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Notes on starting in S-band using a CalAmp 130215



Quoting Andrew Glasbrenner <glasbrenner@mindspring.com>:

> Glad to hear of your success Bruce! I'm thrilled to see one of these
> downconverters in use. Other than a retuning, what did you have to do to make
> it work for satellite? So often I hear the complaint that S band is too
> expensive for the ham on a budget, and we both know thats not true ;-).

I'm most grateful to the group in general, whose collective expertise makes it
possible for newcomers to do these sorts of things on a shoestring. I started
with no equipment for 2.4GHz: my FT-817 was all. I spent around $45.

To make it work, I followed the instructions Woody kindly provided and Roger
posted at http://www.themidwives.org/CalAmp. This entails replacing the xtal and
tuning the filter. I also built myself a super-cheap signal source using VE2AZ's
design, http://www3.sympatico.ca/b.zauhar/SigSourc/SigSourc.htm 

Some thoughts on this signal source. Mine ended up 20khz high once it was
working on the proper harmonic. If one listens to the signal at, say, 440 MHz,
and checks how off the signal is there, it will give a good idea as to how far
off the 2.4GHz signal will be, and in what direction. At this high a harmonic,
this oscillator has quite a bit of start-up wandering. You'd be smart to put it
in a container, but I haven't :-) I didn't need to attach an antenna to the osc.
or the d/c to hear a CW signal from the unit. As long as N connector on the d/c
and the oscillator 'antenna' lead were close to each other (< 1cm), I could hear
the signal eve before tuning the comb filter. Of course, if you're taking this
approach, there's no point in fiddling with the xtal trimmer cap.: the signal
source probably has a wider range of error than the trimmer, anyway. That said,
my rig ended up in the doppler ball-park. Perhaps I was lucky.

Here are some random thoughts on KJ4SO's notes, written from the perspective of
a old hand at HF homebrew who's cracking open microwave equipment for the first
time:

1. I found solder wick worked much better than a solder-sucker in getting all
the solder off the built-in connectors.

2. The connectors are, in fact, built-in. Don't try turning the hex head on the
outside of the case to make the connectors recede. I just ended up with a
messy-looking F-connector.

3. I thought I'd be clever and keep the comb filter soldered to the pcb while
doing the xtal replacement. Bad idea: the weight of the filter partially bent it
off, and I was lucky it didn't do any damage. Listen to Woody on this.

4. I, too, had to Dremel out the holes for the xtal leads to fit into the pcb.
Others might want to stipulate with ICM that this job requires fine leads.
However, there are a couple of pads around this part of the pcb, so even if a
pad lifts (as it did for me), you'll probably be ok.

5. I found I could test for the xtal frequency (on 80MHz) and hear 2.4 Gig
signals using 13.8v input. So if, like me, you don't have a source for the
higher voltage specified, you can nevertheless start on-bench testing with your
usual 12v. I'm now using a 18v transformer that rose from my junk box like
Excalibur, but I'll be testing to see if I can return to the 13.8 source later.

6. Tuning appears to be largely a matter of screwing inwards. Now, I'm using my
FT817 and this funky signal source, which swings around with every drought in my
house, so take my words with a grain of salt. It seemed that most were not very
'peaky': 1/8 of a turn made little difference. On the whole, it seemed that the
sweet spot was just a bit before the screw bottomed out, at which point the
signal would certainly drop to nil. I took the advice given and smoothed out the
bottoms of the screws.

I built a bias tee, too. Since the gain is so high at this stage, I didn't worry
about anything pretty: just an inductor in the lead and a capacitor across the
connectors.

Finally, I bought a N-male bulkhead connector and built a K3TZ patch antenna
around it. This connector is the kind with the 1/8" brass post and the teflon
insulation around it. I pared the insulation down so that there was none above
the plate of the connector, then I used a Dremel cut-off wheel to trim the post
to the height required by the separation between the two elements of the patch
antenna, taking into account the thickness of the material being used. Because
of the size of the post, I drilled a larger-than-specified hole in the patch
element, and I didn't do such an elegant job of soldering the plate to the post.

For all these compromises, it all works. Other hams like me, who have only been
on U/V FM birds, owe it to themselves to try the S-band in LEO. It is amazing
how small one's antenna can be and still receive the bird. Of course, on can
omit any of the steps above by purchasing the d/c, bias tee and/or antenna. I'd
say the patch antennas have the greatest mark-up; that's the one I'd build
myself in any case.

> I can assure you the S transmitter stays on the entire time the mode runs.
> The PL is only capable of gating the audio to the S tray, not switching the
> transmitter. Usually L/S is run without tone access, so when no one is
> transmitting up to the sat, the downlink is just repeated static...sort of
> hard to differentiate from normal noise. If the PL access is on (and I don't
> think it is) you'll only hear a carrier with no audio between transmissions.
> I can't explain really why there was no signal when no one was talking
> though.

Is it possible that the discriminator in my FM rig can not detect a signal in
the transmitted static and hence the meter dropped to nothing between
transmissions? I've observed this twice since, and think it is important for
folks to know if they're testing out a new setup. On 9600 baud downlinks, my
handheld (a THD7A) 'hears' something much like static, but the signal meter
jumps, letting me know that I've picked up the downlink (this even when the
internal tnc is not on). With the s-band from AO-51, I can't tell there is a
signal there until someone transmits.

> With SSETI almost literally sitting on the pad (or actually in the hole!) all
> us S band Junkies are about to get a "daily driver" to play with.

That's been a large part of my motivation. I note on the comprehensive 
Übersicht aller Amateurfunksatelliten
http://www.amsat-dl.org/journal/adlj-tab.htm (which can be read by anyone as
long as they *pretend* they can read German,) that the SATEDU project is
planning a U/S transponder, too. The K3TZ patch purports to be LHCP, however,
opposite of SSETI's patches. I'll have to build a RHCP one, or perhaps a helix
instead, before it gets switched to transponder mode. Alas, there's no cheap
route to a 38.4kb/s modem (PacComm and Symek are it, right?)

73, VE9QRP
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