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I'm in the process of building an SDR-based satellite ground station using a discone. The discone is chosen due to its wider frequency range. Additionally, a dual-band LNA with around 20 dBi in both VHF and UHF be included. The question is, will this setup be enough to receive beacons from orbiting CubeSats (from the link budget perspective)? Are there any factors apart from link budget (e.g. polarization) that should be considered?

Thanks.

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The disccone is probably a poor choice of antenna for a satellite ground station, as it doesn't have very good gain to start with, and its radiation pattern is mostly pointed at the horizon, with a null in the upward direction.

You would be better with an antenna that had a hemispherical radiation pattern, or a directional antenna with an azmuth and elevation rotor.

If you insist on a wide band antenna, you might be better off with a bowtie antenna with a reflector underneath it.

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  • $\begingroup$ What sort of gains to you have in mind with the other antennas you mention? $\endgroup$
    – AG5CI
    Feb 14 at 14:50
  • $\begingroup$ Any gain is better than the big null a discone will have right in the middle of the sky. $\endgroup$
    – user10489
    Feb 15 at 2:22
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The ionosphere doesn't reflect VHF and UHF signals, which allows earth stations to communicate with satellites, but the ionosphere does tend to unpredictably rotate the polarization of signals passing through. The worst-case scenario is that you have an linearly-polarized antenna oriented 90° from the polarization of the signal (e.g. you have a vertically-polarized antenna, but the signal is horizontally-polarized), which would attenuate the signal by about 30 dB. For that reason, permanently-mounted antennas (on moving aimable mounts) for ground-earth communications are almost always circularly polarized. (If you plan to hold the antenna by hand, then you could rotate the boom of a linearly-polarized antenna to maximize the signal.) Circularly-polarized antennas can be polarized clockwise or counter-clockwise (or anti-clockwise if you prefer). This AMSAT web page says that the polarization can switch during a pass, so being able to switch the antenna from one circular polarity to the other can be useful. (I didn't know that.)

Amateur radio satellites often have enough gain that a simple antenna such as a quarter-wave vertical can be enough for the earth station. I'd think that cubesats would use much less transmit power than amateur radio satellites. An antenna with more gain than a discone would probably be optimal.

AMSAT is an organization dedicated to amateur radio communication with satellites, and they have a web page about cheap satellite antennas that hams can build here. Of course you may need to communicate on other bands, and you may not have the skills or equipment to "home-brew" your own antenna; if you're on a tight budget and are limited to commercially-available antennas, then I'd recommend a linearly-polarized yagi on a handheld boom, with the boom rotated to maximize the signal. If you are open to making your own antenna, there are many online articles about building ham-band satellite antennas. Antenna designs typically scale well, so a design for one band can be adapted to work on double the frequency, for instance, by halving the size of the elements and the spacing between the elements.

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This is most commonly done with a multielement yagi array that is aimed up at the location of the satellite in the sky. This arrangement gives you far more gain than a discone and also rejects noise from other sources on the ground. You trim the element lengths to match the exact frequency of the satellite you wish to hit. The ARRL Antenna Book explains how they are built.

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I think the answer is almost certainly no.

This paper presents a pretty detailed analysis of Cubesat link budgets at 150, 220 and 2200 MHz and takes into account a comprehensive set of factors, including rain attenuation, polarization effects, ionospheric scintillation, doppler shift, and antenna depointing.

The study shows that you will need earth station antenna gains of around 15, 24, and 38 dBi, respectively, to support a link margin of 6 dB. A typical discone antenna (e.g. 118-3000 MHz scanner antenna) will have a gain of less than 5 dBi, so you would end up with negative link margin (i.e. the link will fail).

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    $\begingroup$ Wouldn't the LNA (20 dB) help? $\endgroup$ Feb 14 at 1:39
  • $\begingroup$ Not really. Receiver noise floor and how it relates to front end gain and noise figure isn't that intuitive. The LNA gain needs to be sufficiently large that it dominates the gain in the receiver front end. It's effect is that the noise floor of the receiver is dominated by the LNA's noise contribution (which is designed to be low). There is a discussion here. I think basically you could say that LNA gain applies to both signal and system noise, so it washes out in the link budget calcs. $\endgroup$
    – AG5CI
    Feb 14 at 14:47
  • $\begingroup$ That says that (in the case of 2m) it uses a 15dBi antenna and has a 13dB link margin for the case of 9600bps packet. Which is actually more margin than you need to begin with, but it gets even better if you consider the lower rate / more robust modes that a lot of cubesat telemetry uses, for instance the Fox birds' 200bps DUV. $\endgroup$ Feb 24 at 6:42
  • $\begingroup$ @KC2G Interesting point. Ran across this paper discussing the different modulation schemes. I still wouldn't think a discone antenna has enough gain, but perhaps need to revisit. $\endgroup$
    – AG5CI
    Feb 24 at 13:00

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