[amsat-bb] Source for 3" (76mm) pipe for dish mount?

Zach Leffke zleffke at vt.edu
Mon May 9 23:26:46 UTC 2016

Hi Scott,

DISCLAIMER:   Personally, I think its relevant because it has to do with 
increasing your skill in the art of radio and satellite communications.  
More specifically, I think the skills needed (building/finding LNAs, 
building antennas, modem development, tuning for doppler, link budgets, 
etc.) are directly related to the Amateur Satellite hobby even if your 
practicing 'out of band' relative to what folks might be used to on this 
list.  Case in point, AMSAT's nasabare.txt TLEs include spacecraft (such 
as the NOAA POES birds) that have no Amateur Radio equipment on board, 
but they do it because people tend to have overlapping hobbies. So I'm 
posting my response to the list, but if others disagree and prefer the 
conversation be moved off list I'm more than happy to comply.

I too have an interest in the GOES birds, inmarsat, and basically 
anything from airplanes (ADSB), to ships (AIS), to NWS high altitude 
balloons, to LEO birds, to MEO/HEO birds, to GEO birds. (basically 
'other' stuff that is not necessarily ham radio specific, but in the 
same vein).

Source for cheap-ish preamps/LNAS:  minicircuits.com
Basically for LNAs, I found that minicircuits.com has a number of 
'decent' low noise amplifiers that can be used for receiving these types 
of signals.  they usually cost anywhere from 30-75 bucks (usually around 
50) depending on what you are trying to accomplish and the individual 
specs for the band of interest, and are typically wideband enough to 
cover multiple non-contiguous chunks of spectrum (Like 1550 MHz signals 
and 1691 MHz signals in L-Band).  They also sell pretty decent (And 
cheapish, maybe $20) bandpass filters that may be of interest if you are 
in a high QRM environment.

For L-Band (Inmarsat, Iridium, etc) I made a bi-quad feed that covers 
Inmarsat and the SARSAT frequencies around 1550 MHz.  I have played 
around similarly with 0.75m to 1.2m primestar type offset antennas 
mainly for the GEO birds.  The biquad plus a 0.75m offset reflector plus 
minicircuits amp plus RTL-SDR plus GNU Radio was all that was needed to 
observe/capture recordings of the Inmarsat 'Fleet Safety Net' broadcast 
service (an open and public service, not private communications).  
Helicals are also good choices because they are simple to build and 
inherently wideband (forgiving of manufacturing error).

My most recent interest is in the NOAA POES SARSAT transponders (Same 
birds that carry the 137 MHz APT transmitters).  Distress beacons 
transmit around 406 MHz up to the birds (EPIRBs for boats, ELTs for 
Planes, PLBs for hikers).  They rebroadcast that down at L-Band.  They 
have a bent pipe transponder that simply rebroadcasts the beacon as well 
as a digital downlink that contains information about the beacons from 
the onboard processing systems.  Same transmitters/data processors exist 
on the GOES birds as well.  Also the system and the spec is VERY well 
documented and public (google COSPAS-SARSAT and look around for the 
"technical documents" section).  I'm trying to develop a graduate level 
(or 'really good undergraduate' level) student project (maybe in the SDR 
course as a class project?) to code up some open source out-of-tree 
modules for GNU Radio to do the full demodulation/FEC decoding/data 
presentation for this.

Similarly 1690-ish MHz is an interesting band because there are three 
'targets of interest' to me.  The NOAA POES (15, 17, 18) and NOAA GOES 
birds all transmit HRPT in this band.  Additionally, the NWS high 
altitude balloons that fly twice a day from the 50-ish (no sure about 
the count there, but theres a lot of 'em) locations around the country 
also operate close to this band.  So one antenna/preamp/RTL-SDR 
combination could potentially net you access to some GEO birds, LEO 
Birds, and HABs.  (We're fortunate in Blacksburg to have an NWS station 
about a quarter mile from my office at the airport that launches the 
balloons, others' mileage may vary on that one).

For both the NOAA balloons (more specifically Lockheed Martin - Sippican 
Radiosondes, LMS-6s) and the Inmarsat Fleet Safety Net, there are 
Windows decoders that give you free 30-day trial, then you have to pay.  
Again, I'm hoping to find a way to turn this into an SDR class project 
(or satcom, or summer REU program, or however I can pull it off) to have 
students gain experience developing modems for this.  My intent is to 
develop free and open source GNU Radio modules for these decoders (Train 
ATCS is also on the list, but not relevant to this discussion) and then 
post them on github for others to use with GNU Radio.

I'd be happy to swap notes with you (or anyone else on the list) 
concerning these systems.  I've done a lot of legwork compiling the 
various sources for the specs, frequencies, etc. for these systems and 
would be happy to send links/docs your way.  And again, even though I 
think its relevant I could understand if others on the list don't agree 
and I'd be happy to move the conversation off list if anyone objects.

Happy signal hunting!

-Zach, KJ4QLP

Research Associate
Ted & Karyn Hume Center for National Security & Technology
Virginia Polytechnic Institute & State University
Work Phone: 540-231-4174
Cell Phone: 540-808-6305

On 5/9/2016 5:54 PM, Scott wrote:
>>>> If you can supply photos of links of the actual dish we may be able 
>>>> to help further.
> ... I found a pic online that looks right; it's a 1m Primestar dish.  
> I cropped out just the dish - here is the link:
> https://www.dropbox.com/s/hk7dlqot0z78thb/1m-dish.jpg?dl=0
> ... I have previously been using an old round DirecTV dish with 
> various home-made and purchased antennas to receive Inmarsat and a few 
> other signals but obviously needed something larger to experiment 
> further.  Both Vivaldi and patch style antennas have worked well with 
> the DirecTV dish.  I'm not completely sure what I will do with this 1 
> meter dish, but I don't have any plans for tracking at this time.  A 
> stationary mount will be fine.
> Having already gained some familiarity with Inmarsat signals, that is 
> one option.  It may very well be beyond my reach, but I would like to 
> educate myself about what is available on the GOES-13 satellite as 
> well.  I even have a commercial FTA TV LNB to experiment with.  It 
> seems like the upcoming amateur satellites will require familiarity 
> with more gain & higher frequencies as well, so I have a lot to learn 
> there.
> LEO telemetry cubesats and the like are my main interest, but I like 
> the idea of having a dish to experiment with as well. Certainly 
> appreciate everyone's input!
> If satcom topics like Inmarsat & GOES-13 are not of interest on this 
> mailing list, by all means please feel free to contact me directly if 
> you have any pointers!
> -Scott
> ==========================
> If you are not using it in a fixed position, but for low earth orbit
> tracking on s-band, like I am, I can give more advice. I am using a
> 1.2 metre dish. In particular the modification for larger feeds,
> calibration due to offset feed and real world low earth orbit tracking
> problems.
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