Equipment
The Cal Poly earth station has a lot of equipment associated with it. Because we have two satellites that will be orbiting the earth at the same time, we needed to figure out a way to downlink data from both satellites. Instead of trying to share one earthstation, we decided to build another earth station right next to it. It would be totally separate for redundancy purposes. The only thing they share is a 12volt power supply. We have plans to build a backup battery system. We named one station Hertz and the other Marconi to keep them straight in our minds.
Hertz
Hertz was our first earthstation, built mostly by Trent AE6HD to talk with CP1. It consists of a Yaesu 847 radio hooked up to a 2m and 70cm yagi, and a HF antenna.
Antennas: The tower on the roof is a Rohn JRM23810 non-penetrating roof mount. The base is approximately 5 feet per side and the center pole is 10 feet long. While the center pole is 2-3/8" in diameter, it is just small enough to fit our Yaesu G-5500 rotor system. A M-squared 436CP42 antenna is used for 70cm, and connects to the SSB Electronics preamplifier via LMR-400 coax. While LMR-400 is not the greatest coax for that high a frequency, the run is short. After the preamplifier, a 50 foot run of 5/8" heliax goes thru the roof to the radios. The long run of Heliax ends with a short piece of LMR-400 for flexibility, so the heavy coax doesn't get kinked. For more information on our antennas, check out our antennas page.
Radio: The radio that we use for Hertz is a Yaesu 847. It is a radio specifically designed for satellite work. It has 2 VFO's, which allows you to transmit on one band while receiving on another band. This is ideal for Mode J (2m up/70cm down) satellites. However, most digital modes occur exclusively on one band, so this feature is only good for voice work. It has 4 antenna jacks on the back for HF, 6m, 2m, and 70cm. The 847 also has lots of knobs on the front, which sounds silly but lets you control most of the features without resorting to menus.
Radio Interfaces: We use a homebrew break out box to key the transmitter and take all of the audio out of the back of the radio. A schematic can be found here. It is a really simple device that just makes connecting the radio to the computer very easy. We made individual cables, but after a while it was just a pain to keep connecting and disconnecting the cables from the radio every time we wanted to switch between 1200 and 9600 baud. The difference between 1200 and 9600 baud is where the audio is tapped: the 1200 baud can be just straight speaker audio (after the filters), and the 9600 baud data must be taken before the filters (discriminator-tapped). Using the PKT port on the back of the radio, both the 1200 and 9600 audio is available, and the radio takes the audio at the appropriate places. Another advantage of using the PKT port is that the sound still comes out of the internal speaker; when the computer is directly plugged into the SPEAKER port, the internal speaker is bypassed.
We also use the CAT interface on the back of the FT-847 to tune the radio. The connector on the back of the radio is just a standard DB-9 serial port. It works really well, but a null-modem adapter must be used or it will not work.
Computer: We have a standard PC for data collection. It is running WindowsXP, and has various pieces of software on it. We mostly use MixW for decoding digital data. Check out our software webpage for a complete list of all the software we use. The most important piece of software is the anti-virus engine. Mostly we have been virus-free, but every once in a while a virus comes across the network and attacks the computer.
Block Diagram: Here is a block diagram of Hertz.
Marconi
Marconi is our second earthstation, built primarily to handle communications with CP2. It consists of a lower-gain yagi hooked up to an Icom 910H.Antennas: Marconi's single antenna is a Msquared 436CP30. We decided, mostly due to cost, to only have 70cm capability on this earthstation. When we get a 1.2GHz antenna or dish, it will go on the other side of this boom. It has less gain than Hertz's antenna, but the field-of-view is considerably larger at 30degrees. We have another JRM23810 tower on which everything is mounted. It also uses a short piece of LMR-400 to connect to the SSB Electronics preamplifier, same model as Hertz. The RF then goes down LMR-600 coax to the radio. We use a Delta 2N antenna switch just before the radio so that we can use an internal antenna for testing purposes. The internal antenna is a cheap 430MHz mag-mount antenna. For more information on our antennas, check out our antennas page.
Radio: Marconi has an Icom 910H radio. It seems to be the radio that most CubeSat groups are standardizing on if they are using amateur frequencies. It is an OK radio, not great. Make sure to spend the money and buy a new one; the early units (with low serial numbers) had serious problems and should not be used. Make sure to buy a service manual.
Radio Interfaces: We built another one of the breakout boxes described earlier. Surprisingly, the pinout is the exact same. Before we built the breakout box, we used a RigBlaster. It is essentially the same as the breakout box we created, but it has some level conversions and plugs into the MIC connector on the front of the radio. While it may be a good idea to buy one to get an earthstation on the air in the quickest time, someone in your cubesat team will know how to solder a 2N3906 and key the transmitter that way.
Block Diagram: Here is a block diagram of Marconi.
Updated 21 May 2006