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My other hobby, ham radio
tanner0441:
Hi
I did post previously but my post and Mary's reply seem to have disappeared. However if you think that your beam had a very narrow beam width Chris, try 3cms with an Andrew's four foot prime focus dish. it was the only dish I had at the time. According to the spec it was a quarter of a degree. The same as the transmitter on the satellite transmitters, at over 44,000 miles that is OK, at a few tens of miles you're really pleased you included a panning signal in the design.
I still prefer VHF/UHF to HF but I live in a VHF black hole. If ever this part of North Wales need an enema this is where they will put the funnel, RF wise.
I like your idea though Chris of phased dipoles, and if i make a phasing harness I bury the joints in hot melt glue, and spray them with freezer to stop it running off.
Brian
Mary B:
I would never keep something like that up in the winds out here on the prairie! I have a 10 foot dish mounted on 6 feet of rohn 25... tower section bent and it stripped a heavy duty gear reducer I used for az drive.
Been thinking about 10ghz eme. Use a 4 foot offset dish like you have. Be a lot easier to keep up in the wind!
tanner0441:
Hi
If you want a lot of gain on a dish prime focus or offset look at one where the LNB mounts through the dish and there is a hyperbolic reflector, the signal hits the dish and focuses onto the hyperbolic unit and back into the LNB.
I think Echo star make them for DBS systems, the gain is a lot higher than he basic dish.
If you want an interesting evening try the maths for hyperbolic curves,
Brian
Mary B:
Pass on the math!
I would go offset dish, less ground noise picked up by the feed due to spillover.
tanner0441:
Hi
If you want a quiet dish then you want a very shallow dish they have a much lower G/T (gain/temperature) figure. The problem with dishes is if they deviate from a true parabaloid by more than 1/80th of a wavelength then they will squint and fire lobes all over the place, and on 3cms if you use a mesh material the holes can't be bigger than 2.5 to 3mm or again you will get lobes of the hole edges.
I have a phased printed array from a marine radar setup. It has 72 circular quarter wave pads in 24 by 3 on a GRP substrate, with a flat plate reflector they have a beam width of 6deg horizontal and 27deg vertical beam width. this can be narrowed down by firing it into an orange peel reflector or put barn doors on it like a horn.
Brian
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