Hello everyone,
I am a student at Cornell University, and a member of the Cornell University Satellite Project Team (http://cusat.cornell.edu). We are preparing for a launch in June, but problems were found with our RF system, so I'm trying to solicit help from those with experience, expertise, and/or equipment related to spacecraft communication/antenna design on the amateur 70-cm band. If you have any or all of this, please read on.
Some backstory: last year, we won the University Nanosat-4 competition, and the prize was a free launch. Right now, we are aiming to be put on the next launch of the Falcon 1 rocket by SpaceX. Both parts of the satellites have been assembled, and are undergoing further testing with the AFRL in Albuquerque. While that seems to be going along well, our latest test of the RF system doesn't look good, and we don't have the equipment or expertise to correctly diagnose the problem or find a solution, nor do we have much time. So, I apologize if this email is rather disjointed, but there are many things going on at once. I'll be glad to clarify anything I can. Also, I think it would be best if you assumed that we don't know anything about anything, because something that is obvious to you may not be very obvious to us.
The problem surfaced on Friday, when we performed a ridge test to verify that our RF system was adequate, and found out that it is not. We had a the satellite's antenna attached to a structural prototype of the satellite, and brought it to a hill roughly 4km from our ground antenna. We stuck attenuators on the ground station, between the antenna and the pre-amp in order to simulate the path loss we would experience in space (LEO, 330x685, 9.1 degree inclination), and transmitted from the satellite. By our calculations, we needed roughly 57 dB of attenuation in order to be confident that we could hear the satellite while it was at the edge of the horizon, but we lost signal after 10 dB. It is possible we made mistakes in the design or construction of the satellite's antenna, or in the configuration of the ground station, so I would greatly appreciate if you could look over what we have and did, and see if you could point out what we've done wrong, what we can do to further test this, and what potential solutions are. If anyone is in the Ithaca area, I would be more than willing to show you our setup.
On each satellite, we have two square loop antennas made of 12-gauge copper wire, 8.3125 cm per side, corner fed. At the feed point of the antenna, we have a matching circuit as follows:
(antenna)-----------------------+----C2---+--------------TX/RX |_________________C1______L________Ground
C1=56uF C2=33uF L=10.7nH
According to our EZNEC model, this comes pretty close to matching impedence with our 50-ohm, RG316 coax, 1/2 wavelength long, which feeds into a Kenwood TH-D7AG, modified to fit in a metal box suitable for flight. For the test, we didn't use the flight radio, but connected the same antenna to an unmodified TH-D7 held outside the prototype structure (if that has any significance). Since the best SWR meter we have is an MFJ 269 we don't really know how to use, we have not been able to verify that the antenna + matching circuit perform the same as in the simulation, or have the same impedance. I just now found someone at Cornell with a network analyzer, and I hope he will let me use it. What else should we do to test the antenna? Did we even choose the 'right' kind of antenna for what we want to do?
On the ground station side, we have the following equipment:
Antenna: M2 436CP30 70-cm, circularly polarized Yagi Coax: RG-8, 50 ohm Pre-amp: KP-1-440 70 cm in-shack GaSa FET Pre-amplifier (Oops, just found out from ARRL Handbook we need a mast-mounted one. What mast-mounted pre-amps are available for 70-cm? Can we simply weather proof what we have and stick it on the antenna's mast?) Transceiver: Kenwood TS-2000 TNC: Kantronics KAM-XL Rotator: G-5500 with GS-232B controller Rotator control software: NOVA for Windows Radio control software: Ham Radio Deluxe
Unfortunately we're not entirely sure about how to fully use all of our equipment, or if we're using them correctly. It's possible that we have something off and we aren't able to fully step back and see all the 'little things.' Would it be possible for someone to take a look at what we have and show us best practices and/or help us with a sanity check? We can flip through several different manuals and check different websites for help but it might be better to have some grizzled experience working alongside us.
Thanks for any advice or help you can provide. We're excited to be working on this and learning as we go along, but we could really use a guided push towards success.
73 -Nate Parsons KC2SVI