Hello Questions on my current L/S helix feed. 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? Where can I find a good design for a notch filter for S band to reduce desense? Where does the focal point want to be on the helicies? Or is it pretty forgiving. Any other information is appreciated. 73 Bob W7LRD Seattle
It should be easy to make a high-pass filter that removes the 1.2 GHz signal. A piece of waveguide with two coaxial to waveguide transitions will reject everything longer than 1/2-wavelegth in the long dimension of the guide.
73,
John KD6OZH
----- Original Message ----- From: w7lrd@comcast.net To: amsat-bb@amsat.org Sent: Monday, October 23, 2006 23:46 UTC Subject: [amsat-bb] L/S helix feed
Hello Questions on my current L/S helix feed. 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? Where can I find a good design for a notch filter for S band to reduce desense? Where does the focal point want to be on the helicies? Or is it pretty forgiving. Any other information is appreciated. 73 Bob W7LRD Seattle _______________________________________________ Sent via AMSAT-BB@amsat.org. Opinions expressed are those of the author. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: http://amsat.org/mailman/listinfo/amsat-bb
Bob
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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Howard Long -
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