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Re: Mode-B vs Mode-J



Although in principle I don't disagree with any of Bob's statements, I must protest that an omni antenna user CAN INDEED access mode J satellites reliably.  I have been for years.
It does *require* a good preamp on the U-band downlink.  I honestly believe that 99.9% of the "new user" issues on mode-J come from simply slapping on a whip antenna, hooking up the coax, and let's go...nowhere.

The other thing about mode-B (U-uplink) is the radar interference...it obviously is a significant impact on AO-40 operations at 60k km...what will it be to a small bird that's so much closer?  Or did AO-40 just land on an unfortunate sub-band on 70cm?

For reference, my setup:
V-band uplink...M2 HO loop at 8', 100W
U-band downlink...M2 HO loop at 8', coax cavity BPF, ARR GaAsFET preamp
Rig: FT-100
J-Sats worked:  FO-20, FO-29, AO-27, UO-14


> 
> From: Bob Bruninga <bruninga@usna.edu>
> Date: 2003/06/19 Thu AM 06:43:02 EDT
> CC: amsat-bb@AMSAT.Org
> Subject: [amsat-bb] 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
> 
> 
> 
> 
> ----
> Sent via amsat-bb@amsat.org. Opinions expressed are those of the author.
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> 

Scott Townley NX7U
Gilbert, AZ  DM43di

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