MODE B U/V TRANSPONDERSOne of the interesting aspects of a mode B U/V transponder is : because of the 2m output a lot of radio amateurs ( beginners ) can receive the signals, coming from the satellite transmitter, easy. On 70 doppler effects and path loss cause higher values. And perhaps the used (t)rx delivers on 70 more RX noise ..... Besides, the behaviour of 70cm waves is more critical also concerning antenna's ( adjustments, tunings etcetera ) and unwanted objects ( causing reflections ). The 2m transponder output power could be higher now, comparing for instance, SO-50 * and HamSat ( will it ever be launched ? ), maximal ca 7 times ( ca 8.5 dB ), 140 mW or 21.46 dBmW versus 1000 mW or 30 dBmW, in theory ofcourse, if only one station ( hi hi ) should work via the HamSat transponder. No QRM from VHF telephones because of the 70cm uplink ! No desense filters are required, third harmonics in receiving systems do not exist if using mode B U/V ( suppose in the direct neighbourhood another radio amateur works over the satellite ! ). Now, it will be not difficult to receive stations working over the satellite. A very nice and easy way of 'making acquaintance with' a satellite ! * comparing SO-50 and OSCAR-E ( a splendid project ! ) even delivers ( in theory ) a much bigger factor, maximal ca 57 times ( ca 18 dB ), 140 mW * or 21.46 dBmW versus 8W or 39.03 dBmW. However, both have downlinks on 70, so here are no differences in ( downlink ) path loss. * If you want to hear a short audio part, received ( only with a small antenna under roof and without preamp, ofcourse the signal is weak now ) from the 140 mW ( max 250 mW ) SO-50 transmitter, click here ..... ( Only possible if visiting the site, for the present not working in i-mode ). Another important point is : because of the higher required conditions on 70 it is necessary to make dopplershift corrections, to use rotator interfaces and so on. A 70cm uplink is more difficult. UHF needs careful preparings. Because of all above the beginner will not try something at once via the satellite. First he is listening and learning ( we hope ). This all means there will be less distortion, the satellite traffic is more calm and convenient. And when the time is there to start a transmission via the sat ( 70cm uplink, mode B ), the absence of third harmonics in the RX area is an advantage, a desense filter is not needed. If you want to see a few formula's having to do with ( LIN ) mode ( B ) transponders, go back an click one of the lower submenu buttons. TRANSPONDER FREQUENCY TRANSLATIONSConsidering a ( LIN ) mode B transponder, if known : the downlink center frequency fc(dwn), the uplink center frequency fc(up) and the bandwidth B we can find the two passbands : upband:flowuhf=fc(up)-0.5B&fhighuhf=fc(up)+0.5B T (1) = fc(dwn)+fc(up) ( 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. DS02=DS70/3 and DS70=DS02•3 ..... Concerning a NON INV transponder : A2 RX=[TX+DS70]-T+DS02 Because of the LIN MODE B NON INV
function of this transponder we also can find : DS70=0.75[T-(TX-RX)]. If it concerns mode J : DS70=0.75[(TX-RX)-T]. If wanted 2m doppler values ( DS02 ), replace 0.75 by 0.25 ..... And a LIN MODE B INV function
delivers : DS70=1.5[T-(RX+TX)]. Perhaps you are able now to construct the mode J formula if necessary. You have to find : DS70=1.5[(RX+TX)-T]. And if wanted again 2m doppler values ( DS02 ), replace 1.5 by 0.5 ..... Some calculations ( examples ) you will find in : the VUSat transponder ( lowest submenu button ). PATH LOSS & SNRYou will find the ( here ) used
formula's in THE SATELLITE handbook, ARRL edition ISBN
0-87259-658-3 8-12 & 8-13. The used calculation
versions deliver rather good results ! First the path loss ( dB ). Path loss is : 32.4+20logf+20logrho. f is the used ( up or down ) frequency ( MHz ) and rho is the slant range ( km ). Second the power sum, ps ( dBmW ). Power sum is : actual transponder power ( dBmW ) plus satellite antenna gain ( dBi ) plus ground station antenna gain ( dBi ) minus path loss ( dB ). Remark : using dBi if comparing iso, for instance a 0.5 wave dipole in free space delivers ca 2.15 dBi, an antenna with a gain of 6 dBd has 8.15 dBi. Often leaving away d in dBd. Iso and the 0.5 wave dipol are standard references. Third the total received noise power, trnp ( dBmW ) and at the end the Signal Noise Ratio, SNR ( dB ), see THE SATELLITE handbook or below. Signal Noise Ratio is : ps minus trnp. Those who want to calculate all ( analysing the own situation ) : below the rest of the needed formula's. Calculation of total received noise : Ptrn ( trnp above ). Ptrn(mW)=k•Trs•B=>Ptrn(dBmW)=10log(k•Trs•B)=10logk+10logTrs+10logB. In this : k is the Boltzmann factor 1.38•10to(-20) in mW/Hz•K (K=273+C), Trs is the resulting system temperature in K, B is the bandwidth in Hz. Remark : 10logk delivers a -198.6 value. Calculation of resulting system temperature : Trs. Trs contains : the receive system noise ( a hardware result ), cosmic noise ( received by the antenna ) and earth radiated noise going into the lobs of the antenna. ( for instance earth radiated noise at a temperature -5C=268K ). Attention, Trs expressed in temperature, see below. Trs=Tr+Tsky. In this : Tr is the receive system temperature in K, Tsky is the temperature of the sky in K. Tsky is 'seen' by the antenna. Calculation of receive system temperature : Tr. Tr=Ta[10to(Ft/10)-1]. In this : Ta is the ambient temperature ( hardware ) in K, Ft is the system noise figure in dB, concerning the coax, preamp(s) and input(s) in the circuit. Figure 11.3 ( 11-2 ) in the satellite handbook will light up something more ..... Also very clear is : every module must be on the right place as you can see ! The part 10to(Ft/10) in the formula above is the system noise factor fT. Calculation of Signal Noise Ratio : SNR. SNR(dB)=Prs(dBmW)-Ptrn(dBmW). In this : Prs is the received signal power, the power sum, ps ( see far above ). Some calculations ( examples ) you will find in : the VUSat transponder ( lowest submenu button ). EXAMPLE PART 1About up and down frequencies of
VUSat, HAMSAT ( only as an example ! ). 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' ! ). 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 ). |