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Re: Mode-B vs Mode-J VERSUS Mode-S



> Now that I've learned about efficient CP patch feeds on prime focus 
> dishes, I have a monster downlink that cost me a fraction of what a 
> good sat yagi would (that "except for windloading" takes up much less 
> physical space) and I've learned more about antennas and RF on that 
> project than ANY other ham radio project I've ever built/worked on.

Agreed as far as it goes .. what is a patch, really, but a couple of 
pieces of sheet metal, a screw and standoff to hold them together, and 
a coax feed and maybe a CP shunt?  They're pretty demanding as far as 
getting the dimensions right, but if you get the pieces cut right and 
have some knowledge of how they go together, or can follow drawings, 
they're much easier to make than Yagis, no question.

But it seems to me that the question pertains more to the *average* op, 
or even the lowest common denominator.  AO-40 requires a station that 
can hear S-band extremely well, and doesn't require nearly so much 
top-notch performance on the uplink.  The average ham can cobble 
something together that will light up the U-band uplink S9 plus, but is 
likely to have trouble hearing the downlink .. and feedback loops being 
what they are, the already sufficient U band uplink will get cranked up 
until it IS audible on the downlink or LEILA goes off, whichever goes 
first -- either is about equally likely.  I may get flamed for this, 
but the fact of the matter is the *average* op is likely to do this at 
some point along the way to discovering that you really have to work 
extra hard to hear S-band well enough to get a decent power budget, 
either using a super-duper-low-noise downconverter or a BUD.  (nice 
article in this month's QST, btw)

If it were my call .. I'd want to move the downlink to something else.  
I can fully understand that it's desirable in some circles to have a 
certain level of technical expertise be the price of admission, because 
it's well worth having something that takes a little work to achieve, 
but the current setup on AO-40 isn't the best way to do that, because 
it doesn't filter *out* the ops that don't quite have the equipment up 
to speed -- like I said, most VHF/UHF all modes can hit the uplink with 
a modest beam antenna -- but rather encourages them to QRM and/or 
overload the U band AGC.

If you remember the original design (pre-propellant event) of P3D, its 
main mode was supposed to be U-band up and V-band down, and the current 
U/S mode was a workaround to use the remaining TX/RX equipment that 
still worked.  I don't think it was intended to be the sat's primary 
operating mode, and it's certainly not a mode I'd choose to "raise the 
bar" if I wanted to.  Easy to hit and hard to hear isn't the right way 
to go for that -- you want to go the other way.  I'd think it would be 
better to have an L-band or S-band uplink because that would require an 
upconverter at the feed to hit the uplink (you can transmit a low power 
IF from your rig and translate and amplify that at the feed, you know) 
and there'd be plenty of folks that could hear the downlink.  If you 
really want a sat that's for the l33t ops only, that would be the way 
to go.  Personally, I think such a sat has its place, but not as the 
*only* option in its orbital range.  Using the HF analogy, we could 
make some whole bands Extra only, and I'd still be able to talk on 
them, but the folks with Advanced and lower class tickets couldn't .. 
but you know how wrong that would be.  We have a HEO sat, such as it 
is, so if we're putting up other HEO sats, I'd like to see U-band up 
and L-band or V-band down with a relatively wide linear transponder 
passband at least for the next one .. after that we can look at an 
elite-ops-only sat that might have, say, an S-band uplink ..

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