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Re: Building Helix, Have Questions



Tom Smith wrote:

>I am building a helix antenna to illuminate a 46 cm (18 inch) offset TVRO
>dish ("Dish Network 500").  I am looking for a "sanity check" before I build
>it.  It will be for AO-40 S2 receive only.  The plan is:
>
That's a pretty small dish unless you have a really hot downconverter 
(or better yet, a really hot pre-amp).

>6 turns LHCP
>Diameter: 3.98 cm (1.565 inch)
>Turn spacing: 3.12 cm (1.227 in)
>First quarter turn 0.64 cm (0.25 inch) above reflector
>Reflector: 12.47 cm (4.91 inch) square
>Wire: 6 AWG solid copper 0.41 cm (0.161 inch) dia
>
That is about right based on other comments here in the past.

>Do I need to modify the dimensions to use plastic dowels to support?  I want
>to use 2 dowels inside the coil, spaced 180 deg, to support the weight and
>provide a guide for spacing.
>
You do not need to support the helix at all - especially with 6 gauge 
wire for the helix.  Take a look at the photos starting at: 
http://home.earthlink.net/~k6ccc/Photo/Pics/S-band8.html
That and the next several show you my helix which is make from 10 gauge 
and it's plenty solid.

>Can I cover it with PVC pipe?  I want to protect the downconverter from
>electrostatic discharge from inquiring fingers.
>
Will it be anywhere near a location where fingers can get into it?

>How important is it to tune it for 50 ohms if I only use it for receive,
>with a commercial downconverter such as UEK-3000 or RX2400-144?  From what I
>have read, keeping the first quarter turn near the reflector should move the
>impedance closer to 50 ohms.  But I have no way to test it.
>  
>
It should be close enough - except for the purists in the group.  Most 
of us don't have the test equipment to properly test antennas at S-band.

 
73
-------------------------------------
Jim Walls - K6CCC
k6ccc@amsat.org
Ofc:  626-302-8515
http://home.earthlink.net/~k6ccc
AMSAT Member 32537 - WSWSS Member 395


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