If the feeders are truly balanced, and narrow spaced for the frequency in
use, such as window line 450 ohm nominal and 300 ohm nominal at HF bands,
there should be minimal field outside the feeders due to the equal and
opposite currents principle.
A well designed rig, installed properly, will not have RF in the shack.
This means use of a well balanced tuner, and earthing measures that avoid
having the distance to earth as a quarter wave of the band in use or odd
multiples.
This is where many have trouble, especially with unbalanced antennas.
Use of true balanced antennas minimizes problems of RF in the shack, from
both coax and balanced feeders. (The coax feed using a balun at antenna.)
That being said, if a particular rig has sensitive digital logic, modern EMC
methods of beads and choke cores on cables, improved bypassing, twisting
power supply conductors and shielding them, all will help remove the RFI
effects.
Many hams persist in using off balance, off center fed antennas without
balancing and thus will more easily introduce RF into the shack.
Another method little known by hams, to mitigate RF in the shack involves
running the coax feeder through a conduit, (the waveguide beyond cutoff type
of filter). This affords certain off center fed verticals little to no RF
on the coax outer shield. It is a method easily implemented for a single
problem band, although ferrite cable chokes may be easier to implement for
multiple problems.
73,
Stuart K5KVH
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