On 09/20/2013 10:34 AM, Rowland Carson wrote:
> RS list hundreds; obviously it needs to have a central hole of a suitable size
for RG-58 (or probably RG-400 in my case) to pass through, but what about the
other dimensions, and the magnetic characteristics? It's not clear whether the
toroids are fitted to act as a filter, as tuning, or as a balun. If, as I
suspect,
the latter, the magnetic characteristics are likely to be more critical.
The torroids are there do make it clear to the electrons where the
border is between the outer surface of the coax and the antenna leg
connected to it. Without the torroids it is just one long piece of
conductor.
It is not that critical though, but without these torroids you will get
more radiation from the cable itself and it will pick up more noise from
the surroundings of the cable, and since this is an airplane packed with
lots of electronic stuff you will want to prevent that.
The magnetic properties of the torroids are critical indeed. I don't
have a list or manual, part numbers have been disappeared from memory
long time ago, but you obviously need to find something suitable for the
intended frequency range (100-150 MHz). Some googling is in order here.
Or ask a local radio amateur club, the popular 2m-band (144 MHz) is
close to the aircraft frequencies and what will work for them will work
for you too. Changes are that someone has a box full of these things on
his bench and is happy to share a few.
As an alternative you can coil up a part of the coax close to the
antenna, but this is heavier.
Frans
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