| Can Arecibo be heard on a hand-held yagi ? (Apr 10) |
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Domenico I8CVS has made some calculations to determine how well the Arecibo 432.045 MHz signal would be heard on a hand held Yagi.
Although the Arecibo group, callsign KP4AO, say the signal can be heard with a small hand-held Yagi pointed at the moon, Domenico I8CVS reckons a 15 dBi Yagi is required. It will be interesting to see what people achieve in practice. What is the smallest 432 MHz antenna that can produce a detectable signal from Arecibo? Domenico I8CVS has posted these calculations on the AMSAT bulletin board: The specifications of the Arecibo Observatory Amateur Radio Club for the 432 MHz Moon Bounce test on April 16-17 and 18 are as follows: Dish diameter: 1000 foot equivalent to 305 meters Antenna gain: 60 dBi Tx power: 400 W Tx Frequency: 432.045 MHz Since the given ERP is 243,902,443 Million Watts (see below) and since 60 dB is equivalent to 1000000 (one Million) time in power it comes out that the power reaching the feed of the dish is: 243902443 / 1000000 = 243 watt. The rest of the power 400-243 = 157 watt is lost in the feed line. At 432 MHz a dish with a diameter of 305 meters has a -3dB points main lobe angle equal to: Theta = Lambda / Diameter = 0.69 / 305 = 0.00227 rad. The above lobe of the dish at an average distance of 380000 km light up a small circular surface S over the moon wich diameter is: D = 380000 x 0.00227 = 865 km The surface area S = (3.14 x 865^2) / 4 = 5.88 x 10^11 square meters All the radiated power of 243 watt by the dish is now collected over the above S area. The reflectivity of the moon at 432 MHz is the 7% so that the power scattered back isotropically by the moon is ( 243/100 ) x 7 = 17 watt It is like to say that the power reflected back by the moon is 17 watt feeding an isoptropic antenna or 17 watt EIRP or +12 dBW EIRP radiated isotropically by the moon. Since the surface of the moon lighed up by the dish is less then the whole surface of the moon the usual calculation procedure for the EME link considering the isotropic attenuation earth-moon-earth cannot be used here because as seen by the Arecibo dish the diameter of the moon is smaller than in reality and is large only 865 km in diameter not 3476 km as is large in reality the moon. With this is mind we must imagine the dish of Arecibo to be an isotropic antenna with 17 watt applied to it and transmitting all around isotropically from the moon. My ground antenna has a gain G= 15 dBi and an antenna picked up noise of 70 kelvin when looking at the cold sky My receiving system at 432 MHz has an overall Noise Figure of 0.7 dB equivalent to 50 kelvin so that the noise floor KTB of my receiving system in SSB with a bandwidth of 2400 Hz is KTB=1,38 x 10^-23 x (50 + 70) x 2400=3.97x10^-18 watt= -174 dBW Link Budget Calculation: Isotropic power reflected by the moon...+12 dBW Isotropic attenuation for 380.000 Km.. - 197 dB ------- Power received on isotropic earth ant... - 185 dBW Ground antenna gain...............................+ 15 dBi ------- Power applied to ground receiver..........- 170 dBW Noise floor of ground receiver..............- 174 dBW ------- Received Signal to Noise ratio S/N.......+ 4 dB By the way when KP4AO will operate on CW I can switch on the 500 Hz filter on my receiver and here I will gain in sensitivity 2400/500 = 4.8 time and 10 log 4.8 = 6.8 dB so that I gain 4 + 6.8 = 10.8 dB of overall 10 Signal to Noise ratio If I can stake two 70 cm antennas with gain 15 dBi each I can gain about another 3 dB and I can improve the S/N ratio to 10.8 + 3 = 13.8 dB If I can stack four 70 cm antennas with gain 15 dBi each I can gain about another 3 dB and I can improve the S/N ratio up to 13.8 + 3 = 16.8 dB a real very strong signal on CW or 16.8 - 6.8 = 10 dB in SSB Signal to Noise ratio which is considered to be optimal for a comfortable reception in SSB By the way to work EME using a big dish having a lobe with an aperture angle "theta" smaller than the diameter subtended by the moon which is about 0.5 degrees i.e. 0.0087 radiants is useful only for the big dish to hear better those stations using smaller dishes but the big dish to be received better by the smaller one's "must" use more power and not increase the diameter of the dish because as soon as the moon is completely resolved the power scattered back isotropically do not increase increasing the diameter of the dish. In conclusion I believe that ground stations with an antenna gain of 15 dBi and a receiving system with an overall Noise Figure of about 1 dB can easily hear KP4AO on CW and barely in the noise on SSB. Stations with the same receiver overall Noise Figure and antenna gain from 18 to 21 dB can hear KP4AO on CW and SSB without problems. Stations using 100 watt or more and the above antennas ranging from 15 dBi to 21 dBi have chance to make QSO with KP4AO on CW. The above calculation shows that it is very difficult to hear KP4AO or be heard using a small hand-held Yagi pointed at the moon. Have fun 73 de I8CVS Domenico Arecibo on 432 MHz Moon Bounce April 16-18 http://www.southgatearc.org/news/april2010/ arecibo_432_moonbounce.htm |
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