Topband: Coax shielding

[Tom Rauch]

> I thought coax was well shielded until I put 40 db of preamp in the shack.
> The RG-58 picked up broadcast stations at 20 over, with no antenna
connected
> to the coax.  RF chokes on each end of the coax almost solves the problem,
> but my next step is to replace the coax with a double shielded type.

If it is shield ingress, a choke will not help at all. It would make no
difference. Chokes only affect common mode currents, not shield integrity.

I ran a test with cheap Radio Shack coax, the type you could see through the
braid, and the leakage from two pieces 30 feet long that were ty-wrapped
tightly together was below the noise floor of my measurement equipment at
HF, over -100dB for crosstalk.

In many years of building RF equipment, I've never seen cable ingress or
egress that was due to anything other than a broken or high resistance
shield connection, or some other system flaw. Every time I find a problem,
it is with shield connections someplace along the line and not shield
quality.

> RF chokes that work well at 160m must still choke well in the broadcast
band
> to kill the noise.

Noise in the BC band isn't noise on 160, even if it did get in. Unless a
non-linear device in the receiving system is seriously overloading, you'd
never hear about out-of-band noise.

What I'd suspect more than anything is improper feedpoints at the loops, or
a defective cable or shield connections elsewhere. Many reference materials
incorrectly claim the "shield" on a loop shields the element, and worse yet
show the loop "shield" as non-symmetrical or the feedline and a mast or
ground attaching to the shield at a point not opposite of the shield gap!

The shield IS the actual antenna. If the antenna "shield" is not perfectly
balanced, it is a source of ingress from common mode currents on the coax.
An antenna "shield" that is not grounded at exact dead bottom center with
the feedline attaching to the inner wire exiting at that same bottom point,
with a gap exactly in the center of the loop top equal-distant from the
feedline entrance point is sure to be improperly balanced.

A small "shielded" loop really isn't any better or much different than a
single turn open wire when installed correctly. When installed incorrectly,
such as being grounded off center, it is actually worse. That's why I always
avoided shielded loops, after my first few years of playing with them.

73 Tom