After a short study on the antennas used on radioamateur satellites, I have insisted in studying those used on the Cubesat type. The main objective is the design and implementation of the transmission and reception antennas for the Sallesat-I satellite.
Starting on the fact that Sallesat-I will be a Cubesat, what means it will be a cube of 10 cm to be placed in a LEO Orbit (Low Earth Orbit) , due to its lower height, de losses in the propagation path is more reduced than in the geostacionary orbit (remember that a higher height means greater attenuation values and propagation time). The frequencies used in this communication will be a transmission by the satellite (downlink) on 144 MHz (V band) and the reception (uplink) on the 435 MHz (U band).
As a good example, we can mention several satellites as follows:
AMSAT-OSCAR 51 (Echo or AO-51), a satellite stil in operation on frequencies with a downlink on 435 MHz and an uplink on 145 MHz.
LUSAT-OSCAR 19 (LO-19), downlink at 437 Mhz and uplink at 145 Mhz.
Radio Sport – 15 (RS-15), in a semi-operative state with a downlink on 29 MHz (CW/SSB) and an uplink at 145 MHz (CW/SSB).
Monopole and turnstile antennas are quite often used on amateur satellites. This is why I am insisting in analyzing those antennas. For transmission in 144 MHz (2m band), one of the antennas considered should be a turnstile consisting in two crossed λ /2 dipoles connected in such a way they receive signals differing 2/π radians in phase, getting so a circular polarization that allows a perfect reception on the terrestrial station.
For reception, the best antenna is a monopole in 435 MHz (70 cm band).
Due to simple construction and looking at the radiation pattern showed in the below picture, we can understand quite well why it is used so much. To get these radiation patterns, we have considered an infinite and perfect ground, as well as zero losses in the cable.
We have to insist in that this study is not going to end until we have patterns well adjusted to our actual necessities. We have to realize that the input impedance of the receiver antenna as well as the output impedance of the transmitter for the transmission antenna at 144 MHz (2m band, where 2 meters is the wave length) must be in agreement with the input impedance of the receiver and the transmission antenna impedance for a good adaptation.
In what concerns to their physical construction, we have to get a perfect seizure to the satellite chassis, very low loss connections, adapted impedances, and antennas that be “transparent” during the lunch for getting latterly a perfect deployment once the satellite is already in orbit.
Finally, we have to end with a detailed study of the radio link, Earth covering calculus and, definitely, the field intensity achieved on the Earth surface.
Satellite segment
