Question 3: 2.4 GHz or 420 MHz?
The third CrowdSat question comes from SEDSAT-2 Communications team leader Lavina, from Philippines SEDS. Finally, we have a question that is both out of my domain (I am not an electrical engineer) and is not easily Googled...
We are considering using a 2.4 GHz frequency (either with a dipole or a grid oscillator antenna) as opposed to the 420 MHz we had initially thought of. The trouble with the higher frequency, accoridng to our advisor, is that it would contribute to a higher free space loss. We were wondering if there was any way to offset this loss? And if there's anything else we should consider with regard to the 2.4Ghz vs Mhz question?
Not a problem! This is what the CrowdSat idea was really designed to do: bring in people that have real experience and knowledge to answer your questions. Looks like I have to stop faking it sooner than I thought and get some help...Help comes from a friend of mine from University of Illinois who now works at Northrop Grumman. Chris says:
Off the top of my head cubesats usually use the 100s of MHz band because it makes transmitter design and receiver design simple. The parts in that band are cheaper and there will be less interference generated and received by the comm electronics (filtering is easier at lower frequencies). At 2.4GHz, there will be high atmospheric absorption (especially if theres moisture in the air). This will require better coding schemes or more transmit power or both. Simply put, its more cost effective, design is easier, and less power is required with comm in the MHz rather than in the GHz. Try searching under atmospheric absorption to pull a graphic like this: http://www.everythingweather.com/atmospheric-radiation/absorption.shtml. As the wavelengths get into the GHz range, absortion goes way up, and transmission goes way down. They may ask why commercial/government satellites use GHz communications. Its more secure, there's more bandwidth available, and they aren't as power limited as a tiny cubesat.
There's a third, drastically overlooked component to the reasoning that GHZ frequencies are used for satellite comms: Antenna gain! the wavelengths being shorter, require s proportionally smaller antennas to achieve the same gain therefore, satellites with attitude-control such as geo-orbital sats may have very high-gain antennas pointed at earth, or even some specific portion of it, as well the groundstations can also benefit having higgher-gain antennas . Additionally, having high-gain antennas on a satellite reduces the transmitter power requirement, in cases where that's important. A cubesat probably doesn't have attitude-control, or it's rudimentary, or experimental, therefore, GHZ frequencies and high-gain antennas don't make as much sense.
Posted by Auke de Jong, VE6PWN on December 25, 2007 at 10:13 PM EST #