefer
a follow up to the news group as Email is infrequently checked.
Telamon
fru...@erols.com wrote:
The same transformer could be used for a balun or unun.
It looks to me that what it is called is determined by
how it is used.
CW
depending on frequency. Unless you have an antenna tuned to one frequency
and stay there, you will have a wide variation. Consider a point where your
antenna is presenting a 450 ohm impedance. Transformation through your 9:1
transformer and it will be 50 ohms. A perfect match to your 50 ohm coax and
radio. At any other frequency, the impedance will vary and no longer be
matched to your coax. The radio will not see the impedance of the antenna or
of the coax. It will see the combination of the two. You can get around this
by using a length of coax that is cut to an electrical half wave of that
frequency but then you would have to change coax length each time you
changed frequency and in any case, you would still have a mismatch. You
could vary the length of the feedline so the resulting impedance would match
your radio (a common practice with HAM antennas. Cecil, where are you?).
This would, of course, make bandscanning rather difficult. Try putting a
tuner in line with your coax and see just how far you have to tune to make
any significant difference. BTW, rejection of common mode noise is more a
function of the shielding and decoupling from the antenna than any thing
else (you have that down with your transformer).
"Telamon" <telamon_s...@pacbell.net.is.invalid> wrote in message
news:UOKN5.238$FT3.1...@news.pacbell.net...
Telamon
<cma...@sprynet.com> wrote:
I assume you mean to put the tuner between the coax run to the
antenna and the radio. You would then be looking at the
coax / transformer / antenna as a complex impedance network.
This makes sense if you are driving this network with a
transmitter as a practicable matter but I look at it another
way since we are considering receive.
There are three elements to the situation:
1. antenna
2. coax
3. radio
Two nodes:
1. antenna to coax
2. coax to radio
With a 50 ohm impedance input on the radio and 50 ohm coax two
elements and one node is matched. The problem remains matching
the antenna to coax. A transformer would get you in the ball
park impedance wise over a frequency range. A tuner here would
do better but that would be cumbersome and more difficult to
operate. All the RF energy will not make it through the
transformer over a range of frequencies but it will present
a low impedance to the input side of the coax forcing
the desired signal to propagate through the coax
differentially so the radio input will be driven by it.
With the radio end of the coax well matched and the other
end with a termination "in the ball park" RF seen by the
coax shield will have little effect in the way of generating
a differential signal that can drive the radio input. This
is the mechanism that improves the signal to noise.
With little cost and no additional operating complexity
a transformer can do allot of good when the antenna
presents a large mismatch to the coax or because of
design does not cause most of its signal energy to
drive the coax differentially.
Bill Meacham
>
> In article <8u87e5$4nb$1...@slb6.atl.mindspring.net>, "CW"
> <cma...@sprynet.com> wrote:
>
> > In actuality the impedance of that wire will vary considerably from that
> > depending on frequency. Unless you have an antenna tuned to one frequency
> > and stay there, you will have a wide variation.
Very important fact number one.
> > > 1. Not reflecting the desired signal from the radio input.
> > > Using a 75 ohm cable would give you a 50% reflection.
The VSWR is this case is 1.5:1, or around 20% standing
wave. 50% reflection would be a 3:1 VSWR IIRC.
Now, lets go to the charts.
100 feet of modern-day RG6 coax has about 1 db loss at the
top end of the SW spectrum. RG8/9913 somewhat less.
Going to Fig. 13-12 in the ARRL Handbook from 1955, this is
a chart which shows additional loss to be added to the setup
based on SWR based on the cable loss. This is where that
reflected signal disappears to. In this case, the
additional loss is below the scale which drops off at 0.1
db. But to gain some sort of perspective, lets tune off
frequency a couple hundred khz where the SWR will likely be
2:1 and we see a loss of 0.2db....
Don't hit flame reply yet, keep reading.
These relationships should be kept in mind in the context
that an S-unit is generally regarded as a 6 db difference
(3db in some sets).
To suggest that 50% of the signal is lost implies to some
folks that the S-meter is gonna drop from S9 to S4 or 5 and
thats simply not the case.
So there's no magic in using 50 versus 75 ohm cable in this
application. Its even more moot when you get any distance
away from from the design freq of the antenna because the
impedance goes up into the hundreds, if not thousands of
ohms. This is the area where the 9:1 balun fans find their
justification, and maybe rightly so because a much larger
chunk of the band will be representing high impedance than
will the little narrow chunks(s) that offer true 50-ohm
match.
You could really get funky and say that 75 ohm cable finds
'sweet spots' at TWO sides, either direction from the 50 ohm
center and therefore has a wider range of accurate
matching. If the difference is only 0.2 db, that argument
is equally silly.
But the cable factor is only a small portion of the big
picture. There are other factors not included here like the
reactance of the whole set up when it is operated
off-resonance and the general inefficiency of the antenna
element itself when it is operated off resonance.
So whats the cure?
This is why antenna tuners were made. There's no such thing
as a perfect antenna that gives 2:1 match across the whole
spectrum...well, maybe, those zillion element log-periodic
beams do a fairly good job.
Whether the tuner is placed at the receiver, or at the
antenna is another moot argument in this context because the
goal is to match a system full of complex reactances and
impedance variations to the 50 ohms that the receiver is
specced to see. As a general concept, an antenna tuner, in
spite of its name, isn't "tuning" the antenna to some
different resonant frequency. In fact it is simply
cancelling the reactances and providing a forced impedance
match to the out-of-resonance antenna system that is
comprised of the antenna + feedline.
In these days where antenna adjustments on the receiver have
been discarded in favor of a straight 50-ohm match, a tuner
is indispensable.
> > >
> > > 2. Maintaining the coax's ability to function as a transmission
> > > line in rejecting common mode noise.
I can't do the numbers on that one, but yes, it is a factor
IF you live in a noisy environment. This is the only
justification for putting the tuner at the antenna end of
the coax. Its also the justification to use twin-lead or
ladder line so that you can achieve a proper balance on a
balanced antenna. In this regard, true baluns do help.
> > > The source / cable / load impedance needs to be the same to gain
> > > the maximum signal to noise.
I think I know what you're getting at here, but it should be
clarified that the noise referred to is only the locally
generated noise that may find its way into the mismatched
coax, not the antenna, nor does this theory have anything to
do with the regular thermal/RF noise floor.
>
> I assume you mean to put the tuner between the coax run to the
> antenna and the radio. You would then be looking at the
> coax / transformer / antenna as a complex impedance network.
> This makes sense if you are driving this network with a
> transmitter as a practicable matter but I look at it another
> way since we are considering receive.
You can look at it from a directional viewpoint to gain
certain references, but the bottom line is that the receiver
wants to see a properly matched load on its input terminal.
Its a function of proper termination of the internal RF
circuitry and not a directional consideration.
>
--
Bill Meacham
Vieques, PR
Ralf R. Radermacher
> In these days where antenna adjustments on the receiver have
> been discarded in favor of a straight 50-ohm match, a tuner
> is indispensable.
Not to forget that the input filter/preselector networks of most
receivers only perform as designed when they 'see' 50 ohms. In some
cases, a mis-match at the input is transformed all the way through to
the input of the 1st mixer.
Ralf
--
Ralf R. Radermacher - DL9KCG - Köln/Cologne, Germany
http://www.free-photons.de
e-mail from trash accounts (deja, yahoo) automatically rejected
chuck
this is connected right to the center wire of the 50 ohm coax and then there is
a lightning arrester attached to a 10 foot ground rod. the coax goes in the
basement to an antenna tuner ( of arnie coro's design ) then to the receiver (
rca ar-88) and that is it!!!!!!!!!
i still have the 75 ohm coax the ends are proper crimp on ones and are in good
shape, but i will see if i can find any other faults with it and let you know!!
cheers
chuck
CW
antenna (65 foot wire with matching transformer) seemed as though the
signal was getting weaker and the noise level was increasing. I wasn't sure
if it was just location or what. I thought it possible that a neighbor had a
new electrical device that was effecting it. In any case, I just used one of
my other antennas that are located in different areas. Yesterday, I switched
to the problem antenna and it was extremely low signal and the noise was as
if it was in the room with me( my monitor puts out a lot of crap). I decided
it was time to do something about it sow out came the multimeter. Checked
everything and the cable was open somewhere but the outside of it and the
connectors seemed fine. Tried flexing cable and could not find problem so
before replacing 75 feet of coax, I cut about three feet off the receiver
end. That fixed it (fortunately).
A cable can be bad and not look like it and it doesn't necessarily fail all
at once.
--
CW
KC7NODnews:3A0835FD...@aci.on.ca...
Telamon
<ex...@coqui.net> wrote:
> Telamon wrote:
> >
> > In article <8u87e5$4nb$1...@slb6.atl.mindspring.net>, "CW"
> > <cma...@sprynet.com> wrote:
> >
> > > In actuality the impedance of that wire will vary considerably from
> > > that
> > > depending on frequency. Unless you have an antenna tuned to one
> > > frequency
> > > and stay there, you will have a wide variation.
>
> Very important fact number one.
>
>
> > > > 1. Not reflecting the desired signal from the radio input.
> > > > Using a 75 ohm cable would give you a 50% reflection.
>
>
> The VSWR is this case is 1.5:1, or around 20% standing
> wave. 50% reflection would be a 3:1 VSWR IIRC.
Your right about the reflection not being 50% it is more like 25%.
> Now, lets go to the charts.
> 100 feet of modern-day RG6 coax has about 1 db loss at the
> top end of the SW spectrum. RG8/9913 somewhat less.
> Going to Fig. 13-12 in the ARRL Handbook from 1955, this is
> a chart which shows additional loss to be added to the setup
> based on SWR based on the cable loss. This is where that
> reflected signal disappears to. In this case, the
> additional loss is below the scale which drops off at 0.1
> db. But to gain some sort of perspective, lets tune off
> frequency a couple hundred khz where the SWR will likely be
> 2:1 and we see a loss of 0.2db....
> Don't hit flame reply yet, keep reading.
Hey! I'm happy you took the time to respond. Thanks!
> These relationships should be kept in mind in the context
> that an S-unit is generally regarded as a 6 db difference
> (3db in some sets).
> To suggest that 50% of the signal is lost implies to some
> folks that the S-meter is gonna drop from S9 to S4 or 5 and
> thats simply not the case.
Right! The S meter is a log function so it would take a large change
in linear signal strength to move it several S units and we will not
affect it that much by changing from 50 to 75 ohm cable.
> In these days where antenna adjustments on the receiver have
> been discarded in favor of a straight 50-ohm match, a tuner
> is indispensable.
>
> > > >
> > > > 2. Maintaining the coax's ability to function as a transmission
> > > > line in rejecting common mode noise.
>
> I can't do the numbers on that one, but yes, it is a factor
> IF you live in a noisy environment. This is the only
> justification for putting the tuner at the antenna end of
> the coax. Its also the justification to use twin-lead or
> ladder line so that you can achieve a proper balance on a
> balanced antenna. In this regard, true baluns do help.
>
>
> > > > The source / cable / load impedance needs to be the same to gain
> > > > the maximum signal to noise.
>
> I think I know what you're getting at here, but it should be
> clarified that the noise referred to is only the locally
> generated noise that may find its way into the mismatched
> coax, not the antenna, nor does this theory have anything to
> do with the regular thermal/RF noise floor.
> >
The "noise" I was referring to is locally generated noise that the coax
would protect against. It will make no difference in atmospheric noise
received by the antenna nor the thermal noise in the receiver.
> > I assume you mean to put the tuner between the coax run to the
> > antenna and the radio. You would then be looking at the
> > coax / transformer / antenna as a complex impedance network.
> > This makes sense if you are driving this network with a
> > transmitter as a practicable matter but I look at it another
> > way since we are considering receive.
>
> You can look at it from a directional viewpoint to gain
> certain references, but the bottom line is that the receiver
> wants to see a properly matched load on its input terminal.
> Its a function of proper termination of the internal RF
> circuitry and not a directional consideration.
> >
CW responded earlier by mentioning a tuner at the radio for transmit
and I was trying to look at this differently.
Well I'm of the opinion that using a transformer on a long wire makes
in several small ways an improvement in the received signal from locally
generated noise. ( I'm learning to be more specific )
Since many people are going to put up a long wire for short wave
reception I was trying to point out a way to optimize this antenna
in a simple way at low cost.
Using a transformer and 50 ohm coax should be worth the effort.
Do you agree?
CW
between 50 and 75 ohm is of no real consequence. If you will reread my
previous post, I was not suggesting the tuner for transmitting. I was
suggesting putting the tuner in line with your receiving antenna and seeing
just how far you have to tune to notice any appreciable difference in
signal. The tuner can be useful in receive situations but the impedance
range that is tolerable by your receiver is quite broad.
BTW, I had pans for a matching transformer and other useful accessories on
my web page. Well, my web host crapped out and dumped my site and informed
me that they will no longer carry it for free. Does anybody have any
recommendations as to a free web hosting service? I do, fortunately, have it
backed up but I need someplace to put it. Before I went with the one I did,
I had tried others with less than satisfactory results. If anyone has any
suggestions, I would sure appreciate it.
"Telamon" <telamon_s...@pacbell.net.is.invalid> wrote in message
Tom Sevart
"Telamon" <telamon_s...@pacbell.net.is.invalid> wrote in message
>
> The same transformer could be used for a balun or unun.
>
> It looks to me that what it is called is determined by
> how it is used.
That's what I was thinking. I have a balun that was attached to a dipole,
now it is an unun because it is being used on a longwire.
Bill Meacham
>
>
>
> Well I'm of the opinion that using a transformer on a long wire makes
> in several small ways an improvement in the received signal from locally
> generated noise. ( I'm learning to be more specific )
>
> Since many people are going to put up a long wire for short wave
> reception I was trying to point out a way to optimize this antenna
> in a simple way at low cost.
>
> Using a transformer and 50 ohm coax should be worth the effort.
> Do you agree?
>
> --
> Telamon
>
it' situations. If there is any logic to the transformer,
it would likely be as was in the case of the 9:1 example
mentioned before - a relatively wider range of better
matched frequencies.
Here's the deal.
A long-wire, or lets be accurate and call it a "random wire"
because a long wire is a very specific type of antenna
several wavelengths long, has a feed impedance that is very
low at its odd quarter-wave multiples. You're probably
familiar with the classic quarter-wave vertical, which is
equivalent to a random-wire for those who are real stingy
with their wire, has a feedpoint impedance BELOW 50 ohms.
Something around 30. Between that length and a half-wave,
the feedpoint impedance rises drastically and at a half-wave
is somewhat difficult to match from the end. If you've ever
transmitted, trying to load a half-wave ant is nigh
impossible.
This concept repeats itself over and over as the wire is
lengthened. Low z at odd 1/4 wave multiples, hi z at 1/2
wave multiples. So, in theory, you can pick a special
length and get a good match using a fixed matching network,
9:1 transformer or whatever. That works good for hams
because the bands typically have some harmonic relationship
between each other, but for SWLs isn't practical unless you
want a separate random-wire for each band.
Feeding a random-wire with coax tosses theory and a fair
amount of logic out the window because of the inductance and
capacitance of the cable that you are throwing into the
circuit. The cable IS going to become part of the whole
antenna and its impedance is irrelevant because its gonna be
one chunk of standing wave in as much as the antenna element
itself for the better part of the desired tuning range.
Even the length of ground wire becomes an issue in the
un-matched condition.
I'm not gonna comment on using a "transformer" because I
have no personal experience in the various gadgets being
marketed these days as transformers, nor how they function.
I have used regular baluns, coax stubs, and transformers of
that nature. If I had to guess, I would suspect that maybe
the "common-mode" noise rejection could be improved at ONLY
the freqs that are close to say a 30-150 ohm feedpoint
impedance, otherwise, I doubt it. If its a 9:1 true bal-un
xfmr, (notice that they don't call it a 450-50 ohm balun)
move that scale up to 270-1350 ohms which would represent a
much wider range of freqs.
If noise is a problem and you can reduce it by an S-unit or
two without degrading the signal, thats more effective
usable gain than you will ever be able to achieve by
changing coax or worrying too much about impedance matching,
but, that result is frequency dependent.
Now, 50 or 75 ohm cable? If you say 50-ohm Belden 9913/RG8
versus plastic dielectric VCR jumper 75-ohm RG59, ok, its a
better cable. But conversely, if you say 75-ohm RG11 CATV
grade vs. cheapo CB RG58, I'll take the bigger cable because
if there is anything to be gained it will be with the
larger, higher quality cable. The more effective shielding
of the better cable will be a significant factor in local
noise rejection.
The 2 big variables here are:
1. Do you have a true 50 ohm input on your receiver, or is
it an alligator clip on the antenna rod scheme? On the
latter, anything goes and forget everything thats ever been
said about antennas.
2. Actual length of the antenna and coax. If you have 100
feet and the next guy has 120 feet the performances will be
totally different on different frequencies.
All that long-winded discussion goes to say that outside of
a fixed frequency purpose, you're shooting in the dark. Its
the nature of antennas to be experimented with and find what
gives the best results in your situation. The best
improvements are found by getting more height, adjusting the
length (longer or shorter) for your band of interest and
having a good, versatile tuner. Although its inconvenient
in a lot of installations, its best to run the random wire
straight to the tuner and avoid all the potential matching
conflicts and questions introduced by coax and transformers,
not to mention their cost in an application where they
really offer no tangible advantage..
Kevin
> The best
> improvements are found by getting more height, adjusting the
> length (longer or shorter) for your band of interest and
> having a good, versatile tuner. Although its inconvenient
> in a lot of installations, its best to run the random wire
> straight to the tuner and avoid all the potential matching
> conflicts and questions introduced by coax and transformers,
> not to mention their cost in an application where they
> really offer no tangible advantage..
> --
> Bill Meacham
> Vieques, PR
Glad you stated the above, Bill. Been SWLing for one year and tried
various antenna configurations and determined that certain devices in
my house generate RFI even tho the antenna is a RG-58/9:1
transformer/random longwire setup. With those devices (computers, VCRs,
etc) powered off the noise disappears. So, the next antenna I install
will have the longwire go directly to the preselector/tuner.
Regards,
Kevin
Ventura, Calif
Jake Brodsky
wrote:
>
>"Telamon" <telamon_s...@pacbell.net.is.invalid> wrote in message
>news:GSKN5.239$FT3.1...@news.pacbell.net...
>>
>> The same transformer could be used for a balun or unun.
>>
>> It looks to me that what it is called is determined by
>> how it is used.
>
>That's what I was thinking. I have a balun that was attached to a dipole,
>now it is an unun because it is being used on a longwire.
Read Sevick's book and then you'll see the difference. It's very
subtle. I can't explain it here without a good web site and lots of
pictures and I have no desire to plagiarize Sevick's work.
Bill Meacham
>
>
> I was thinking of connecting the coax center conductor
> to the random wire tuning in a number of stations and
> then repeating with the transformer recording the S
> meter readings. I would then have to repeat this
> looking for my local noise sources with this before
> and after technique. This will not be exact by
> any means. Any other ideas here?
>
> --
> Telamon
>
Use fairly diverse freq selections when you make this test.
My suspicion is that you'll find a whole mixture of results
that need rationalizing. Is your receiver a fixed 50-ohm
input or is this via a tuner? Thats a big issue.
--
-Bill Meacham
Vieques, PR
Telamon
(Ralf R. Radermacher) wrote:
> Bill Meacham <ex...@coqui.net> wrote:
>
> > In these days where antenna adjustments on the receiver have
> > been discarded in favor of a straight 50-ohm match, a tuner
> > is indispensable.
>
> Not to forget that the input filter/preselector networks of most
> receivers only perform as designed when they 'see' 50 ohms. In some
> cases, a mis-match at the input is transformed all the way through to
> the input of the 1st mixer.
>
> Ralf
Well I would not know about that. I have a fairly narrow
focus here since my engineering work involves transmission
lines and that was what I was trying to explore here. My
work does not involve radios. I do work with DC to 10 GHz
hardware in a mostly digital environment. My hardware
designs pay close attention to impedance, loss tangents,
VSWR of circuit boards, cables, sockets etcetera to test
IC's. I pay allot of attention to the quality of signal.
It seems to me that going from the 75 ohm to 50 ohm
cable would make on average maybe 1 S unit higher
reading on the receiver and if the coax was utilized
in a way that rejected the local noise in my house
then that would translate to a better signal to noise
since I do have local noise sources.
I do understand that:
1. The impedance of the random wire will change depending
on the frequency of operation.
2. The impedance of the transformer will change over
frequency also.
But I know that the transformer will "improve" the
signal to noise as imperfect as it is and my 40 foot
long 20 foot off the ground random wire works better
with the 3 to 1 ratio transformer in general.
I would like to give the people in this news group
some data but I lack the test equipment to do this.
I was thinking of connecting the coax center conductor
to the random wire tuning in a number of stations and
then repeating with the transformer recording the S
meter readings. I would then have to repeat this
looking for my local noise sources with this before
and after technique. This will not be exact by
any means. Any other ideas here?
--
Telamon
The opinions expressed in this posting are my own and do not reflect the
new Bush-Gore administration where everyday in the oval office AlGore
says "OK that's 2,907,578 to 2,907,579 let's toss the coin one more time."
Telamon
> Telamon wrote:
> >
> >
> > I was thinking of connecting the coax center conductor
> > to the random wire tuning in a number of stations and
> > then repeating with the transformer recording the S
> > meter readings. I would then have to repeat this
> > looking for my local noise sources with this before
> > and after technique. This will not be exact by
> > any means. Any other ideas here?
> >
> > --
> > Telamon
> >
> Use fairly diverse freq selections when you make this test.
> My suspicion is that you'll find a whole mixture of results
> that need rationalizing. Is your receiver a fixed 50-ohm
> input or is this via a tuner? Thats a big issue.
I have a Drake R8 and no tuner. I have neighbors with light
dimmers and other noise sources I battle on a daily basis.
My set up is the already mentioned 40 foot random wire from
the house to the back corner of the yard about 20 foot off
the ground. This is connected with the transformer to 50
ohm cable to the R8. The R8 has 50 ohm and 500 ohm antenna
connections.
I should mention that I live in southern California where
allot of interesting SW signals are not as strong as in
other parts of the country so if I can find a fairly
inexpensive and / or low cost way to make even small
improvements it's a help.
Jake Brodsky
<telamon_s...@pacbell.net.is.invalid> wrote:
>In article <3A0DE17E...@coqui.net>, ex...@coqui.net wrote:
>
>> Telamon wrote:
>> >
>> >
>> > I was thinking of connecting the coax center conductor
>> > to the random wire tuning in a number of stations and
>> > then repeating with the transformer recording the S
>> > meter readings. I would then have to repeat this
>> > looking for my local noise sources with this before
>> > and after technique. This will not be exact by
>> > any means. Any other ideas here?
The differences you're looking for may not be so easy to measure since
band conditions will change and so will the noise. Given that you're
dealing with a random wire in a poorly modeled situation, I'd simply
write the whole thing off as an exercise of Heisenberg Uncertainty.
You simply can't get close enough to a conclusive test to know for
certain if one is better than the other.
>> Use fairly diverse freq selections when you make this test.
>> My suspicion is that you'll find a whole mixture of results
>> that need rationalizing. Is your receiver a fixed 50-ohm
>> input or is this via a tuner? Thats a big issue.
>
>I have a Drake R8 and no tuner. I have neighbors with light
>dimmers and other noise sources I battle on a daily basis.
>
>My set up is the already mentioned 40 foot random wire from
>the house to the back corner of the yard about 20 foot off
>the ground. This is connected with the transformer to 50
>ohm cable to the R8. The R8 has 50 ohm and 500 ohm antenna
>connections.
>
>I should mention that I live in southern California where
>allot of interesting SW signals are not as strong as in
>other parts of the country so if I can find a fairly
>inexpensive and / or low cost way to make even small
>improvements it's a help.
The big thing that a transformer will do for you is to decouple your
coax from the antenna system, thus buying you some distance from the
various noise sources in your house. Keep in mind that even if the
shield does allow some radiation to leak in due to an unbalanced load,
it isn't likely to pick up stuff inside your house unless the coax is
short enough to be around 1/2 of a wavelength.
Your outdoor random wire should also be long enough to pick up a
substantial signal. Minimum size should be at least 1/4 of a
wavelength. Often if these antenna wires are shorter, the efficiency
becomes so poor that the coaxial shield picks up more than the random
wire does, and thus the poor S/N ratio from internal noise sources.
Happy Listening!