SETI League Technical Manual -- AntennasAlthough numerous antenna options exist for amateur SETI stations, by far the most popular approach is to use a three- to five-meter (ten- to sixteen-foot) satellite TV dish, with a prime-focus feedhorn appropriate to the radioastronomy frequencies. Many SETI League members already possess suitable dishes, either currently or formerly used by them with Television Recieve Only (TVRO) systems. Others have friends, relatives or neighbors who have transitioned from C-band TVRO to Ku-band digital Direct Broadcast Satellites, which use the small (18" diameter) dishes of which the RCA DSS system is typical. Often these individuals are anxious to clear their yards of their "unsightly" obsolete large dishes. Ask around; there may be a suitable SETI antenna in your neighborhood, which could be yours for the taking.
Gary Bourgois has published a booklet for the satellite TV hobbyist, which discusses low-cost alternatives for obtaining all the hardware necessary for TVRO. The work includes a useful chapter titled "The Dish". Though specifically aimed at the TV hobbyist, this information will be equally useful to the amateur radioastronomer and SETI enthusiast. Best of all, this information is available online; see The Scrounger's Guide to Satellite TV.
Because the communications satellites used for TV are in Clarke (geosynchronous) orbit, most TVRO antennas are on modified equatorial (polar) mounts. Thus they are fixed in declination, and rotate in right ascension or hour angle. SETI amateurs, on the other hand, generally prefer to mount their antennas as drift-scan (transit mount) telescopes. This requires some ingenuity in mounting of TVRO dishes. Fixed azimuth, and variable elevation, are generally required. You will want to point your antenna due South (if in the Northern hemisphere) or North (if in the Southern), and use whatever rotor mechanism you possess to vary the elevation between the Southern and Northern horizons. Often, zenith (aiming the antenna straight up) provides users with the least noise, interference, and signal blockage. Such an alignment is entirely appropriate to SETI, resulting in a declination equal to the user's geographic latitude. If this seems constraining, remember that any direction in an all-sky survey is just as likely as any other.
Feedhorn options include the cylindrical waveguide hydrogen-line feed distributed by Jeffrey M. Lichtman at Radio Astronomy Supplies, as well as existing dish feeds for the 23 cm (1296 MHz) amateur band and the GOES series of 1691 MHz weather satellites. Bear in mind that, while these feeds will cover portions of the Water Hole, they probably lack the bandwidth required to cover it all. And though circular polarization is desirable for a number of reasons, it is unlikely that you will be able to achieve good circularity over more than a small portion of the Water Hole, in any practical dish feed.
While 10 dB edge illimination of the dish is common for communications applications, radioastronomers usually prefer to under-illuminate their dishes, as a means of reducing antenna noise temperature and sidelobes. For SETI, a fifteen dB edge taper is probably desirable. The SETI League has developed a new feedhorn design with bandwidth and under-illumination in mind.
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