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Ask Dr. SETI ®

Chapter 6: Technology

Dish Mesh Spacing

Dear Dr. SETI:
In your column on parabolic reflector surface accuracy, you state, "The surface inaccuracies should not exceed a tenth of a wavelength at the operating frequency." I agree with this, but I am looking for some reference that will allow me to calculate what will happen to the dish efficiency if I use only a 1/4 or 1/2 wavelength mesh surface. Can you help?

Jim, Michigan

The Doctor Responds:
Here's a rough rule of thumb, Jim: A solid surface reflects 100% (minus feed blockage and illumination spill-over, of course) of the incident signal. That is, it is completely opaque. At half-wavelength mesh spacing, the dish surface becomes 100% porous -- that is, it reflects none of the incident signal. So, for quarter wave spacing (midway between solid dish and half-wave mesh), it stands to reason that the dish should be 50% porous, reflecting half of the signal and passing the other half.

This may sound like only a 3 dB penalty for going to 1/4 wave mesh, but it's actually much worse than it sounds. This is because, in addition to losing half the signal, when you are pointing your dish straight up (toward quiet sky), you end up filling your feedhorn 50% with warm Earth (290 Kelvin), which it sees looking down through the dish. This would raise your system noise temperature by an extra 155 Kelvin (half of the Earth temperature, because the quarter-wave mesh is half porous, remember?) The resulting thermal noise takes (for example) an otherwise 50 Kelvin receiver up to 205 K. The net penalty is thus: 3 dB reduction in antenna gain, plus an additional 6 dB in receiver noise, for a total signal-to-noise degradation of a shocking 9 dB!! This is like reducing your parabolic reflector's diameter by a factor of three, which I think you can agree won't do much for the sensitivity of your radio telescope.

Bottom line: quarter wave mesh just won't work. A reasonable compromise is to use eighth wave mesh, which degrades your overall system only about 1 dB, relative to a solid dish.

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