Topband: isolation transformer(s) in RX antenna feedlines (?)

[Tom W8JI]

>I ran across a sketchy reference to this idea, described as a way to block 
>common mode signals from making it into the shack by cutting off the paths 
>from the coax shield to the RX input.  Is there anything to the idea?
>

I would never consider doing that unless there were special circumstances, 
because any isolation transformer at that location could easily cause more 
issues than it cures.

Years ago, the common system used everywhere was a conventional un-un style 
transformer or an autotransformer. I broke from the standard and used 
primary-secondary isolation transformers at the feedpoint for three reasons:

1.) I sometimes used phased antennas that required 180 degree phase shift. 
Isolating the primary accomplished that all in one device that had to be 
there anyway.

2.) The isolation removed the *direct wired connection between the antenna's 
RF ground and the feedline shield at the antenna*, and kept any unwanted RF 
coupling to a very low value. The connection isolation reduces importance of 
the antenna system's ground quality.

3.) By blocking the dc path, the isolated primary reduces power line 
frequency or dc bias on the transformer, and reduced galvanic corrosion 
issues at the antenna by eliminated the path back to the station ground.

This is all just fine right at the feedpoint, or near the feedpoint.

The shack is different. Any RF ingress problem in the shack almost always 
boils down to how the "boxes" are made and how the coaxial connectors are 
grounded in cabinets. It is much better to do all the connections and boxes 
properly than bring more potential ingress problems into the system.

With coaxial lines, we absolutely do NOT need extreme isolation impedances 
in the shack or just outside the shack.  If the connectors and cables are 
good, and if equipment in the shack is designed even moderately well for 
cabinets and connector shield path, even a few dozen ohms of impedance from 
a bead or two over the cable should cure any problem....if there ever is a 
problem.

Common mode chassis impedances between things in the shack are typically 
very low, so just a few ohms of additional shield impedance on signal lines 
can make a profound difference in noise ingress, if there even is any noise 
ingress.

You don't see any of that stuff in my station, and I sometimes transmit with 
antennas near the shack while receiving. The most I use is a single bead or 
two on lines using phono connectors, because the male-female pressure 
connection (like a BNC) can sometimes develop a few dozen milliohms 
resistance. I can't ever imagine a situation where more than a few hundred 
ohms isolation would be required. If there is such a situation, it would be 
far better to correct the actual cause.

73 Tom