[amsat-bb] Re: L/S helix feed

Howard Long howard at howardlong.com
Tue Oct 24 03:18:47 PDT 2006


> I am building a dual L/S concentric helix feed for 
> a 48x54 inch primestar dish.  The reflector is cut 
> for  L band does that cause a problem with S band?  

Only if it causes a large blockage - on such a large dish, and as I think
it's offset fed, then this is unlikely to make any noticeable difference.
You will probably find  more problems with ground noise from the helix
sidelobes on S band.

> Where can I find a good design for a notch filter for 
> S band to reduce desense?  

I have Charlie G3WDG's filters. The 1.269GHz notch is essential in front of
the DB6NT S band front ends that have no notch: you will fry it otherwise.

On the other hand, with the AIDC3731's ISTR that W0LMD documented these at a
symposium a few years ago as having very deep OOB filters and no 23cm
filtering is generally required.

This is worth keeping in mind if noise figure is bugging you. If you have to
put in a 0.5dB loss filter in front of a low noise 0.7dB NF front end, you
would be better off sticking with a 1dB NF AIDC on its own.

> Where does the focal point want to be on the helicies?  
> Or is it pretty forgiving.  Any other information is appreciated.

To within an inch you will not hear the difference at S band. Of the two, S
band is more critical.

73, Howard G6LVB

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