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Mode-B vs Mode-J



> Just about everyone down here found SO-35 in Mode B was by far the easier
> mode to work.  Uplink Doppler wasn't that much of a hassle, and those trying
> it with "marginal" antennas could use higher power to compensate (something
> the satellite can't economically do).  Also a lot less alligators...  One
> frustration here with UO-14 is the number of people who call CQ, but can't
> hear the downlink.  Mode B largely solves this problem.

Agree completely!

The link between a typical dual-band mobile rig with an OMNI antenna and a
typical mode J satellite transponder has almost a 22 dB link inbalance
between uplink and downlink.  (no wonder the first-timer doesn't hear it!)

The link between the same dual-band mobile rig and a mode B satellite
transponder is just about balanced.  (everyone hears it!)

Here's why (using OMNI antennas on both ends):

Mode J donwlink (UHF) is 9 dB worse due to link equation.
Mode J Downlink (UHF) user has greater coax loss on receive
Mode J donwlink (UHF) satellite transmitter may be only 1 W

Mode J user VHF uplink is 9 dB stonger at Satellite due to link eqn
Mode J user VHF uplink has less coax loss
Mode J user VHF uplink is using probably 50W

Thus, the typical Mode-J downlink performance is 20+ dB worse than the
uplink.  Thus you need beam antennas, skill, and good (not deaf) equipment
to hear it.  That is why you hear so many LID's calling CQ who cant hear
it.

Flip all these around for mode B and instead of having a 20+ dB inbalance
in the uplink and downlink, you end up with a pretty well balanced system
for the OMNI antenna user.  Everyone can hear the 1W satellite down link
10 dB better, and you wont hear so many LID's because now they have to
take a little more care on their uplink settings to be heard.

Mode-B is really quite a nice way to go.

Also, the original reason for Mode-J (Japan) was because it was impossible
to hear the 2m Mode-B downlink in Japan because all the truckers and
CB'ers in Japan were using 2 meter rigs for lots of illegal operations,
making the 145.8 to 145.9 band useless for downlnks.  But that was 20
years ago.  Now they have all migrated to Cellphones and other bands.

But these are just long term design issues.  I fully support ANY
transonder (Mode-J or B) that AMSAT choses to build..

Bob, WB4APR




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