[amsat-bb] Mode J and B on Small Satellites

[Bob Bruninga <bruninga@xxxxxxxx>]

Another thing I observed at the AIAA/USU Small Satellite conference 
was the large number of student satellites (cubesats) that were going 
to use 2m uplinks and 70cm downlinks (mode J) in their micro-powered
designs.   When I asked one of them why, the response was because that is
the way the others did it (pacsats) etc...

My comments on mode J versus mode B (biased heavily by my desire to hear
satellites on an HT  and/or mobile) are as follows:

Mode J was a legacy of the pacsats where the lack of uplink Doppler was an
advantage for the early digital operations and where full-duplex downlinks
avoided any problems with 3rd harmonic desense.  But other than those
issues, Mode J might be a poor idea as it results in almost a 23 dB
difference between the uplink and downlink power budgets using omni
antennas on the ground.  This number comes from the fact that the 2m link
is 9 dB better (to an omni) and the ground user running 50W has 14 dB more
power than a typical satellite 1W transmitter.

In the case of trying to close the link to handhelds and mobiles, mode J
is not a good idea.  This is not to say, however, that mode J may be a
good idea if your satellite transmitter power is so low in the first place
as to require beam antennas on the ground anyway.  Then it will be easier
to track with a UHF beam instead of a 2m beam...

But with the popularity of handheld Satellite operations and the built-in
packet TNC's of the Kenwood HT and Mobile, it might be much better to
design with a 2m downlink if you want everyone to hear your bird... The
downlink will be about 9dB stronger to an HT for the same satellite power.  
Although this puts a bigger burden on the uplink, the 5W HT usually has
more power than the bird and can afford it.

An additional advantage of the 2m downlink is that it may in some small
way, alert the non-amateur interlopers that the 145.8 to 145.99 MHz
frequencies ARE IN USE.  With enough 2m downlinks maybe they will go
elsewhere...

Now that the 2m uplink on UO-40 may be never used, this does give an
opportunity to take a second look at the 2m satellite band,...  Of course,
a mitigating factor is the higher observed noise floor in urban areas in
the 2m band making it harder to hear such a 2m downlink...

Just thinking out-loud...

de WB4APR@amsat.org, Bob

ISS-APRS FAQ:       http://www.ew.usna.edu/~bruninga/iss-faq.html
PCsat Design        http://www.ew.usna.edu/~bruninga/pcsat.html
CUBESAT Designs     http://www.ew.usna.edu/~bruninga/cubesat.html
APRS LIVE pages     http://www.ew.usna.edu/~bruninga/aprs.html
APRS SATELLITES     http://www.ew.usna.edu/~bruninga/astars.html
MIM/Mic-E/Mic-Lite  http://www.toad.net/~wclement/bruninga/mic-lite.html

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