"Bob" wrote in message
...
Hello,

Not too sharp with antenna and noise theory, so please bear with me a
bit.

If I have a run of coax from a Balun right at my outside receiving only
antenna (well grounded at that point) right to my receiver, is it still
possible to have common-mode noise induced into the coax ?

Say from a PC, Modem, Router near the receiver.

Wouldn't the coax shied prevent this ?
If not, why not ?

Can't picture how an emi source field would/could "get-thru" the coax
shield to the inner conductor.

Thanks,
Bob


One key point here is that the coaxial cable shield is grounded at two
points, at the receiver and at the antenna. You might have what is known
as a ground-loop. The best thing to do is to ground the receiver, the
computer, the modem, and the router with leads as short as possible, AND
TO THE SAME GROUND POINT. Since you have a balanced to unbalanced
conversion (Balun) at the antenna, there is no need to ground the coax
shield at this point.

Years ago, I had three grounds in my ham shack: the power line ground at
the service entrance to the house, the tower ground where I had mounted a
rotary coax relay, and a copper cold water pipe running beneath the shack
which connected to the buried galvanized water pipe running from the
street to the house. I measured from 3 to 15 volts AC between the three
grounds. When connecting them together, I measured as much as an amp of
current flowing. I bonded everything together at the water pipe and used
this as the single point ground to which everything else was grounded.

I agree with Cecil that the noise is probably common mode noise. But the
quality of the coax certainly does affect how much signal leaks in or out
of a coaxial cable. I haven't bought Radio Shack coax in years, but it
used to be very poorly shielded. The braid shield looked more like
window screen than it did a solid sheath. This is the reason most CATV
providers use flexible coax with two shields: a foil or metalized plastic
shield covered by the braid shield. Special coaxial cables with high
leakage are often used in long tunnels to provide FM reception in the
tunnel.

73, Barry WA4VZQ