Roy Lewallen wrote in
treetonline:

Owen Duffy wrote:

A conductor carrying RF currents and immersed in soil behaves like a
lossy transmission line. The bahavior depends on the parameters of
the soil (amongst other things), so the effect is different in dry
sand than in wet clay. Current entering the line will be attenuated
along the line. . .

Please permit me to amplify on what Owen said, because it looks like
it might be interpreted. Burying coax in soil has no effect on the
signal inside the coax -- the loss of the coax transmission line isn't
increased. The lossy transmission line Owen refers to is the line made
up of the outside of the coax and, for the other conductor, the Earth.
This is the transmission line which carries the common mode current
and the one which will be made lossy by burying the coax.

Roy, one of the lingering gaps in my post was that I spoke of a single
coax in a plastic conduit. As many readers will recognise, that is an
important qualification.

If you put the coax cable inside a metallic conduit, attenuation of
common mode current on the coax is much lower because it forms a coaxial
TL with the conduit. The next question is what is the CM current on the
outside of the conduit, and that requires examination of the end
conditions on the conduit.

The other issue, is that people often install multiple coaxs in one non
metallic conduit. If they are not bonded together, they may convey energy
with relatively lower attenuation by driving the coax outers in
differential mode.

I recall a project where understanding the configuration of underground
medium voltage and high voltage power entry to a comms facililty was real
important in modelling EMP impact. A neutral screened medium voltage
cable worked differently to three conductors in trefoil or flat. HV
cables were invariably armoured, and the armouring was relevant,
depending on how it was treated at both ends. In some situations, 30m of
underground power entry cleaned it of EMP, in others, it made almost no
difference (though still relevant in modelling).

Owen

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

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

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

"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