Transponder Frequency Translation

Finding the two passbands of a linear mode B transponder. This is possible if we know the downlink central frequency fc(dwn), the uplink central frequency fc(up) and the bandwidth B :


Also we can find the mode B calc factor T ( f,T osc ) :

T (1) = fc(dwn)+fc(up) ( INV transponder )
T (2) = fc(up)-fc(dwn) ( 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 .....

Concerning a NON INV transponder :

A2 RX=[TX+DS70]-T+DS02
B2 TX=[RX-DS02]+T-DS70

Because of the LIN MODE B NON INV function of this transponder we also can find :


If it concerns mode J :


If you want 2m doppler values ( DS02 ), replace 0.75 by 0.25 .....

And a LIN MODE B INV function delivers :


Perhaps you are able now to construct the mode J formula if necessary. You have to find :


And if you want 2m doppler values ( DS02 ) again, replace 1.5 by 0.5 .....

You find some calculations ( examples ) in : the VUSat transponder ( fourth submenu button ).