VUSAT ( HAMSAT ) Example Part 1

Concerning the up and down frequencies of VUSat, HAMSAT.

HAMSAT : downlink centrally : 145.90 MHz,
uplink centrally : 435.25 MHz. Bandwidth 0.060 MHz.

If the values are correct, we find these two passbands :

435 MHz : 435.220.000 - 435.280.000 ( uplink band )
145 MHz : 145.870.000 - 145.930.000 ( downlink
band )

and a mode B calc factor T ( f,T osc ) :

T (1) = 581.15 INV transponder
T (2) = 289.35 NON INV transponder

Some simple mathematical manipulations deliver the 'doppler shift compensated translation equations' ( INV transponder ) A1 and B1 .....

A1 RX=T-[TX+DS70]+DS02
B1 TX=T-[RX-DS02]-DS70

DS02 is 2m doppler and DS70 70cm doppler value.
DS02=DS70/3 and DS70=DS02•3 .....

An example : suppose there is a transmission to the satellite on 435.25 MHz ( centrally, the middle of the band ). The LEO doppler value ( on 70cm, DS70 ) for that moment is -8 kHz ( a LEO LOS situation ). The coming back signal ( downlink ) on the ( ground station ) antenna has now a frequency :

formula A1 RX=T-[TX+DS70]+DS02=581.15-[435.25+(-0.008)]+(-0.008/3)=145.90533etc. MHz.

Doppler values concerning LEO satellites are changing less if AOS or LOS situations exist ( concerning overhead passes, also see ). Therefore 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 formulas I worked out. Some very good results ( examples ) : see THE SATELLITE handbook ( ARRL edition ISBN 0-87259-658-3 4-3fig4.2 & 4-4fig4.3 ..... ).

Do not think that an error in the result ( example above, the downlink frequency ) exists. Perhaps you expected a much lower value because the satellite was moving away. Because of the combination concerning 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
B2 TX=[RX-DS02]+T-DS70

An example : suppose someone transmits on 435.25 MHz ( centrally, the middle of the band ) and the LEO doppler value ( on 70cm, DS70 ) for that moment is -8 kHz ( a LEO LOS situation ). The coming back signal ( downlink ) on the ( ground station ) antenna has now 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 ), centrally, in the middle of the band, but the downlink signals are different. The signal, arriving at the transponder antenna, is not centrally anymore ( because of the doppler influences ! ). The signal is not anymore existing in the middle of the band. Both transponder types give exclusively the same result if Fup=Fcenter.

If, after the launch, now the frequencies are not quite correct ( because of 'drift' of the oscillatorfrequency ), is it easy to find a new T-factor. Perhaps you work with special equipments with very good tolerances. After measuring and analysing you can find the new T-factor. Do not forget the doppler values which act during the measurements !

VUSAT ( HAMSAT ) Example Part 2

Here some more calculations ( based on VUSat/AMSAT bulletins ) around the HAMSAT transponder. You will find also the used formulas in THE SATELLITE handbook, ARRL edition ISBN 0-87259-658-3 8-12 & 8-13.
First we calculate the path loss : suppose the satellite is directly above the ground station ( so now the slant range is minimal ). The 70cm ( uplink ) path loss is :

And the 2m ( downlink ) one : 32.4+20logf+20logrho=32.4+20log(145.95)+20log(917)=

Note : in this site you can find a small script ( JAVA ) which calculates the formula above. To see : click here

This is a difference of approximately 10 dB ! 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. We assume that this is meant in the concerning information. Besides, in this calculation we use both a small groundstation antenna, and a sperrtopf. The first antenna has a gain of 10 dBi. And the this way called sperrtop has 3 dBi. The sum is :


Remark : because the satellite downlink antenna gain probably is not 16 dBi, we use here another, average value, for instance 5 dBi. The sum is :


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' antennae will function fine. Another example ( more detailed, see the used formula's in path loss & snr calcs ) : this time a transponder of which the TX power has been assumed approximately 250 mW FM or ca 24 dBmW. The satellite downlink antenna gain in the first example was 16 dBi. We again assume that this was meant in the used informations. If yes, it is a very good one ! Now we choose a value of 5 dBi.

And we use, in this calculation, a very small groundstation antenna, having a gain of 3 dBi ( half wave sperrtopf ). The temperature, seen by this antenna, is for instance 155 K, in most of the cases it a lower value will be. Bandwidth is 6 kHz ( TRX A 510 E, NBFM selectivity ). Suppose, that all equipment parts have been correctly established ( lined up in a very correct way, also see satellite handbook 11.3 11-2 ! ). The system noise figure might be circa 2.5 dB. We found already the path loss. It was ca 135 dB. Room temperature is 21C. You find below the further calculations.


A very good result ! No problems concerning the received signal. Remark : the VUSat info gives also a received carrier power of ca -107 dBm ( dBmW ). It means for sure that this value delivers a maximal transponder power output. The calculation antilog-107dBmW/10 ( in 50 ohm ) delivers a value of circa 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 !

Note : in this site you can find a script ( JAVA ) which calculates all the formulas above. Click here