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Re: c-band TVRO dishes for AO-40


I would not "detune the feed point".  Go for the best gain you can - there 
will be times when it is useful.  I have found that the linear satellite 
positioners used to move the dish for C-Band operation will seem quite slow 
and easy to adjust at S-Band.  Slightly more tricky at K-Band, but still 
quite useable.

I agree you will need to adjust the "elevation" with a second actuator.  If 
possible, convert the mount to simple el/az.  My system allows 0 - 35 deg 
elevation and 50 deg azimuth "throw".  I can start the azimuth at a number 
of predetermined points, allowing a few hours operation before needing to 
reset the azimuth start point (manually).

At 15:51 07/05/02 -0400, Bruce Nolte N3LSY wrote:
>A second consideration is that you may
>want to "detune" the feedpoint a bit, since a large dish will tend to
>peak very sharply on the signal.

I experimented with "side by side" feeds in my 10ft 0.3 f/D dish and found 
that there is a significant drop in gain with this scheme owing to the 
S-Band and L-Band beams being pointed in different directions. The best 
compromise is to use two coaxial helix antennas, the L-Band one being wound 
"around" the S-Band one.  Sun noise measurements show that neither helix 
seems to affect the performance of the other to any great degree.  This 
system was first reported by ON6UG years ago.   My implementation of it is 
described at http://www.g3wdg.free-online.co.uk/dual.htm and it seems to 
work well.

>The L-band uplink feed should not pose a problem, just mount it next to
>whatever type of feed you want to use for the downlink.


Charlie G3WDG

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