On 11/20/2018 3:10 AM, Rob Atkinson wrote:
If the ground system is at or below grade, you don't need a choke.
You DO if your radial system is limited, as you've stated below.
If
you find you do need one then that means you don't have enough radials
down. This is an example of ham antenna work where it is useful to
take a look at medium wave broadcast instead of re-inventing the
wheel. Broadcasters do not have chokes on their feedlines. That's
because they have a lot of radials and the feedline is (nowadays) coax
under ground.
AND because they don't have to LISTEN on that antenna.
But even if your feedline is laying on the ground you
can avoid having to fool with a choke provided you have enough
radials, so that the return current to the feedpoint is divided so
much that whatever is on the feedline is insignificant. And, you need
plenty of radials anyway, if you want to have a decent antenna. Put
down at least 60 radials and see if you have a problem with RF on your
coax....probably not enough to matter.
Right, but few hams have great radial systems for 160M, and most hams
don't live in the middle of nowhere where there's no local noise. A
choke matters when there are a few radials, short radials, elevated
radials, etc. N6LF has done extensive research, both modeling and
measurement, on radial systems. One of his results is that with a few
radials, balancing current between them has a strong effect on losses,
and a combination of radials and an earth connection is a bad thing. Use
of a feedline choke is a good thing in that sort of antenna system.
Hams are ferrite choke and
balun crazy...use them in many cases where they aren't needed....
Some hams have more demanding operating requirements than others. Chokes
DO matter, often a lot, for reducing inter-station interference in
multi-transmitter environments, like Field Day, SO2R contesting, and
multi-op contest stations. They can also matter when you have local noise.
73, Jim K9YC
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