My question then is:
1. Does a multiple sets of grounding blocks for the RG--6 outside the
shack hooked to a common point ground hurt anything?
2. Does a toroid ring on each side of the grounding block with 12 turn
pass throughs hurt anything?
Just curious.
Herb Schoenbohm, KV4FZ
On 11/29/2014 9:36 AM, Tom W8JI wrote:
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
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