VUSAT ( HAMSAT ) EXAMPLE PART 1About up and down frequencies of VUSat, HAMSAT. HAMSAT : downlink center 145.90 MHz, If these values are correct, we find the two passbands : 435 MHz : 435.220.000 - 435.280.000 ( uplink band
) T (1) = 581.15 INV transponder Some simple mathematical manipulations deliver the 'doppler shift compensated translation equations' ( INV transponder ) A1 and B1 ..... A1 RX=T-[TX+DS70]+DS02 DS02 is 2m doppler and DS70 70cm doppler value. An example : suppose somebody transmits on 435.25 MHz ( center band ) and the LEO doppler value ( on 70cm, DS70 ) for that moment is -8 kHz ( a LEO LOS situation ) : we will see the coming back signal ( downlink ) on the ( ground station ) antenna with a frequency : formula A1 RX=T-[TX+DS70]+DS02=581.15-[435.25+(-0.008)]+(-0.008/3)=145.90533etc. MHz. Because doppler values are changing less if existing LEO AOS/LOS situations ( overhead pass, also see http://www.qsl.net/vk3jed/doppler.html ), sometimes it could be convenient to make such a calculation as above. It is possible to construct a SATELLITE
MODE B FREQUENCY TRANSLATION
CHART, included doppler shift compensation if you
want ( see the small formula's I worked out ). Some very
good results ( examples ) you will find in THE SATELLITE
handbook ( ARRL edition ISBN 0-87259-658-3 4-3fig4.2
& 4-4fig4.3 ..... ). Do not think there is an error in the result ( example above ), the downlink frequency. Perhaps you expected a lower one ( satellite moving away ). Because of the combination of this transponder type ( mode B INV ) and the doppler effects the current result exists. Concerning a NON INV transponder, T ( f,T osc ) = 289.35 : A2 RX=[TX+DS70]-T+DS02 An example : suppose somebody transmits on 435.25 MHz ( center band ) and the LEO doppler value ( on 70cm, DS70 ) for that moment is -8 kHz ( a LEO LOS situation ) : we will see the coming back signal ( downlink ) on the ( ground station ) antenna with a frequency : formula A2 RX=[TX+DS70]-T+DS02=[435.25+(-0.008)]-289.35+(-0.008/3)=145.88933etc. MHz. Remark : both uplink frequencies are the same ( in example 1 and 2 ), center up, but the downlink signals are different. The signal, arriving at the transponder antenna, is not center anymore ( because of the doppler influences ). Both transponder types will only work the same if Fup=Fcenter. If ( after launch ) it appeared the frequencies are not quite correct ( because of a run away of the oscillator frequency ) it is easy to find a new factor T. And perhaps you can work with special equipments with very good tolerances. After measuring and analysing you can find the new factor T ( do not forget the doppler values on the 'measuring time' ! ). VUSAT ( HAMSAT ) EXAMPLE PART 2Some more calculations ( based on info from VUSat/AMSAT bulletins ) around the HAMSAT transponder. You also will find the used formulas in THE SATELLITE handbook, ARRL edition ISBN 0-87259-658-3 8-12 & 8-13. First the path loss, suppose the sat is direct above the ground station ( slant range minimal ). The 70cm ( uplink ) path loss is : 32.4+20logf+20logrho=32.4+20log(435.25)+20log(917)= And the 2m ( downlink ) one : 32.4+20logf+20logrho=32.4+20log(145.95)+20log(917)= Remark : a ca 10 dB difference ! Then the calculation of the sum of : the transponder power plus the satellite antenna gain plus the ground station antenna gain minus the ( already found ) path loss. Suppose a transponder TX power of 1000 mW SSB or 30 dBmW ( a very ideal situation ). The satellite downlink antenna gain is 16 dBi, presuming this is meaned in the informations ..... Besides, used in this calculation, a small groundstation antenna, having a gain of 10 dBi ( also a sperrtopf with a gain of 3 dBi hi ). The sum is : 30dBmW+16dBi+10dBi(3dBi)-135dB=-79dBmW(-86dBmW). Remark : because the satellite downlink antenna gain probably is not 16 dBi, using here a more usual value, for instance 5 dBi. The sum is : 30dBmW+5dBi+10dBi(3dBi)-135dB=-90dBmW(-97dBmW). We know already a lot now hi. Because it concerns a mode B transponder, a SWL will receive ( with the same 'hardware' ) the signal, coming from the satellite, in most cases, ca 10 dB better ( calculation path loss ) ! Besides, the satellite power could be on a higher level, comparing, for instance, the SO-50 ( transmits 140 mW ) and Hamsat. The value -90dB ( or -97dB ) is excellent. At the end the calculation of total received noise power : copied here a value, used in the satellite handbook, pages 8-12 and 8-13 : -138.5dBm, this is a usual one. The SNR ( Signal Noise Ratio ) will be : -90(-97)-(-138.5)=48.5(41.5)dB ..... Very good. HT antenna's will work. Another example ( more detailed, see the used formula's in path loss & snr calcs ). This time a transponder TX power is supposed of 250 mW
FM or ca 24 dBmW. The satellite downlink antenna gain in
the first example was 16 dBi, presuming this is meaned in
the informations. If yes, it is a very good one ! Now
choosing a value of 5 dBi. Below the calculations. Prs=24dBmW+5dBi+3dBi-135dB=-103dBmW. A very good result ! No problems about the received signal. Remark : the VUSat info also gives a received carrier power of ca -107 dBm ( dBmW ). It means for sure this value brings on a maximal transponder power output. The calculation antilog-107dBmW/10 ( in 50 ohm ) delivers a value of ca 1uV. Thus a transponder input ( signalstrenght ) of 1uV or better causes a maximal possible power output ( the actual and final power output depends on the number of active stations working at the same time over the transponder ). |