Re: [amsat-bb] RE: Prime-Star Dish - Now Sky dish too

[ae517@xxxxxxxxxxxxxxxxxxx (Russ Renaud)]

>I finished my 2.4Ghz dish helix feed over the weekend. I used 1/4"
>copper tubing wrapped around 1" PVC pipe as a form.
>
>I 've used 5 turns for what I was told was the correct illumination on
>a DSS style (18") dish.
>
>The shim that I soldered to the first 1/4 turn is wider than 6 or 8mm
>(more like 10mm). How critical is the width of the shim to the overall
>match (I have nothing I can use to test the match at this point)? I
>used 1/16" Teflon as a spacer between the shim/tubing and the Hammond
>box I am using as a cup.

I just finished building a stressed parabolic reflector, and need to build 
a helix, so I've been watching amsat-bb for similar info, and of course,
reading 'mode-s, the book'.  BTW, I used hardware cloth for the dish
surface, and, even using gloves, my hands look like I've been playing
with a bobcat.

According to the book, #8 wire should be used for the helix, but there's
also mention of #12 wire being used.  G3RUH used the inner conductor
from some coax.  Can I use #12 for a 5 turn helix?  If I use copper
tubing, how does one solder the tubing to the N connector?

>From what I've read, the purpose of the shim is to match the 140 ohm
impedance to the 50 ohm impedance your preamp expects to see.  If this
is a 1/4 transformer, then using a wider shim would decrease the 
Zo of the transformer section, changing the impedance of your match.
(downwards, I think).



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