[Amps] RF in the Audio
km1h at jeremy.mv.com
Mon Sep 26 06:16:37 PDT 2011
>> One test I rarely see mentioned is to test your coax first.
>> Leave in place and terminate the far end in 50 or 75 Ohms and then tune
>> bands recording any crud frequencies. Then add a bead balun and
>> Record any differences in signal levels. If you have a quiet receiver
>> any pickup with the antenna is likely in "antenna mode" as I like to call
>> it. Many get confused with all the technical terms used. Any additional
>> picked up in "interference mode" will be small and easily eliminated at
>> shack end with another bunch of beads and hopefully a decent RF ground.
>> ### Ok, this is a new one on me. This is a damn good idea. Are you
>> to terminate the coax at top of tower with a 50 ohm plug ? I would have
>> that would result in zero local noise pickup. Any small, shielded dummy
>> load should work.
> Don't forget about the transfer impedance of the shield, which is a
> measure of the perfection of the uniformity of the shield. In short, the
> higher the transfer impedance, the more differential voltage that
> appears on the coax as a result of current on the shield. The lower
> limit of the transfer impedance is the shield resistance at the
> frequency of interest.
> BUT -- the role of common mode coupling is different -- it is a coupling
> path between the shield and the antenna itself.
> 73, Jim Brown K9YC
If you want to know what is happening as you make changes you need to
establish a set of benchmarks first. You and I have the test equipment to do
a lot of the measurements, most do not.
Many may be surprised to learn how leaky their 20 year old (or even newer)
coax is. All runs here are 3/4 thru 1" CATV hardline thru 1296 with quad
shield RG-11 foam for runs around rotators and up to antennas. Same at the
house end. Above that its 7/8 Heliax and ultraflex LMR.
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