Common mode choke - best position?

[Guy G4DWV 4X1LT]

I have seen, in various places, 3 different positions for a common
mode choke in an antenna's feed-line.
1) Between TX and ATU, 2) Outside the shack (next to wall) and 3)
various other places in the feed-line.

I currently have 2 loops of coax with 4 ferrite (type 31)
"Polo-shaped" cores each side (I can supply a photo if my description
fails) just after the coax leaves my ATU. I can move it further away
from my operating position to where the coax leaves the shack.

What do you think would be the best? Outside would be a PITA, but not
impossible. My antenna is a 30m end-fed, approx 3-4m above the ground.
--

73 de Guy G4DWV/4X1LT

[Brian Howie]

In message <hi3u2dd8jg9qkbk89...@4ax.com>, Guy G4DWV 4X1LT
<no-e...@tardis.com> writes

I'd put it up at the antenna end of the coax. It's supposed to stop
unbalanced currents causing the feedline to radiate. I've a common mode
choke in my 6m delta-loop up there for that reason.

Have you a counterpoise from the business end of the coax ?

https://www.sotabeams.co.uk/efhw/ says between the transceiver and the
antenna tuner.

Brian
--
Brian Howie

[Catweazel]

I have some of these on my own (unbalanced) antennas - always seemed
best to me to have a choke nearest the source of the unbalance, i.e. the
antenna & never felt the need to put it anywhere else; there's a nagging
thought in my head that it may not matter too much where it is placed as
it will have some effect - the objective is to minimise the (unbalanced)
rf current on the coax braid as best as one can.
HTH

---
This email has been checked for viruses by Avast antivirus software.
https://www.avast.com/antivirus

[Stephen Thomas Cole]

Pardon the silly question but "common mode choke" and "RF choke" (and other
folk call the same a "choke balun") are one and the same, right? If so, I'd
agree that right at the antenna is the sensible place for it as the whole
point is to reduce/remove reflected current from the coax.

A side point; why do radio amateurs have so many different names for the
same things? It's really maddening when you're new to the hobby!

--
STC / M0TEY /
http://twitter.com/ukradioamateur

[David Woolley]

On 12/12/17 09:40, Stephen Thomas Cole wrote:

> Pardon the silly question but "common mode choke" and "RF choke" (and other

RF Choke is a more general concept, and is more commonly used in single
wire situations, on a circuit board.

> folk call the same a "choke balun") are one and the same, right? If so, I'd

Balun comes from BALanced to UNbalanced transformer, but the devices in
question are not really acting as transformers. Prefixing with "choke",
is an attempt to distance oneself from the transformer interpretation.
In true transformer case, there would be a DC path between both wires,
but no DC path from the transmitter to the antenna.

Common mode choke is a more accurate description of how they actually work.

> agree that right at the antenna is the sensible place for it as the whole
> point is to reduce/remove reflected current from the coax.
>
> A side point; why do radio amateurs have so many different names for the
> same things? It's really maddening when you're new to the hobby!
>

It's often because, like here, technology was developed to achieve a
function in a different way, but the name was adapted from the old way
of doing it, and then developed into a more accurate description.

It can also be the result of marketing twisting a meaning, or people not
really understanding the real meaning. SDR might be an example;
amateurs use the term for an analogue front end and ADC, which may have
no software at all, but, professionally, the term refers to the complete
receiver. Consequently, whilst most current commercial rigs are SDRs,
amateurs don't recognize them as such.

There is also the development of a certain jargon amongst amateurs, e.g
the stress on SWR, whereas, for many purposes, the professional world
would use the term return loss. This is also a case where a very old
measurement technology (measuring voltage on an open line) has been
replaced by more direct methods.

Finally, two different people could invent essentially the same
solution, but give it different names.

[Guy G4DWV 4X1LT]

On Tue, 12 Dec 2017 07:31:05 +0000, just as I was about to take a
herb, Brian Howie <br...@b-howie.demon.co.uk> disturbed my reverie and
wrote:

>Have you a counterpoise from the business end of the coax ?

The counterpoise is connected to the Balun at the feed point. It was
added some time after the antenna was commissioned. It made damn all
difference :-((.
--

Merry Christmas, Happy New Year, and may your God go with you!

73 de Guy G4DWV/4X1LT

[Brian Reay]

Dave has answered your main points but, just to add to your knowledge,
the kind of choke being discussed here is also sometimes known as a
Collins Balun. I don't know if that derives from someone who came up
with the idea, is after a common manufacturer etc. but it is sometimes
used.

[Brian Reay]

On 11/12/2017 23:02, Guy G4DWV 4X1LT wrote:

I'd look at your RF earthing first.

For an End-fed, you need a good, low impedance, RF earth. Trying to
'choke off' stray RF without a good RF earth like herding cats,
especially if, as it seems, your ATU is inside, fed by coax.

I also have an End-fed (also around 30m). I use a remote ATU at the
feedpoint, fed by coax from the operating position. I've also have a
good RF earth- a series of wires run out under the drive, patios, along
the fences, etc. and bonded to both a ground rod and the PME earth
point. I've had no issues with stray RF, no chokes required etc. I've
recommended / set up the same system numerous times for others and never
heard it have issues.

[Brian Reay]

For a loop, which is ideally fed with a balanced feeder, where you have
placed the choke is better.

However, Guy has an End fed and, while a choke can help he'd be better
off ensuring he has a good, low Z, RF earth/counterpoise which may
obviate the need for a choke, although one could be added to 'mob up'
any final issues.*

Likewise, having the ATU at the antenna feed point, reduces Coax loss
(the ATU does the matching at the antenna end, the coax sees a better
match (perhaps not 'perfect') and tx should be 'happy'. If the tx has an
internal ATU it can always be used to 'mop up' any remaining mismatch
if, at the tx end, you aren't getting 50 ohms. With my set up (I use an
SGC remote Tuner- although SGC prefer the term 'coupler'), I rarely need
to use the internal tx ATU. The SGC nearly always achieves a near 1:1
match but there are are few times I see perhaps 1:1.5. Not really a
problem but the internal ATU can be used if you are bothered by it.

The SGC (or similar devices) will feed loops etc but not coax fed antennas.

Having owned and used 'local', internal, and remote ATUs, I much prefer
remote ones.

*I tend to treat chokes as a remedy for a problem. If you can avoid the
problem, you don't need the remedy.

[Guy G4DWV 4X1LT]

On Wed, 13 Dec 2017 11:02:39 +0000, just as I was about to take a
herb, Brian Reay <no...@m.com> disturbed my reverie and wrote:

>*I tend to treat chokes as a remedy for a problem. If you can avoid the
>problem, you don't need the remedy.

Hi Brian,

I do have RF issues and I have done all that I can with the antenna.
When weather permits I will replace the RG-213 with RG-58 coax as I do
not need the former and the '213 is much more difficult to handle.
With the RG-58 I will affix a choke to the feedpoint and also have one
in the shack.

[Guy G4DWV 4X1LT]

On Tue, 12 Dec 2017 19:31:20 +0000, just as I was about to take a

herb, Brian Reay <no...@m.com> disturbed my reverie and wrote:

>I'd look at your RF earthing first.
>
>For an End-fed, you need a good, low impedance, RF earth. Trying to
>'choke off' stray RF without a good RF earth like herding cats,
>especially if, as it seems, your ATU is inside, fed by coax.

Another job for the summer ;-).

[Brian Reay]

On 13/12/17 12:22, Guy G4DWV 4X1LT wrote:
> On Wed, 13 Dec 2017 11:02:39 +0000, just as I was about to take a
> herb, Brian Reay <no...@m.com> disturbed my reverie and wrote:
>
>> *I tend to treat chokes as a remedy for a problem. If you can avoid the
>> problem, you don't need the remedy.
>
> Hi Brian,
>
> I do have RF issues and I have done all that I can with the antenna.
> When weather permits I will replace the RG-213 with RG-58 coax as I do
> not need the former and the '213 is much more difficult to handle.
> With the RG-58 I will affix a choke to the feedpoint and also have one
> in the shack.
>

I would recommend you keep the 213 but insert a short section of '58 you
can use to wind round the choke. Also, if possible, pull the 213
through some new hose pipe, sealing the ends.


Rationale: I'm assuming you are trying to avoid a remote ATU. The 213
will, slightly, reduce coax losses compared to '58. The hose will
protect the '213- I've used the same coax for decades with no measurable
deterioration in loss. The '58 section is easy to make into a choke.

You could also try a coax choke, wind about 5 or 6 turns of coax in a
circle, perhaps 20cm i/d . There are ways to calculate the dimensions
and various tables but, from experience, those are a good starting
point. You can use '58 or '213, even 213 will form a coil that size. The
diameter isn't critical but keep the 'input' and 'output' apart.

There are other designs- '58 on a bit of PVC drain pipe etc.. ('213 has
too large a bend radius to wide on small pipe). Probably 15 turns from
memory but it is some time since I used one like this so check on line.

I'm recommending you keep the '213 as, with the coax on the 'wrong side'
of the ATU they may be (probably is) a high SWR and therefore you want
to minimise coax loss.

There is a lot of 'myths' re SWR. The real issues are keeping the Tx
'happy' (the ATUs job) and feeder loss. With the coax on the 'wrong
side' of the ATU, it can't help with the latter so you need to take
other steps.

[Stephen Thomas Cole]

David Woolley <da...@ex.djwhome.demon.invalid> wrote:
> On 12/12/17 09:40, Stephen Thomas Cole wrote:
>
>> Pardon the silly question but "common mode choke" and "RF choke" (and other
>
> RF Choke is a more general concept, and is more commonly used in single
> wire situations, on a circuit board.
>
>> folk call the same a "choke balun") are one and the same, right? If so, I'd
>
> Balun comes from BALanced to UNbalanced transformer, but the devices in
> question are not really acting as transformers. Prefixing with "choke",
> is an attempt to distance oneself from the transformer interpretation.
> In true transformer case, there would be a DC path between both wires,
> but no DC path from the transmitter to the antenna.
>
> Common mode choke is a more accurate description of how they actually work.
>

Thank you for that detailed and comprehensive answer. Much appreciated!

>> agree that right at the antenna is the sensible place for it as the whole
>> point is to reduce/remove reflected current from the coax.
>>
>> A side point; why do radio amateurs have so many different names for the
>> same things? It's really maddening when you're new to the hobby!
>>
>
> It's often because, like here, technology was developed to achieve a
> function in a different way, but the name was adapted from the old way
> of doing it, and then developed into a more accurate description.
>
> It can also be the result of marketing twisting a meaning, or people not
> really understanding the real meaning. SDR might be an example;
> amateurs use the term for an analogue front end and ADC, which may have
> no software at all, but, professionally, the term refers to the complete
> receiver. Consequently, whilst most current commercial rigs are SDRs,
> amateurs don't recognize them as such.
>
> There is also the development of a certain jargon amongst amateurs, e.g
> the stress on SWR, whereas, for many purposes, the professional world
> would use the term return loss. This is also a case where a very old
> measurement technology (measuring voltage on an open line) has been
> replaced by more direct methods.
>
> Finally, two different people could invent essentially the same
> solution, but give it different names.

Another thing might be amateurs simply using all of these terms
interchangeably out of ignorance or laziness, of course.

[Guy G4DWV 4X1LT]

Hi Brian,

Thanks for that reply. I am situated very close to the feed point of
the antenna* and the use of RG213 is overkill. I got some when getting
back on the air and just went got the best. I assume that mil-spec
RG-58 is not worth it or is it? The '58 I have is regular stuff that I
bought when one of the retailers was having such a good discount on it
that it would have been rude not to buy it.

There are other advantages in that I will not have to drill any more
holes though the wall for feeders as using '58 will give me a lot more
space in the one hole I already have. Plus the ease of having baluns.
I will have one at the antenna's feed point and one in the shack.

*I am downstairs at the back of the house and the feed point is above
me and off to the side, 1-2 yards below the guttering.

[Brian Reay]

On 15/12/17 17:35, Guy G4DWV 4X1LT wrote:
> Hi Brian,
>
> Thanks for that reply. I am situated very close to the feed point of
> the antenna* and the use of RG213 is overkill. I got some when getting
> back on the air and just went got the best. I assume that mil-spec
> RG-58 is not worth it or is it?

It is a meaningless term.

There are, of course, Mil-Specs for things, including cables, etc but,
in itself, saying coax is 'Mil-Spec' means nothing. This has been
covered several times in various areas.

Essentially, you need to look for good quality, from a reputable source,
look for good 'coverage' of the braid etc.

>The '58 I have is regular stuff that I
> bought when one of the retailers was having such a good discount on it
> that it would have been rude not to buy it.
>
> There are other advantages in that I will not have to drill any more
> holes though the wall for feeders as using '58 will give me a lot more
> space in the one hole I already have. Plus the ease of having baluns.
> I will have one at the antenna's feed point and one in the shack.

Obviously convenience is an issue. However, running '213 in a hose means
you only do it once, chances are it will last as long as your do. I'm
used stuff I put in a hose in 1989 or so. Last time I checked (last
summer), I couldn't measure any loss in it over the spec.

The hose is 'tired' but still OK.

You can run '58 in a hose but sealing it isn't easy.

[Guy G4DWV 4X1LT]

Many thanks, Brian.

[Brian Reay]

On 19/12/17 07:19, Guy G4DWV 4X1LT wrote:
> Many thanks, Brian.
>

No problem Guy.

How did the shelf go? Were you able to cut the sides down?

Sorry I couldn't hang around- we had a long drive after leaving you.

[Guy G4DWV 4X1LT]

On Tue, 19 Dec 2017 16:19:45 +0000, just as I was about to take a

herb, Brian Reay <no...@m.com> disturbed my reverie and wrote:

>No problem Guy.
>
>How did the shelf go? Were you able to cut the sides down?
>
>Sorry I couldn't hang around- we had a long drive after leaving you.

No problem Brian. There are a few things I need and want to do to the
Yaesu rigs before they will be in their final positions.

In my day job I worked with Perspex quite a lot every day as that is
what dentures are made of. I know that Perspex can be a sod to work. I
would have to check out what I would use to shorten the sides. I would
rather leave things as they are rather than ruin it.

[Brian Reay]

On 22/12/2017 01:42, Guy G4DWV 4X1LT wrote:
> On Tue, 19 Dec 2017 16:19:45 +0000, just as I was about to take a
> herb, Brian Reay <no...@m.com> disturbed my reverie and wrote:
>
>> No problem Guy.
>>
>> How did the shelf go? Were you able to cut the sides down?
>>
>> Sorry I couldn't hang around- we had a long drive after leaving you.
>
>
> No problem Brian. There are a few things I need and want to do to the
> Yaesu rigs before they will be in their final positions.
>
> In my day job I worked with Perspex quite a lot every day as that is
> what dentures are made of. I know that Perspex can be a sod to work. I
> would have to check out what I would use to shorten the sides. I would
> rather leave things as they are rather than ruin it.
>

You could always cut two slots in the desk top and drop the sides
through. ;-)

You laugh but, in my travels, I saw some weird and wonderful things
which would make that look normal.

[David Woolley]

On 22/12/17 01:42, Guy G4DWV 4X1LT wrote:
> In my day job I worked with Perspex quite a lot every day as that is
> what dentures are made of. I know that Perspex can be a sod to work. I
> would have to check out what I would use to shorten the sides. I would
> rather leave things as they are rather than ruin it.

If this is sheet poly methyl methacrylate, and not too large, the modern
way of doing accurate cuts would be a laser cutter. Parts of the Fab
Lab movement are likely to be able to provide access to these, free for
non-commercial projects (normally only one day a week is available for
non-commercial use.

[Brian Reay]

The material in question is quite thick as 'perspex' goes- at from
memory 8mm or so. Would the kind/power (output) of cutters these places
are likely to have cope?

I don't disagree laser cutting is 'the' way to cut such things, in the
past I've had access to machines via various routes. Since retiring, I'd
have to 'visit' old haunts etc. ;-)

I had half thought of offering to cut the sides down but time got in the
way and I wasn't sure what Guy's needs were. I'd have used a fine saw,
with things all 'clamped', a straight edge as a guide etc., followed by
finishing with 'wet and dry' and some compound/polish to restore the edge.

I'm sure Guy will be fine, his 'day job' skills will almost certainly
mean his techniques for working with perspex are anything but 'shoddy'
;-) He just needs to think of it as a pair of dentures for a Great White
(I think they have teeth). To give you some idea of scale, it was (is?)
a printer stand-at least that is what we used it for- I assume homemade.