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Re: FM bird logjam



Emily,
I think the big advantage is that with the lower noise floor and a smaller 
size gain antennas on 70 cm less satellite power is required with mode J 
than  mode B. The problem is in Southern California where there is lots of 
Radar  interference on 70 cm, is received from the Military, but that is 
tempered with intermod on 2M from the commercial moble service band.
Art,
KC6UQH
----- Original Message ----- 
From: "Emily Clarke" <emily@clarke-design.com>
To: <brobertson@mta.ca>
Cc: <amsat-bb@amsat.org>
Sent: Monday, January 02, 2006 1:21 PM
Subject: Re: [amsat-bb] FM bird logjam


> At 11:12 AM 1/2/2006, Bruce wrote:
>>I've been on the SSB sats only for two months or so, and this is one of 
>>the
>>counterintuitive results I've observed. Because of their reputation for
>>being 'intermediate' birds, I assumed that communications through FO-29,
>>VO-52 and AO-7 would require more power / better antennas than what I was
>>used to with AO-51. In fact, FO-29 is a far better DX bird for me: with 
>>the
>>same antennas and the same 5w out, FO-29 can be worked lower to the 
>>horizon
>>on SSB (not to mention CW). It is my impression that VO-52 and AO-7 would
>>like more power on the uplink, and I'm presently without a preamp for 2m,
>>but I've had great fun on CW using these, and can certainly hear both 
>>quite
>>well. I suspect this is partly because of the mode involved. FM appears to
>>require a minimum signal to provide any intelligence at all.
>
> Single Sideband (SSB) is very efficient compared to almost any other form 
> of modulation.  Both FM and AM transmit a carrier wave that is modulated 
> by either frequency changes or amplitude changes.  SSB not only suppresses 
> the carrier wave but removes one of the sideband lobes (since they are 
> mirror images of each other.)  So the efficient use of power is a big plus 
> for SSB and this efficiency is shared by both your transmitter and the 
> satellite's, which doesn't need additional power to duplicate the carrier 
> or the redundant sideband.
>
> There is nothing inherently different using Mode B (U/V) versus Mode J 
> (V/U) however there are a few things that can account for the impression 
> that they need more power.  AO-7 is nearly twice as high as other 
> satellites (so the path loss is greater) but VO-52 is lower.  AO-7 in Mode 
> B has almost twice as much power as VO-52, but less in Mode C (same 
> frequencies as Mode B - just lower power).
>
> Here are some other things that are different between Mode B and Mode J 
> that affect me personally.
>
> 1) The Icom 910 puts out 25 percent less power on UHF than VHF, so if I 
> want to put out the same power (say 3 watts - the lowest setting on 2M) on 
> AO-7 that I put out on FO-29 I have to increase the RF power setting. 
> This isn't "cranking up the power" - it's just moving it up to the same 
> level 3W level.
> 2) The LMR-400 I use between my rig and antennas has more loss on UHF than 
> VHF, so I am losing more power on UHF to get the signal up to the antenna. 
> It's probably moot since my 70cm antenna has more gain.
> 3) I have a lot of trees in the way.  At 70cm these provide some 
> attenuation that they do not at 2m.  I'm restricted to using a vertically 
> polarized UHF beam, which unfortunately is the worse polarization. The 
> bulk of the mass of trees in my area (redwoods, Monterey pines and 
> eucalyptus) is also vertical.
>
> One other note - it's probably personal preference but I really enjoy the 
> tone of an SSB transmission more than FM.
>
> 73,
>
> Emily
> ----
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