Earth Station Antennas

Antennas are the most important part of your earth station. For beginners, we suggest an omnidirectional antenna. An omnidirectional antenna simplifies the building of the earth station tremendously, as no rotors or rotor interface are needed. Omnidirectional antennas are fine for most of the amateur satellites in orbit right now, but you will probably have a hard time hearing your CubeSat with one.

One of the big reasons to use specialized antennas is to have circular polarization. Most satellites use circular polarized antennas because the satellite is spinning. The difference between horizontal and vertical polarization is (theoretically) infinite, so if the orbiting antenna is horizontally polarized and your antenna on earth is vertically polarized, you will not receive anything, no matter how much power is transmitted from the satellite. This is bad. So satellite builders went with circularly polarized antennas, because the difference between right-hand circular and left-hand circular polarization is only 3dB, much more manageable than infinite.

Omnidirectional

A quadrifilar helix antenna consists of four quarter-wavelength or half-wavelength elements fed with 90 degrees phase difference. The polarization is circular, and the beamwidths are often greater than 90 degrees, covering a big chunk of the sky. The ever present "eggbeater" antenna is based on this design.

Eggbeater - Jerry K5OE talks about how to build an eggbeater antenna, almost the same design as the QHA. Also check out his Eggbeater2 antenna, which receives better towards the horizon that the original eggbeater. Both of these antennas can be built in an afternoon, with minimal tools and a hardware store nearby.
Quadrifilar Helicoidal - An on-line calculator from John Coppens to determine all of the measurements for a quadrifilar helicoidal antenna. Also go up a directory and check out his main page on the QHA.
Resonant Quadrifilar Helix Design(1.1MB, PDF) - By C.C. Kilgus. This Technical Feature from the December 1970 edition of Microwave Journal talks about designing helix antennas for VHF and low UHF. Different feed methods are discussed, as well as dual-frequency operation. Radiation patterns are shown.

We also have a Comet discone antenna, but we do not recommend using it to talk with satellites, as it is linearly polarized and usually the radiation pattern is pointed toward the horizon. Discone antennas are "wide band" antennas, which means that they try to cover all frequencies, but in the process they poorly perform on all bands. We suggest you stay away from these type of antennas, as they bring a lot of interference into the earth station.

Direcional Antennas

Once you start to get comfortable operating your earth station, time to upgrade the antennas! Directional antennas focus RF energy in one direction, allowing you to talk to satellites that are farther away. The same is true for receive, directional antennas can pick up weaker signals provided that they are pointed in the right direction. Think of a directional antenna as a flashlight; it is the mirror and lens that focus the relatively weak light into a bright spot.

Yagi:
Most earth stations use Yagi-Uda antennas, which are directional antennas. As you can see from our pictures, we use yagi antennas here at N6CP. Yagi antennas come with different numbers of elements depending on how much gain is required. More elements means more gain, but a smaller beamwidth.

Since we have two earth stations here at Cal Poly, we decided to go both ways when selecting our antennas. Hertz has high-gain antennas with a small beamwidth, and Marconi has a low-gain 440MHz antenna with a wide beamwidth. The high-gain antennas require very good pointing accuracy , and the low-gain antenna does not have to be pointed as accurately.

Polarization: Yagi antennas can be either linear or circularly polarized. Yagis with only one row of elements are linearly polarized (either horizontal or vertical depending on which way you mount them), and yagis with two rows offset by 90 degrees are circularly polarized (either right-hand or left-hand circularly polarized, looking down the antenna). For satellite communication, circular polarization is a must because the difference between right-hand (RHCP) and left-hand (LHCP) circular polarization is only 3dB. Yes, that is half your signal, but the difference between horizontal and vertical polarization is theoretically infinite. However, real world values for the difference between horizontal and vertical polarization are around 30 to 40dB, over a thousand times more loss than the difference between RHCP and LHCP.

Elements, Gain, and Beamwidth: The number of elements on a yagi is directly proportional to the gain of the antenna. More elements means more gain but smaller beamwidth, so there is not an advantage to a 100 element yagi because pointing accuracy must be very good. Our high-gain 70cm antenna has 42 elements (which is quite a lot), a gain of 16.8 dBdc, and a beamwidth of only 21 degrees. Our smaller yagi on Marconi has 30 elements, a gain of 14 dBdc, and a beamwidth of 30 degrees circular. When you combine in the fact that the pointing accuracy of our rotors is not good on a windy day, one can see there are tradeoffs in each antenna. The lower-gain antenna can search a section of sky quicker, but won't be able to hear really weak signals.

Rotors: We use the Yaesu G-5500 rotor system here at Cal Poly. It is pretty much the only cheap commercially available az/el rotor system. To get the G-5500 interfaced with the computer for automatic tracking, we use the Yaesu GS-232A or GS-232B. The system works great but is very expensive; the full system costs around $1200.
Dishes:
Dishes are the next step up from yagi antennas. However, the benefits of a dish are often not worth the cost at these low frequencies (around 430 MHz). A dish becomes feasible in the 1.2 GHz range. The overall system cost goes up dramatically when using dishes because there are not many amateur radio dish and accessories suppliers around; you will probably buy commercial gear which is very expensive.