"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