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Power:

Since the power available from the solar cells will be a maximum of 15 Watts and an average of 6 watts the transponders must be designed for a minimum current drain for maximum RF power out.

The design goal is to obtain 1 Watt RF output power for 4 Watts DC input.

 

Frequency Choices:

Since we are limited to 1 watt EIRP by the available power, we must limit ourselves to 10 Metres, 2 Metres, or 70 CMs for the downlink.

 

The AMSAT-ZL committee therefore have decided that KiwiSat will have 2M down links and uplinks on 70 CMs and 23CMs.

 

A 2 Metre down link has a number of advantages.

1) 9dB greater field strength than 70 CMS.

2) It can use antennas that do not need to be deployed. 

3) Almost all current satellite users are equipped for this mode.

4) Access to a wider "non-satellite" community if F.M. can be used.

5) More efficient conversion of DC to RF.

6) A 70 CMs uplink less susceptible to interference from unlicensed transmissions than 2 M. 

 

The 70 CMs uplink / 2 M downlink is known as Mode U/V or Mode B and has

been used in many satellites over the years.

 

Linear Verses FM Transponders:

Linear advantages:

1) Linear mode allows more than 1 station to use the transponder at a time.

2) Little power is used when the transponder is not in use. 

3) Doppler shift is handled by the ground station.

4) Current satellite users are all ready equipped for this mode from AO10 and AO13. 

 

Linear disadvantages:

1) More complex ground stations than with F.M.

2) Strong uplink signals will activate the agc and prevent weaker stations from accessing the bird.

 

F.M. advantages:

1) Dual-band handhelds can be used with portable yagi antennas. 

2) New users can be recruited using their existing equipment.

3) Transponder easier to construct with readily available chip sets.

4) Transponder power is concentrated on one user.

 

F.M. disadvantages:

1) Only 1 station can use the bird at any time.

2) The strongest uplink signal will capture the bird.

3) Full transmitter power is used at all times (unless turned off).

4) Higher average power consumption than a linear transponder.

 

Bearing all this in mind the AMSAT-ZL committee have decided that KiwiSat will have both linear and FM transponders.

 

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Page last updated 22/07/2002