SatSig topic:

Ku 7,7m Sci Atlanta feed mod for Cband

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Jan 17^th, 2008 at 6:29pm

I need to modify an old (1989) Ku 7,7 m Scientific Atlanta casegrain dish (model 8181 ?) to Cband.
Tried to find Cband Feedhorn with no results.
Can anyone help finding parts?

Jan 17^th, 2008 at 7:25pm

Contact ViaSat for advice. They took over SciAtlanta antennas.

Do you have copies of the handbook and drawings for the existing antenna? It may describe Ku and C band feed options, including possibly different sub-reflectors also.

If you have no success, please come back here for advice.

Best regards, Eric.

Jan 17^th, 2008 at 7:44pm

Thanks Eric
My Handbook and drawings are for Ku only. I don't know if I can change only the feed ( I believe so). I know its difficult to find a spare feed for this old antenna but it worth a try before buying a new one.
Thanks again

Jan 17^th, 2008 at 8:50pm

Using the same subreflector, the focal point is at a fixed location which is the phase centre of the existing Ku band feed.

Feeds vary, the phase centre may be in the middle of the flat radome window or it may be further down inside the feed. It depends on the flare angle of the conical horn.

If the flare angle is wide the phase centre and focal point is deep down inside. If the horn is very long and the flare angle is shallow then the phase centre and focal point is at the aperture.

Note that the diameter of the aperture of the feed gives the beamwidth of the radiation pattern towards the subreflector. It is important that the beamwidth (edge illumination at say -16 dB down) matches the view of the edges of the subreflector, otherwise the feed directly interferes with adjacent satellites.

If you replace the Ku band (centre freq 12 GHz) feed with a C band (centre freq 5 GHz) feed it will need to be much larger diameter aperture, about 2.4 times larger, so as to properly illuminate the sub reflector. See Antenna beamwidth calculator which gives the pattern beamwidth at -15 dB down. A feed is just a small diameter antenna, with a relatively wide beamwidth and low gain. Measure the diameter of your existing feed.

Best regards, Eric.

Jan 17^th, 2008 at 10:31pm

Thanks again Eric.
Probably I need to check if I must change the 'FEED' together with subreflector.
In any case it would be ideal to keep mount assembly,motors reflector and hub.