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

[Herbert Schoenbohm]

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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