I seem to remember reading that running coax underground acts like a current
choke. However, my memory has many missing bits these days. Is this a true
statement about coax underground? In my case the run is about 40' (in a
conduit that is about 10" under ground). I am interested only in the normal
HF bands.

Bill
W2WO

"Bill Ogden" wrote in
:

I seem to remember reading that running coax underground acts like a
current choke. However, my memory has many missing bits these days.
Is this a true statement about coax underground? In my case the run
is about 40' (in a conduit that is about 10" under ground). I am
interested only in the normal HF bands.

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.

Your coax is similar to some extent in respect to the common mode current.
Placing a single coax in a plastic conduit reduces the loss to some extent
compared to surrounding the cable directly with soil.

Owen

On Fri, 17 Oct 2008 17:23:05 -0400, "Bill Ogden"
wrote:

I seem to remember reading that running coax underground acts like a current
choke. However, my memory has many missing bits these days. Is this a true
statement about coax underground? In my case the run is about 40' (in a
conduit that is about 10" under ground). I am interested only in the normal
HF bands.

Hi Bill,

The coax shield is no different in loss as is a ground radial wire of
the same dimension. Visit any site that offers the data of Brown,
Lewis, and Epstein to observe that for their experiments at 3MHz, the
amount of radial conductivity drops like a rock well in advance of the
end of the wire (where it would be forced to zero by being open).

You might get it to conduct a bit further by having insulated it from
ground (the conduit), but it is still easily within ground's deadly
embrace.

73's
Richard Clark, KB7QHC

Bill Ogden wrote:
I seem to remember reading that running coax underground acts like a current
choke. However, my memory has many missing bits these days. Is this a true
statement about coax underground? In my case the run is about 40' (in a
conduit that is about 10" under ground). I am interested only in the normal
HF bands.

Bill
W2WO

I believe Richards' comment is 100% correct. However, I would still run
a current-choke/balun/unun, unless the antennas radiation pattern was
designed around the outer shield acting as a counterpoise ...

Regards,
JS

On Oct 17, 5:45*pm, Owen Duffy wrote:
"Bill Ogden" wrote :

I seem to remember reading that running coax underground acts like a
current choke. *However, my memory has many missing bits these days.
Is this a true statement about coax underground? *In my case the run
is about 40' (in a conduit that is about 10" under ground). *I am
interested only in the normal HF bands.

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.

Your coax is similar to some extent in respect to the common mode current..
Placing a single coax in a plastic conduit reduces the loss to some extent
compared to surrounding the cable directly with soil.

Owen

I have heard that burying your cable will keep RF from coming back in
the shack. From what you say it seems like this is possible but at the
expense of using some of your precious watts to warm the ground.

Jimmie

JIMMIE wrote in
:

I have heard that burying your cable will keep RF from coming back in
the shack. From what you say it seems like this is possible but at the
expense of using some of your precious watts to warm the ground.

The effect of the buried cable will be to influence the magnitude and
distibution of the common mode current on the feedline.

Yes, the common mode current flowing on the feedline in the ground will be
mostly converted to heat if the line is sufficiently long. Minimising the
current flowing into the underground section helps system efficiency.

But the measure has been effective in reducing RF levels in the shack.

Owen

JIMMIE wrote:

I have heard that burying your cable will keep RF from coming back in
the shack. From what you say it seems like this is possible but at the
expense of using some of your precious watts to warm the ground.

Jimmie

The same is true of any other kind of resistive common mode choke. But
it's an impedance matching situation, so the loss doesn't necessarily
have to be significant. Remember that the loss is I^2 * R where I is the
common mode current and R is the choke resistance. If R is low, the loss
will obviously be low. The choke won't be very effective, however,
unless it also has a substantial amount of reactance. If R is very high,
the loss will also be low because it will cause I to be low. The only
time that loss really becomes a significant fraction of those "precious
watts" is when R is in the vicinity of the effective common mode source
and load impedance. A properly designed common mode choke will have a
high enough impedance (resistive, reactive, or some combination) to
reduce common mode current I to a low value -- that's the objective
after all.

Roy Lewallen, W7EL

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 Lewallen, W7EL

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