Your thoughts on antenna/coax coupling.

[DXman]

Concerning a single polarity beam, is there really much to worry about on 11 meters?

Does power make any difference?

How can you tell if coupling is taking place?

Can just a short <span style="font-size:x-large;">U</span> of the coax at feedpoint be sufficent to stop it?

Can one just seperate the coax a couple inches from the boom using a spacer of some sort to help in stopping the coupling?

Does coupling take place at the feed point or along the boom?



Thanks,

<span style="color:red;font-size:x-large;">DXman</span>


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[Moleculo]

The chances are high. That's also called Common Mode interference. A simple U in the coax isn't going be enough, I don't think. I would at least make a coax air choke with 8 or so turns of the coax, about as big across as a coffee can.


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[Moleculo]

You have to use some kind of matching system in combination with a balun. The gamma match by itself will not prevent common mode current. You mentioned that you are running the coax along the boom of the antenna. That's a sure way to pick up common mode interference. My guess is that your antenna performance is degraded over it's potential. Check out the HyGain baluns...they're supposed to be pretty decent...but whatever you use, make sure it's a current balun.


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[Moleculo]

I'm not quite following you Marconi. The fact that the HyGain antenna is balanced is precicely why you need the balun. The antenna is balanced, the coax is not. Am I missing something?


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[Moleculo]

OK, I missed the part where his antenna had a gamma match. In that case, a coax air choke should do the trick. But you really shouldn't have the coax running along the boom like that.



That U matching balun deal is a Delta Match. It acts as a a 4<img src=http://www.wwdx.org/smilies/posticon1.gif ALT=":1"> balun. So for 50 ohm coax, it would have a terminating impedance of 200 ohms. The U has to be an electrical 1/2 wave. In this case, the U is often supported by the boom and it is coiled up. Check the ARRL Antenna book, section 26-24 for information on this balun. I don't believe you would want to use one with a gamma match.


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[Moleculo]

Larry, it's certainly worth a try. The idea is to put the choke as near to the feed point as you can. It's easy to make, so why not try it?


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