[TowerTalk] Chokes and RFI in a Vehicle

[Jim Brown]

On 4/14/2012 2:19 PM, Rick Stealey wrote:
> Would a current balun (choke?) be a possible solution to a problem of "RF on your audio, OM" in my mobile installation?

Yes, if RF coupling is truly the cause, and it should go at the base of 
the antenna. Multiple turns of RG58 around a single #31 core would be 
quite effective.  If RF was getting in through a Pin One problem on one 
of the cables, winding multiple turns of each cable through a core large 
enough to hold the turns would be a good move.

  There is, however, another very different cause of what SOUNDS like RF 
feedback, but is actually modulation of the V- conductor by TX current. 
This modulation, which is the envelope of the RF signal, can be added to 
any unbalanced connection of audio equipment to the transmitter. One 
very good solution is to get DC power for the interconnected equipment 
from transmitter end of the DC cable that runs from the battery to the 
transmitter.

Although it may seem counter-intuitive, bonding the rig to the auto 
frame is NOT a good thing, because it provides a second path for return 
noise currents.  That's why a two-conductor cable running straight to 
the battery is so important -- indeed, twisted pair would be best if 
relatively large gauge copper was available.  Zip cord is, in general, 
lousy for RF rejection.

73, Jim Brown K9YC