QRP on the birds


From: F. Kevin Feeney (fkf1@cornell.edu)
Date: Fri Jun 04 1993 - 05:28:29 EDT

Hi Vikki,

Glad my blatherings were encouraging. Here's some info for you.

AO13 is an inverting transponder. The frequency profile runs like this...

Uplink Downlink

435.425 145.975 top of passband

435.505 145.895 mid passband

435.575 145.825 bottom of passband

The recommended EIRP was .5-1.0Kw to be heard, but experience has shown
that that puts up a fairly stiff signal. If I run 100 watts to a 14 dbi
antenna, I can gain compress the satellite transponder at good squint
angles on voice peaks or CW.

CW and digital runs mostly from the bottom up to about 880 or so. Voice
above that. I think the slowscan people are hanging out on 960. Usually a
semi-dxer group on 890 and Rex and crew on 940.

Oh the beacon is 812 and runs 40 baud ascii stuff interspersed with 10 wpm
cw and 45 baud rtty.

Squint angles - Oscars 13 and 10 have gain antennas with a certain
beamwidth that are oriented off the bottom of the satellite. The Mode S
(2.4 Ghz) and Mode L (1.2 Ghz) antennas have the highest gain, and
narrowest beamwidth. Mode B is probably the broadest, with a good footprint
of about 20 degrees beamwidth. The squint angle is a combination of the
angle the satellite is making with the earth at the moment, and your offset
from the center of that footprint. Ummm, how to explain this...

Ok, AO13 has an elliptical orbit with an apogee of 36000 Km and a perigee
of about 1200 Km. Divide the orbit into 256 parts, starting at 0 at
perigee. Apogee is 128, and at 255 you're back next to where you start.

The satellites attitude would ideally be a Blon/Blat of 180/0 which would
point the antennas at the center of the earth at apogee (directly at the
subsattellite point) and would be off a little on rise and fall. In
practice they often tilt the sattellite so that the antenna is much better
during the latter part of the orbit to get the required sun on the solar
cells. That's usually a winter time thing.

(If you have instant track, you can go into the orbital profile mode and
see this all really well. Much better than I am explaining it )

Anyway, what it means is that even if the satellite is right overhead, it
might be pointing at england and you might be out of the prime 20 degree
beamwidth footprint. Mostly that means less gain, but it can also mean
reversed polarization (LHCP instead of RHCP as in the main lobe). If you
are using linear antennas, by about 30 degrees of squint you can hear the
spin modulation. By 40-50 degree's it's pretty unusable. (with linear

Oh yeah, they also schedule the prime pointing time for those higher gain
modes like S and L (though L is broken at this point - don't know if it's
permanent or not) so mode B always takes a back seat, but we still get some
good time and its usable for hours at a time.

Typically I find about 3-4 hours of the pass in the summer, when the
antennas point pretty much straight down, that you can use it with little
antenna pointing and linear antennas. (you lose 3db by not being CP, so
you build your simple linear antenna 3db longer). Lots/most of the european
stations use linear antennas on it - CP seems to be more of an American
thing. I've used both, they both work, CP is nice if you can manage it but
not a necessity unless you want to use the bad squint angles.

Mode B is usually scheduled for about 9.5 hours of the 11 hour cycle,
through perigee (where for about 40 MA units (of the 256) they switch to
omni antennas for broadest coverage since the losses are lower. Modes J, L
and S are scheduled through the best pointing angles and usually take about
1.5 hours. They move the schedule around some.

Now to make a liar out of me, I'll copy in the last schedule as I got it.

>L QST *** AO-13 TRANSPONDER SCHEDULE *** 1993 May 10 - May 31
>Mode-B : MA 0 to MA 130 ! Omnis MA 250 - MA 60
>Mode-BS : MA 130 to MA 180 !<- S transponder; B trsp. is ON
>Mode-S : MA 180 to MA 190 !<- S transponder; B trsp. is OFF
>Mode-LS : MA 190 to MA 195 !<- S beacon + L transponder
>Mode-JL : MA 195 to MA 210 ! Blon/Blat 210/0
>Mode-B : MA 210 to MA 256 ! Move to attitude 120/0, May 31
>Please don't uplink to Mode-B during MA 180-190. Doing so will interfere
>with Mode-S operations. Mode-S will be ON for nearly 3 hours, from MA 130
>to MA 195. New Mode-S stations appear daily. During MA 130-180, Mode-S
>stations will have to endure the coupling from the Mode-B users operating
>between the downlink passband between 145.880-145.920 MHz. Either work
>between them, use them as test signals, or go to cross-band operations.

I see that they've expanded mode S to 3 hours vs the 1.5 hour - I told you
they moved it around. :-)

From what you said of your station, you should have a good shot at working
it with at least CW. I think you said you could make 10 watts on 435 and
had a receive preamp for 2 meters. get some 1 by 2 and welding rod and
make a quagi for 435 and use the shortest, best coax to get it to the rig.
My satellite antennas are on a RS tripod with a 10 foot mast right outside
the window with a coax run of less than 25 feet of 9913. Then build a quad
or quagi for 2 meters (quad is shorter and not as wide) and put the preamp
right at the antenna. If you can swing your current mast, just put a
crosspiece on it that can tilt up (I made the one we use for field day with
a door hinge) and arrange it so the weight of the antennas tilts it forward
to the horizon. Then attach a tilting arm and some string to pull it up to
the required elevation. I made an elevation meter with a school protractor
and a string and nut.

I think you could work the satellite with a fixed elevation of 30, but for
a lot of the good apogee time you probably want to be higher. If you can
get to the antenna to make a manual elevation adjustment, I think it would
be worth it. If not, you might want to pick a fixed point higher, like
60-70 degrees rather than 30 which is good for LEO birds. Your going to
want to work 13 when it's near apogee with good angles.

I think you asked about some other points, but I don't have your mail in
front of me. I'll end this one here and see if it helps at all. Tell me
more about your 70cm/2m gear.

Kevin WB2EMS

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