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Re: 2mtr dish (additiona)l

At 08:38 PM 10/1/03 +0800, Keith Bainbridge wrote:
>First of all many thanks for all the email responses to my original post,
>there are some clever people out there !!
>I've decided to go with the dual helix feed for mode S & L and to build a
>separate uplink antenna for 70 cms ( still undecided on a 14 turn helix or
>17 ele each side crossed yagi)
>Now there is a separate problem, for a 0.656 f/d dish some of you recommend
>a 6 turn helical for each band and others say 2.5 turns or three turns, who
>is right ??
>Also I have searched the net for any drawings of a dual  S/L helix feed and
>drawn a complete blank.
>Once again I would accept any useful information.
>Again , many thanks for your previous responces.
>73 de Keith VK6XH


Can't help with links for dual-band helix feeds (others will, I am sure).

But your question on number of turns, I can!  I am using a f/d = 0.7 dish
and use a 6-1/4 turn helix feed.  I actually computer-modeled the design
before construction to check the pattern.  I get about -15 dB on the
main-beam of the helix in the direction of the dish edge so that is pretty
good for low noise reception (I hear the AO-40 xponder noise floor to
squint of 10 deg.).  BTW I use a 3-1/4 turn helix on my 8-foot (2.5m) dish
at 1420 MHz for radio astronomy.  That dish has a f/d= 0.38.

I will someday install a dual-band (S/L) patch-feed on the big dish!

My experience using a HB 18-turn helix for satellite uplink on UHF has been
disappointing (AO-10).  I would urge you toward the yagi!

73, Ed- AL7EB
mode S:  33-inch (85cm) offset feed (helix) dish=>MKU-232=>unmod Drake
2880=>FT-847 (123 MHz IF)
mode U:  FT-847=>60w PAat antenna: M2 436CP42 U/G (16.5 dbdc)
mode L:  FT-847=>DEM 144/1268 conv  (no antenna at this time)
I am not QRV at this time for AO-40   

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