Telamon wrote:

There is not much mystery to coax shielding. If putting ferrite on the
coax is making a difference then you may not have a proper termination
on the end. This is fixing a symptom not the problem. Disconnect the
antenna on the far end and using very short leads place a 50 ohm
resistor in place of the antenna across the coax end. Make sure no
conductors are near that resistor like the end of the disconnected
antenna. Tune the receiver through the band and you should hear nothing
other than the radios birdies and internal noise. Any question here
where the noise is coming from disconnect the coax from the radio to see
if it is still there.
snip
--
Telamon
Ventura, California
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Several minor points.
Very few receivers has a true 50 Ohm input Z.
With exception of active antennas, ver few antennas present a 50 Ohm
Z to a coax.

Real world experience, several years ago I was hired to design and
install
a WWV time recovery system. My initial effort used a dipole cut for
10MHz.
I uses a 1:1 balun to match the balanced dipole to the unbaanced coax.
The receiver was coupled to a dedicated PC ISA card and I had major
problems
with PC RFI getting back into the antenna. Experimenting with a
portable SW
receiver showed the PCs RF was going up the shield.

With the antenna disconnected, and with or with out a 75 ohm load, I
had no RF
from the PC gtting into the receiver. This was in late 1986. I was
able to use a
feritte rod wound with (maybe) ~20 turns and a variable capacitor to
resonate the
LC to 10MHZ. This showed me that even with a matched antenna, and
decent
receiver, R2000 (I am prone to using equipment I am familiar with) RFI
flowing up
the coax could be a source of major interference.

I finally went with a homebrew active antenna but I still had to use
the 10MHz trap.

But even with ~100' of every coax I checked, terminated into the proper
Z, with a
1.5:1 matching transformer for the use of 75 ohm coax to feed the
receiver, I had
some engress from local (within ~10 miles) MW stations getting into
through the
shield. This lead to my research on "transfer impedence". I still don't
have any
more "understandable" data on that effect. Lots of math that causes my
ears to
bleed.

Based on my experiences I think that more people have self inflicted RF
via their
coax then commonly accepted.

As John Doty points out, a very good shield RF supression can be gained
by
simply placing the coax under at least ~12' of soil. If the soil dies
out completly,
this effectiveness will be degraded.

Terry

Terry