An Introduction To Amateur Satellite Ground Station Antennas
There are many kinds of satellite antennas that will get you on the birds (some better than others) and allow you to have lots of fun. If you are new to amateur satellites, though, all the options may be confusing, or worse, a disincentive to try the birds. It's really not that bad. I offer below some facts and some opinions about the state of the art today and I hope it helps you try some new things and have some fun. I have arranged the information in a progressive outline, allowing you to visualize a step-by-step methodology, gain some experience, and make incremental improvements each time you try something new.
Hearing the Birds:
Often, the first challenge is just to hear the satellites. The easiest thing to try first is an HT with 70 cm capability and a "gain" antenna. HT antennas 500-900 mm long (often collapsible) were very effective for UO-14 and SO-35, but are a little weak for AO-27 and SO-50. If you have a mobile dual-band FM radio, try a ¼ wavelength 2 m antenna on 70 cm. These antennas are 3/4 wavelength at 70 cm and offer considerable gain above 30 degrees elevation (deaf below that).
If you want to hear a little better, try a small 70 cm beam. I have plans here for a little 3-element Handi-Tenna. The very popular Arrow Antenna is a convenient and very effective portable antenna. Cushcraft makes a 3+3 dual-band beam with a built-in duplexer, suitable for both portable or fixed station use. All of these antennas are "linearly polarized." For a great discussion on linear v. circular polarization, see The Amateur Satellite Handbook, by Martin Davidorff, K2UBC (available from both ARRL and AMSAT). Small quads, quagi's, and helix's are also workable at 70 cm, but a little more awkward to handle.
Working The "Easy Sats":
Of course, "easy" is a relative term. See the AMSAT web site for some general introductory articles. The next step for a home station might be to try a simple, omnidirectional circularly polarized antenna. The most common of these is the M2 eggbeater. You can also build a popular version of this antenna, but I have found them to be ineffective at low elevation passes (most passes are below 45 degrees 90 percent of the time). An improved version of this classic design, the Eggbeater II, will give pretty fair results from horizon to horizon, especially if combined with a preamp. The basic Eggbeater II design is fixed right-hand circularly polarized (RHCP), leaving it susceptible to the "fades" common in satellite downlinks (you will not hear 100 % of the pass), but is still an effective, simple antenna--and MUCH better than the "classic" eggbeater. Other antennas in this class are the turnstile, the quadrifilar helix array (QHA), and the Lindenblad.
The next step up is to buy or build a higher gain antenna and rotate it to match the satellite's position (azimuth). Gain in the 6-7 dBi range, corresponding to a 60 degree beamwidth, is about the maximum that can be utilized without needing elevation control. Unfortunately, there are no commercially available circularly polarized antennas available in this size/gain range. If you don't mind building somethng from scratch, I recommend the TPM II antenna as a perfect solution for working all the LEO's. The TPM II antenna does not require an elevation rotor or even accurate pointing (you can do it manually with no trouble). A crafty and inexpensive automatic azimuth rotor system can be easily constructed using WB4APR's design. I built my TPM II with coaxial relays to switch the circular polarity, allowing me to optimize both downlink and uplink. I can hear and work ALL the LEOs from horizon to horizon using this simple to build antenna and a manual TV-type rotator: I have literally thousands of contacts and have Worked All Continents, WAS, and VUCC with this antenna..
Working AO-10 (not heard since March 2002):
You can work AO-10 with any of the antennas described above when it is near perigee (less than 10,000 km or so with an omni antenna). To work it further out, though, requires considerably more gain. A typical "OSCAR class" station uses 100 Watts on 70 cm for the uplink into a 40 element antenna (20 x 20) and a 22 element (11 x 11) downlink antenna (with mast-mounted preamp). Both antennas are circularly polarized and usually switchable. The most common models here in the US are KLM, Hy-Gain, and M2, but many people also use large linear antennas (or arrays of linear antennas). These antennas require both azimuth and elevation control, most often from a Yaesu or Kenpro rotator. The narrow beamwidth of these antennas also requires precise pointing, making computer control of the rotator almost mandatory: popular devices include FODTrack, Kansas City Tracker, Uni-Trac, and others. This setup is the standard of excellence in satellite antennas today.
Working AO-40 (not heard since December 2003):
AO-40 operation is pretty attractive: a high-altitude orbit that repeats every 4 days, lots of DX, and lots of interesting and educational technical challenges. Mode U/S was the most popular combination, but L/S was favored by many of the operators. Smaller UHF ground station antennas than those used for AO-10 can be used: many use 70 cm uplink antennas in the 10 dBi range. Most any Yagi-Uda beam in the 4-6 element range will work. For L-band, 23 cm, antennas in the 20 dBi range are required if you only have a 10 W rig. Many L-band ops use amplifiers with 40 or more Watts (for BIG! signals). Many operators are taking a wait-and-see approach to AO-40, but if you want to put something up now, you will probably want to consider a small parabolic dish for S-band, 13 cm, as a starter system. For about $200 you can get a 3' BBQ dish and a low noise MMDS downconverter modified for 2 m. The microwave bands lend themselves to experimentation, so expect to see lots of new and interesting antenna designs published. Almost all of the future satellites have some plans for mode U/S or L/S.
Article by K5OE originally available a http://members.aol.com/k5oe/new_ant_intro.htm