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RE: Re: AMSAT-BB-digest V2001 #103

Hello David

> We tested the 23 and 13cms versions at the Amsat UK colloquium last year
> found the impedance way off 50 Ohms. Return loss was between 6 and 10dB.

Yes I think they were mine that you tested!!

I've since modified them, pushing the feedpoint right down against the
reflector, and glue-gunning the first 1/4 turn right onto the reflector
(insulated of course). It's a very very stiff helix, and so needs a helping
hand. At 23cms I got the VSWR down to 1.3:1 from 5:1. I assume that as both
now perform (empirically) far better than before I must have done something
right! I have not tried the shim matching section yet - haven't quite worked
out where to find the brass shim everyone talks about.

Before doing the mod, you're absolutely right: they were behaving similarly
to a leaky dummy load with lots of reactance.

Considering they're commercial, I would have hoped for none of this fiddling
about with the Sandpipers in the first place.

I'll be using the Wimo's. The 23cm Wimo has a 1.2:1 VSWR or less right
across the 23cms band, and although I don't have the means of measuring VSWR
on 13cms, I assume the 13cms unit to be similarly high performance from the
empirical tests. The Wimo's _do_ have matching sections.

73 Howard G6LVB

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