Hello I have this 12 foot dish waiting for "something". I want a dual L/S dish system. Given the beamwidth of a 12 footer would be small. What would happen if the feed were placed at a spot other than the focal point? I realize I would be not using the dish to it's full potential. Would that broaden the beam width? I also have a four footer which would be more applicable. I am planning for the next HEO and retirement in six months. 73 Bob W7LRD Seattle
If you get creative and mount the feed in such a way that you can slide it in towards the dish, you can select your beam width and effectively gain at will by adjusting the illumination of the dish. (If you only illuminate 4 feet of your 12 foot reflector, you should have the same gain and beamwidth as the four foot dish, just with a longer focal length) I used an acrylic rod about two or three feet long on a C-Band TVRO dish, and it saved a lot of repointing when the feed was moved closer to the dish. It was also interesting to see the increase in signal strength as the feed moved closer to the focal point.
The only real advantage your four foot dish has over the 12 is being smaller and lighter, so it's easier to point and has less wind loading. If you can mount and maneuver the 12, do that. I used a 16' dish for a while on AO-40 with excellent results.. but mounting and positioning it was not a trivial task (for me at least)...Especially in my case since it was installed on the west coast of Florida in very sandy soil. I could dismantle it for hurricanes, but I still had to plan for any unexpected storms when mounting it. In the end, I found a 90cm dish preferable because I could position it by hand with minimal effort, not spend all my time repositioning, and it was also easily removed and stored during hurricanes. I doubt that will be as much of a problem in Seattle!
The only thing to remember is that some TVRO dishes have a mesh that isn't very good above C band, particularly the older fiberglass ones, but you should be fine if you intend to use it for L/S.
73s, Jason N1XBP
P.S. - I wouldn't have gotten anywhere with my big dish if it wasn't for W0LMD's webpage:
http://www.ultimatecharger.com/dish.html
w7lrd@comcast.net wrote:
Hello I have this 12 foot dish waiting for "something". I want a dual L/S dish system. Given the beamwidth of a 12 footer would be small. What would happen if the feed were placed at a spot other than the focal point? I realize I would be not using the dish to it's full potential. Would that broaden the beam width? I also have a four footer which would be more applicable. I am planning for the next HEO and retirement in six months. 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
If you get creative and mount the feed in such a way that you can slide it in towards the dish, you can select your beam width and effectively gain at will by adjusting the illumination of the dish.
Don't get too creative; the focus point exist only in one place. Gain will collapse rapidly and radiation pattern gets distorted and that makes the dish more prone to receive interfering signals around. This is of course highly depending on feed system itself.
(If you only
illuminate 4 feet of your 12 foot reflector, you should have the same gain and beamwidth as the four foot dish, just with a longer focal length)
Better use narrower beamwidth at the feed. (Underillumination) This will not degrade the performance of the dish. Only gain will be lower but beamwidth and even F/B-ratio can be increased. To illuminate just 4 feet area use narrower feed like helix. Calculate (or guestimate) the turns needed. Please read Paul's W1GHZ excellent "Online Microwave Antenna Book".
73s, Jason N1XBP
73s, Jari OH3UW
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