原文由 n3ox 在
http://www.moxonantennaproject.com/construction.htm#balun notes
balun notes: the desired result at the antenna feedpoint is to make equal currents flow in the element on each side of the split. if the antenna is perfectly symmetrical and not close to anything, this will just happen, even with coaxial cable. you get equal and opposite currents at all times on the antenna element halves and in the coaxial cable conductors as well.
you can upset this balance by building the antenna asymmetrically or getting one side closer to the surrounding objects (trees, houses, other antennas) than others. if you upset this balance, then current may flow on the *outside* of the coaxial cable as if it were part of the antenna. this is a common situation, and is not the end of the world. however, the coax becomes part of your antenna, and the currents flowing on the shield affect the radiation pattern.
this can be a bit of an issue for highly directional antennas. the nice moxon pattern will be changed by the coax. for better or worse, who knows. you might never notice.
with a properly designed choke or current balun, you can force the currents on the element halves to be equal simply by making it "impossible" for them to flow on the coax shield. such a balun could be a simple coil of coax (of the right diameter and number of turns) or a number of ferrite beads slipped over the outside of the coax (this is what i use).
if you're not noticing any pattern problems (low front-to-back ratio) or if those pattern problems don't bother you, don't worry about the balun. if you are noticing pattern problems, put a balun on. such a balun can also keep electrical noise from the house from being *conducted* onto the antenna by the coax shield. sometimes an antenna with a choke balun will be quieter than one without. the more you care about your antenna's directional pattern (long boom yagis, etc) the more you need to worry about making sure the feedline doesn't become part of the antenna and radiate in unwanted directions.
dan,
n3ox
