[TowerTalk] feedpoint choke for inv L

[Jim Brown]

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