Mode B U/V Transponders

One of the interesting aspects of a mode B ( U/V ) transponder is :

because of the 2m output a lot of starting radio amateurs ( novices ) can receive easily the signals, coming from the satellite transmitter. The path loss values ( dB ) and the doppler differences ( f ) are small.

On 70 these are larger. A receiving is more difficult. And perhaps the used TRX produces now also more rxnoise. Besides, the behaviour of 70cm waves is more critical, also concerning antennae : adjustments, tunings etcetera, and unwanted objects, causing reflections.

Moreover, the 2m transponder output power is sometimes higher ( can be more easy higher on these frequencies ). Comparing for instance, SO-50 * and HamSat, maximal ca 7 times ( ca 8.5 dB ), 140 mW or 21.46 dBmW versus 1000 mW or 30 dBmW, in theory of course, if only one station would work via the HamSat transponder.

The Unit dBmW ( or shortened : dBm ). This site contains a few small javascripts. Which have sufficient exactitude and carry out desired calculations ( for example concerning 50 Ohm impedances ). Click here or here. Or see : Java Applications - OnLine Calculations - Relative & Real Power or Voltage & Impedance REF .....

No QRM from VHF telephones because of the 70cm uplink !

Assume, another radioamateur, in the direct neighbourhood, is working over the satellite. Because of the use of mode B no desense filters are required because third harmonics do not disturb now the reception. The harmonics are not present in the rxband.

Now, it is not difficult to receive the stations, which work via 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 satellites have downlinks on 70, so here are no differences between ( downlink ) path loss.

* If you want to hear a short audio part, received from the SO-50 transmitter ( 140 mW, maximal 250 mW ), click here ..... The file concerns a receiving with help of a small antenna under roof and without preamp, of course the signal is weak now. The audio is only available if you visit the PA3GJE or PC1L website. For the moment it does not work via the I-MODE website.

Another important point is :

because of the higher required conditions on 70 it is necessary to make dopplershift corrections, to use rotator interfaces and more. A 70cm uplink is more difficult. UHF must be prepared carefully. Because of the above mentioned reasons the novice starts not at once certain transmissions over the satellite. First he will only listen. And more learn before he uses actively any satellite.

All this means more operating practice. The satellite traffic will become more correct.

And when the time is ripe to start a transmission via the satellite ( so, 70cm uplink mode B ), the absence of third harmonics in the rxband is an advantage. A desense filter is not needed.

Finally, if you want to see a few formulas concerning lineair mode B transponders, click one of the lower submenu buttons.