Post by Eric Johnston on Jan 17th, 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.
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.