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Author Topic: Coax Choke on OCF Dipole  (Read 3790 times)
KL3HY
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Posts: 117




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« on: November 24, 2011, 08:49:38 PM »

I've been reading up on OCF dipoles since that's what I'm using for my first HF antenna, and one thing that seems to jump out at me is that they tend to radiate on the feedline.  I'm thinking I might try to put a choke (like what's described here: http://www.hamuniverse.com/balun.html ) up by the balun on the center support.

The only interference I've experienced in my house has been on my wife's cheap computer speakers, but I'm also hoping that the choke might also drop the background noise level somewhat. Is that something that's reasonable to expect?

Another option that I've read about is making use of that feedline RF and installing the choke about 30' down from the balun, so that 30' of coax acts as a vertical radiator, sort of like the ladder line in a G5RV antenna.  I hesitate on this option because that vertical radiator will be right up against the metal mast, so I'm thinking that will largely negate any benefit it might otherwise provide.

I'd love to hear any opinions you might have regarding the use of a vertical radiator, or the effects of a choke.

Thanks,
Mike
« Last Edit: November 24, 2011, 09:01:24 PM by KL3HY » Logged
K4RVN
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Posts: 758




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« Reply #1 on: November 24, 2011, 10:10:16 PM »

Mike I did build the ugly balun which is actually an air choke. I used a pvc coupling from Lowes for the coax to wrap around.
I used plastic ties to hold the coax in place and installed it as close to the antenna as possible. I had about 20 ft of spare coax on my feed line so did not have to do anything but coil the coax around the coupling, tie it in place and reconnect my pl259 to the antenna switch for 5 bands.
It is presently on my 5 band quad RG8X feedline. It works well for me. I have a field strength meter in my shack right next to the feed line and it reads zero now. Prior to that I had some RF floating in the shack. I also improved my ground system so that could have also reduced some unwanted RF in the shack. I use the quad on HF only.
It would be a good project for you to try even if it does not meet your expectations. That's the fun in amateur radio
for some of us who like to try things we build.

Frank
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VK2FXXX
Member

Posts: 102




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« Reply #2 on: November 24, 2011, 11:12:18 PM »

Gday
Have a look at this page
http://www.karinya.net/g3txq/chokes/
you can measure the actual effect on your feedline with on of these
http://vk1od.net/measurement/icm/index.htm
have fun
Brendan.
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G3TXQ
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Posts: 1464




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« Reply #3 on: November 25, 2011, 03:34:25 AM »

I've been reading up on OCF dipoles since that's what I'm using for my first HF antenna, and one thing that seems to jump out at me is that they tend to radiate on the feedline.  I'm thinking I might try to put a choke (like what's described here: http://www.hamuniverse.com/balun.html ) up by the balun on the center support.

The only interference I've experienced in my house has been on my wife's cheap computer speakers, but I'm also hoping that the choke might also drop the background noise level somewhat. Is that something that's reasonable to expect?
Mike,

An OCF is inherently an unbalanced antenna which has a tendency to drive common-mode current onto the outside surface of the coax braid - you need a really good choke to combat that. The so called "ugly balun" is a poor choice for this application: it maintains high choking impedance over a relatively narrow bandwidth; and its impedance is reactive rather than resistive - that means it could even increase the CM current in some situations. For best results you need a ferrite-cored choke.

73,
Steve G3TXQ
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WX7G
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Posts: 5920




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« Reply #4 on: November 25, 2011, 05:11:09 AM »

No a coax choke is not the thing to use on an OCF dipole. At best a coax choke, when set to be self resonant, is a one band device. On the other bands it can appear as a high-Q inductor or capacitor.

What is needed with an OCF dipole is a high impedance choke balun or a galvanically isolated transformer.
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WB6BYU
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Posts: 13040




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« Reply #5 on: November 25, 2011, 07:30:32 AM »

If you look at the charts on G3TXQ's web page (linked to earlier) you'll see that an air-core
coiled-coax choke with the right number of turns may work for  a couple of close bands, like
20m + 15m, if that is all that is causing your problems.  (Though most OCFD designs don't
work well on 15m anyway.)  But to be effective over the full range of the antenna really
requires that the coax be wound on an appropriate ferrite core.

Don't bother with the version that puts the choke further down the coax - at best it might
make a bit of an improvement in signal on one or two bands, and it makes it much more
difficult to choke off the RF properly.  You may actually need two chokes spaced some
distance apart on the coax, or something similar.

That's not to say that an OCFD won't work without the proper choke, but you may have
to deal with common mode current issues on some bands.
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N4RSS
Member

Posts: 258




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« Reply #6 on: November 27, 2011, 08:58:58 AM »

7 turns of rg8x through fair rite type 31 giant clamp on part 0431177081 from mouser will do the trick. This is what I used on my OCF
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K1ZJH
Member

Posts: 901




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« Reply #7 on: November 27, 2011, 09:19:26 AM »

OCF antennas require a good current balun, that will force equal currents into each leg of the antenna.
Voltage baluns will try to equalize the voltage, which is exactly what you don't want to do on an OCF
antenna. Make sure you use the right balun for that application.
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KL3HY
Member

Posts: 117




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« Reply #8 on: November 30, 2011, 10:58:57 AM »

Thanks very much for all the info.  In hindsight I should've provided some more detail.  My antenna is a commercial one I bought from Mapleleaf Studios, and I verified it has a 4:1 current balun.  Given that I already have a current balun, would I get any benefit out of something like one of these:

Balun Designs 1113u
MFJ-915 Line Isolator

Both appear to be 1:1 current baluns.  MFJ uses more marketing glitz to promise great benefits, but I'm not sure I trust that.   Grin



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K4RVN
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Posts: 758




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« Reply #9 on: November 30, 2011, 12:06:14 PM »

Since you have a store bought OCF antenna with a 4 to 1 balun, A common mode choke such as the MFJ -915 would probably solve the RF on the coax cable. Just install it right below the 4 to 1 OCF balun as close as possible would be my suggestion. Also the experience and solution as posted by N4RSS is one to be considered as the cost of rolling your own is comparable to buying one from MFJ. If you have some spare coax on your feed line, the N4RSS solution would cost you about 27 bucks for the core and shipping from Mouser as they had no minimum order the last time I bought from them.
An air choke in series with the 4 to1 such as the ugly balun might also do the trick just wound around a bottle then tied together with nylon ties if you have the extra coax on your feed line. Have fun and let us hear how it all turned out for you.

Frank
« Last Edit: November 30, 2011, 08:33:53 PM by K4RVN » Logged
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