[amsat-bb] Link budgets for upcoming HEO/GEO sats

Sat Apr 9 22:34:10 UTC 2016

Estimates suggest aiming for 8 watts of RF and a re-purposed
3-foot-diameter satellite TV dish to be able to get into the bird. It would
be nice if only 3 - 4 watts were enough to do the job, because there are
inexpensive 3 - 4 watt linear amplifiers designed for First Person Video on
radio-controlled "drone" aircraft like this:

http://www.ebay.com/itm/262249565979?_trksid=p2057872.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT

So, time will tell. It is possible that with a slower digital transmission
rate the bird might hear well enough for the lesser power.  I expect I will
try one of these low-power amps, and might even consider driving a pair of
them into an appropriate combiner to get twice the power if one by itself
does not make the grade. They are small and light, but would require
protection from weather. On the other hand, I should have 10 watts
available if I need it, once I perform these modification on a much bigger
and heavier surplus power amplifier:
  http://www.g4fre.com/avantekpa.htm
(In fact, I have two of these beasts awaiting modification. I think they
are much too large and heavy for my tripod-mounted rover station, but one
of them might be doable on a fixed station.)

It would be hard to equal the gain of a dish by using a helix. Once upon a
time, I thought that the helix was the greatest antenna in existence, just
make it longer and longer to get outrageous amounts of gain. But the more I
investigated the less excited I got about building a humongous helix
antenna. (In fact, at one point, I was dreaming about a 2x2 quad array of
helices of great length, but I never actually attempted to build one.) On
the other hand, I have been amazed by the ability of a scrap-heap satellite
TV dish (less than 3 feet in diameter) and a couple of watts of power to
make terrestrial 10 GHz SSB QSO's over distances of well over 100 Km, at
least when I stopped using a dish that was bent like a Pringles Potato
Chip, causing it to lose 10-20 db of its gain! I expect even greater things
when an un-damaged dish is pointed up into the sky, away from trees and
hills!

For the 10 GHz downlink, a possible low-cost scenario might work out to be
a direct-to-home satellite TV dish of approximately one meter in diameter,
and a PLL-based LNBF, which would mix the 10.5 GHz satellite signal down to
the 600 MHz or so range, and the  receiving that with some sort of SDR,
possibly even a RTL-SDR USB dongle, though perhaps a better SDR might be
required. The downlink is going to be DVB-S2x which is NOT the native
format of the inexpensive USB dongles. DVB-S2x requires a bit of
computation ability on the computer controlling the SDR, especially if the
VLSNR (Very Low Signal to Noise Ratio) extensions are needed. It is also
possible that we can find (and suitably modify) an inexpensive
direct-to-home satellite TV receiver. AMSAT also expects to have a familiar
USA company building complete ground station transceivers, but those will
likely command a bit of a (well-deserved) premium price. We are actively
pursuing a lot of different choices, so stay tuned!

>From what I have heard, the bird will be in a geosynchronous orbit, NOT a
geostationary orbit, so periodic dish re-positioning of some sort will be
necessary. Since the primary payload onto which we get to piggyback is
owned by a government agency that likes to keep secrets, an exact proposed
orbit is not widely discussed, if at all. So it is hard (for me, at least)
to give more specifics at this time. You won't need the
horizon-to-zenith-to-horizon pointing ability needed to chase a satellite
in Low Earth Orbit, but some positioning in both azimuth and elevation will
be needed. High precision will be more valuable than a wide range of
pointing directions. In other words, being able to tweak the position by a
degree at a time to fine-tune the pointing would be more helpful than being
able to move the dish by 30 or 40 degrees. I wish I could be more specific,
but I can't at this time. On the other hand, one of the other projects in
the works is the NASA-sponsored CubeQuest Challenge, and for that bird you
would need to be able to point at any part of the sky where you observe the
moon, since the package is going to be headed for lunar orbit! We are
trying to make these two birds compatible with the same ground station
equipment.

Sorry for the vagueness, but I hope it helps a little bit.
73 de W0JT/5, EL09vu

On Sat, Apr 9, 2016 at 12:42 PM, Jim Barbre <jbarbre at xmission.com> wrote:

> Hi all,
>
>     Given that people are currently working on groundstations for the
> upcoming HEO/GEO sats, I am in hopes that at least some of the following
> questions can be answered.
>
> What kind of power will be required on the 5GHz uplink?
>
> Does anyone currently make a linear amp or upconverter that can be mounted
> at the antenna for the 5GHz uplink?
>
> Will I be able to use a helix antenna on the uplink or will I be limited
> to using a dish?
>
> Same question with regards to antenna usage for 10GHz downlink.
>
> What size dish?
>
> With AO-10 and AO-13 I could point my antennas at the satellite and not
> have to move them for an hour or longer. These new sats will be using
> microwave frequencies. What kind of realistic expectation do I have of
> being able to point a dish at the satellite before having to tweak it?
> Otherwise said, will a rotator be absolutely necessary?
>
> Thanks for the info. Looking forward to working the next generation of
> HEO/GEO sats.
>
> 73
> Jim Barbre
> KB7YSY
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