1740
« on: July 14, 2013, 2252 UTC »
A choke at the antenna feedpoint can help the antenna from receiving noise picked up by the outside of the coax shield further down the feedline. As already noted, it is recommended to at least wind an "ugly balun" (really an air-core UNUN) with coax, though note ugly baluns are often rather limited in bandwidth - YMMV depending upon length, diameter, coax type, etc.
Coax is built as a two conductor feedline, but it is more like three conductors from an RF standpoint: center, inside shield, and outside shield. The outside shield is the most exposed to external EMI/RFI.
I like to use active antennas like the DXE-ARAV3-1P as good examples of decoupling the feedline shield. Active antennas tend to work or fail based upon N, since S is already usually low due to the short antenna element and a typically even worse RF ground/counterpoise. Feedline shield isolation is explicitly called out as an important design consideration in the specs. Without effective isolation, the coax outer shield can become part of the counterpoise, right along with delivering unwanted locally picked up noise to the antenna feedpoint, thus further degrading S/N.
Of note, it works the other way around as well. If the outer shield becomes part of the antenna, then "RF in the shack" can become a problem when transmitting.
I use multiple active antenna designs - mostly verticals - due to lot size considerations. I can personally recommend the MFJ-915 RF isolator from personal usage, but admittedly, it is kind of overboard for just a receiving antenna. It is little more than a bunch of ferrites on a piece of coax, placed in a PVC pipe. I suppose it could be replicated for less money, especially with lower grade ferrites if not transmitting.
It is much the same reasoning for a 4:1, 9:1, or whatever baluns on non-resonant antennas. Not only are they doing impedance changing, but they are acting as feedline shield isolators as well.
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