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#11
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Current chokes
Bill Ogden wrote:
I am the original poster of the question about the current-choke effect of underground coax. I mentioned HF bands, with multiple coax runs (and control cable runs) inside a 3" PVC conduit about 10" (top of PVC) underground. The conduit is about 50' long. This is probably a very common situation. Do I understand the sum of the comments correctly? - A single buried coax (not inside a conduit) will experience a significant current choke effect. - Multiple coaxes (and other cables) inside a non-metalic conduit will experience widely varying effects, depending on lots of parameters that would be difficult to quantify in common usage. One of my coax runs is for a half-sloper that is not very well matched at any frequency. I detect no RF feedback into the shack (checking with the little MFJ RF current meter, among other things). I had been wondering if I should place current choke(s) on the coax on the tower, but I gather there is probably no need for them. Bill - W2WO The instructions for my half-sloper (Alpha Delta DX-B) say that an RF Choke (6-8 turns of the cable in an 8" coil) should be used at the feedpoint (6-8 feet from the top-hat). http://www.qrz.com/hampix/l/i/k6dil.1221924885.jpg |
#12
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Current chokes
On Mon, 20 Oct 2008 09:59:38 -0400, "Bill Ogden"
wrote: I detect no RF feedback into the shack (checking with the little MFJ RF current meter, among other things). Hi Bill, That should be the end of it, then. I had been wondering if I should place current choke(s) on the coax on the tower, but I gather there is probably no need for them. It would define a plane beyond which current is substantially reduced which would raise the current available for contributing to radiation. 73's Richard Clark, KB7QHC |
#13
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Current chokes
"Bill Ogden" wrote in
: I am the original poster of the question about the current-choke effect of underground coax. I mentioned HF bands, with multiple coax runs (and control cable runs) inside a 3" PVC conduit about 10" (top of PVC) underground. The conduit is about 50' long. This is probably a very common situation. Bill, Go back and read your post. I don't think there is anything that states or hints multiple cables, nor did you state the size and type of conduit. The larger the conduit, the lower the common mode attenuation for a given conductor size as more of the dielectric subject to the highest field strengths is low loss air. Do I understand the sum of the comments correctly? - A single buried coax (not inside a conduit) will experience a significant current choke effect. - Multiple coaxes (and other cables) inside a non-metalic conduit will experience widely varying effects, depending on lots of parameters that would be difficult to quantify in common usage. Broadly, don't expect that multiple cables work as a single coax. I gave some reasons in my earlier post. One of my coax runs is for a half-sloper that is not very well matched at any frequency. I detect no RF feedback into the shack (checking with the little MFJ RF current meter, among other things). I had been wondering if I should place current choke(s) on the coax on the tower, but I gather there is probably no need for them. The common mode current is a standing wave. (Confusing concept for some!) That means that measuring its amplitude at one point using the "little MFJ RF current meter" gives only a limited perspective. Nevertheless, you have other indications that there is not a problem. Perhaps you should leave it alone if it is working adequately. A further benefit of a buried feedline not already mentioned is that it does protect the conductors somewhat from radiated energy Owen |
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