[Amps] RF in the Audio

Carl 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 
>> the
>> bands recording any crud frequencies. Then add a bead balun and 
>> reterminate.
>> Record any differences in signal levels. If you have a quiet receiver 
>> then
>> 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 
>> crud
>> picked up in "interference mode" will be small and easily eliminated at 
>> the
>> 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 
>> saying
>> to terminate the coax at top of tower with a 50 ohm plug ?  I would have 
>> thought
>> 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.


More information about the Amps mailing list