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*Subject*: Re: [amsat-bb] Coverage Area of 10Ghz*From*: Dr Thomas A Clark <clark@xxxxxxxxxxxxxxxxxxxx>*Date*: Wed, 24 Nov 1999 16:13:03 +0000

James KD4DLA wrote: > > Just wondering if someone could answer a couple of questions for me? > > I am wondering what the coverage area (footprint) for three centimeter > (10 Ghz) dish(0.6 meters in size with a 0.45 F/D ratio) feed putting out > 1 watt mounted pointing straight down on the International Space Station > would be? Would this be the same as on two meter and seventy centimeters. Well, here is how to do a "back-of-the-envelope" calculation to answer your questions. In the following there are some small simplifications in the trigonometry, but they only introduce errors at levels of 25%. Regarding dish beamwidth, dish diameter and wavelength are all that are important -- the xmtr power level and f/D are irrelevant. A 60 cm diameter dish at 3 cm wavelength is 20 wavelengths in size. Therefore the beamwidth of the dish (full width @ half max) will be about (3/60) = (1/20) radian or about 3 degrees. Let's consider the satellite in question 500 km away. A (1/20) radian beam will have a footprint of about (500/20) = 25 km in size. > Also wondering how bad the doppler shift would be using different modes > (SSB, CW, FM)? Also on two point four, three, and five point seven GHZ? Again this is pretty simple to calculate and the answer doesn't depend on the mode. A satellite in earth orbit has a speed (tangential to the orbit) of about 7 km/sec. It's not be surprising to see that when it is at the horizon and rising upwards, we see a good fraction of this velocity "head on" -- lets call it about 4 km/sec. For your 3 cm wavelength case, this means that the satellite is moving towards you with a speed that amounts to about (4 km/sec) ------------------ = 133,000 wavelengths/second (3 cm wavelength) which means that the downlink signal has a Doppler offset of 133 kHz high in frequency. When it is setting it is moving 4 km/sec away from you, so it has a Doppler offset of about 133 kHz low in frequency. Therefore from rise to set the frequency has "swooshed" by about 266 kHz in a time of about 15 minutes or about 1000 seconds for a LEO satellite like ISS. Therefore you would have to move your receiver's dial at a rate of about 266 kHz/1000 seconds or about 266 Hz/second. Since SSB gets to be unintelligible when you a off frequency by more than a couple of hundred Hz and your SSB radio has filters with a bandwidth of only ~3 kHz, you would have to keep tuning the dial very rapidly!!!!! While on the topic of the satellite's speed, I noted earlier that the footprint of a 60 cm dish would be about 25 km in size. Since the satellite's tangential velocity is about 7 km/sec, a spot on the earth zip thru bthe beam in 3-4 seconds. You asked about how this scales with frequency -- the answer is that all these effects scale directly. Since 2.4 GHz is about a quarter of 10 10 GHz, the Doppler effects scale down by a factor of 4, and the beamwidth of the antenna is 4 times bigger. Hope that helped -- 73, Tom ---- Via the amsat-bb mailing list at AMSAT.ORG courtesy of AMSAT-NA. To unsubscribe, send "unsubscribe amsat-bb" to Majordomo@amsat.org

**References**:**Coverage Area of 10Ghz***From:*James Clay French KD4DLA

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