Telamon wrote:

Practically speaking there is not much you can do about the receiver
input not being 50 ohms but it should be close enough to terminate the
coax to the extent that the the coax would be able to shield properly
unless it is a really bad design.

So with the coax terminated with a 75 ohm resistor (for a 75 ohm system)
instead of the antenna the receiver still picked up a local AM station
AND the station was not there if the coax was disconnected from the
radio input? Both tests need to pass for you to feel confident that this
was a case of the local station making it past the shield. It's possible
that the local stations signal could be on the mains making it way into
the PC radio. If you are correct about this then it would be a pretty
rare situation.

Dry ground does not have mobile ions for electric fields to push around
thereby absorbing the energy.

--
Telamon
Ventura, California
---------------------------
The MW engress was noted in experiments in 3 diverse locations with
several different
receivers ranging from a R390, AOR7030+, R2000, and, a R5000. In one
location I used
also used several ham (Kenwood) general coverage transceivers. And yes
the MW
signals, with the exception of my home QTH, were only present with the
coax
connected to the receiver. And even at home the signal level jumped
enough to
clearly show the engress was fromthe coax. Interestingly a borrowed
~300' spool
of TV triax was reeled out and with only the outer shield groudned at
both ends,
with the inner shield grounded at the receiver end, with the far end
groudned or
ungrounded, had no engress.

The WWV dipole experiment was ~20 years ago and I used a borrowed
R5000,
a R392 and my trusty R2000.

At my home QTH I do get one MW station that is about 3 miles away and I
swear
they must throw a lobe, they are ND daytime only, right across me. The
AOR7030+
would receive this MW station on external 12V without an antenna. Only
on internal
power, wiht the antenna input terminated, did the AOR reject the
bugger.

I built a rat wire (1/4" square openings) faraday cage and can get the
local MW on
my R2000s, my R390 and my DX398. Faraday cages do not offer 100%
isolation.
It is weak but there. This is with all receivers powered from gell
cells and the antenna
terminated in a 50 ohm non inductive load. The signal was week, but
still there, just
barely, but there none the less.

I am working on trade to get a ~15 year old HP spectrum analyser that
will allow
me to measure the real signal strength of various events. I am also
working on
another trade for tuneable RF voltmeter that measures from 0.1uV and
from 10KHZ
through ~15MHZ. Not perfect, but at least I should be able to get
repeatable numbers.

Like I said this whole excercise has made me very interested in
"transfer impedence".
Science is not magic, but sometimes the rules are arcane enough to look
like magic
until you get enough info to start making sense.

For most applicaton minor coax imperfections shouldn't mater. Unless
the local noise
floor near the coax is on the order of volts per meter any leakage will
be, I think,
pretty minor. But when trying to reject strong local noise emitters
like PCs VCRs and other QRM any leakage can be problematic. There are
days I swear I am going to
move to the woods and live in a mud hut to et away from all the EMI
crap.

Terry

RHF wrote:
Terry R2000SW,

Up-date us when you have more to say on the subject.

~ RHF
.
.
. .

----------------------------------------
If I ever get less confussed then the more confussed I am rapidly
getting
I will be happy to pass the info on. There have been a few times in the
last
year of so that I thought my brain was going to start leaking out my
ears.

"Transfer Impedance" is a very exotic topic and barely covered in my
1972
text on "coax, waveguides and feedlines".

Terry

RHF wrote:
Terry R2000SW,

Up-date us when you have more to say on the subject.

~ RHF
.
.
. .
.
-------------------------------
The "best" single page I have found is at a Microwave magazine and it
gives math that almost makes sense.

http://www.mwrf.com/Articles/Print.cfm?ArticleID=10017

This ought to reduce to a set of simple numbers that would allow one to

input outside (of the coax shield) signal levels in V/mtr, TI and see
just
how much or how little RF will sneak inside through the braid and foil.

I have built a simple test jig on a 10' long 4"4 wodden post with a 6'
noise
injection antenna that runs parallel to , and 1" from a coax under
test. I
used wood and plastic golf tees to keep stray metal from effecting the
test
results. I am using a RF antenna noise bridge through a emitter
follower
buffer and a 16:1 step down to couple the buffer to the loop. I haven't
had
time to do more the a quick and simple test and all I can say is it
appears
to be working.

As time and weather permit I will post the results.
At this point I am convinced that the simplest solution to engress is
to burry
the coax. Your copper pipe idea sounds good.

Of course tossing out the PC and moving to the woods in my proposes mud
hut still has a lot of promise. Noise engress only matters if you have
noise.

Terry