[Moon-net] tnx and choke ring question de SM5LE

[Marko Cebokli]

Hello Sven,

A VE4MA style "choke" is actually not a choke, but a resonant structure, so 
you should not change its dimensions very much, if you want top performance.
It is parasitically excited (like a yagi director) and its field (a coax 
higher mode) together with the round wg fundamental mode field produces a 
nice clean and symmetric pattern.

You can use a small (narrow, say 3cm) CHOKE instead. It must be lambda/4 deep,
air lambda, since this is an air filled coax line. This will only prevent the 
currents spilling onto the outside of the feed, and thus reduce some 
sidelobes, beneficial for noise on receive. However, the mainlobe will still 
have the unsymmetric shape, that comes from the round wg fundamental mode 
field distribution.
I guess absorber would have a similar effect on the pattern as a lambda/4 
choke, but could worsen your noise on receive because of losses! 

I have worked with "vertex plates" some 15 years ago, designing antennas for 
PTT links. You can improve the match with an "vertex plate", at the cost of 
some increase of the sidelobes and some very small gain loss.
The increase is mainly in the near sidelobes, might not increase your noise 
too much.

73, Marko S57UUU



On Thursday 22 March 2007 22:05, SM5LE wrote:
> Hi all
> Tnx all for such encouragement for mesh septum and Homepage etc! Tnx Tnx.
>
> Question:
> 1.
> Is possible to make a choke ring smaller (diameter) with some dielectric
> material (frigolite or something) or does the losses "eat up" the benefit
> to such a ring?
>
> 2.
> Can I use some absorber material instead of a ring ?
>
> 3.
> I have heard that one can have a coon of reflection material in the center
> of the dish to avoid reflection to get into the feed. Have any tested that
> ?
>
>
>
> Small rings needed for small dishes - hi
>
> 73 and Supertnx  /Sven Sm5LE
>
>
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