Parameters of the Arecibo telescope...

[Ningey]

I have found some information regarding the Arecibo telescope (which interestingly enough also has a transmitter available).
Now I wonder what the gain (in dB) of the antenna would be, not including any amplifiers and neglecting any losses.

[`Eylan Ayfalulukanä]

The gain of a parabolic dish antenna, in dB is 10log(pi²*diameter²/wavelength²*efficiency)

Where
diameter is in meters
wavelength is in meters
efficiency is a value between 0 and 1. For a prime focus antenna like Arecibo, .55 is a reasonable place to start, although I suspect that it is a bilt lower due to its huge, cable-suspended reflector. The accuracy of the surface shape is very important for dish antennas, and becomes more critical with increasing frequency.

Arecibo is actually a spherical, not a parabolic antenna. this allows 'aiming' the antenna a little, by moving the feed assembly around. I am not sure what effect this has on gain and efficiency. It is quoted that Arecibo's gain at 2.38 GHz is 70 dB, or a ratio gain of 10 million.

They transmit from Arecibo from time to time, for a variety of reasons. Sometimes, it is to use it as a radar. I believe that have used it to transmit to spacecraft as a last-ditch effort to communicate with them when they are malfunctioning. Once in a while, they transmit an 'ET phone home'   message to anyone or anything out there that might be able to copy it.