[Date Prev][Date Next][Thread Prev][Thread Next] - [Date Index][Thread Index][Author Index]

Re: 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
----
Sent via amsat-bb@amsat.org. Opinions expressed are those of the author.
Not an AMSAT member? Join now to support the amateur satellite program!
To unsubscribe, send "unsubscribe amsat-bb" to Majordomo@amsat.org



AMSAT Top AMSAT Home