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Cheap satellite transponder

A low cost homebrew transponder styled after the Es'hailSat ham payload. S-band to X-Band

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This project was created on 05/26/2014 and last updated a month ago.

Description
So the Qatar Satellite Company is going to launch a communications satellite in 2016 to a geosynchronous orbit. Among the traditional payloads is going to be two Amateur radio payloads. These are two Linear transponders that input on 2.4GHz and output on 10.450GHz.
As the launch is still two years away I want a good test bed for testing my solutions for it.
Thus, I want to make my own transponder with somewhat similar specifications. The uplink is going to be around 2.4GHz and the downlink is going to be around 10.450GHz.
My plan is to use surplus parts as much as possible and keep the total cost of the system well under 200eur, preferably under 50eur.
In addition to just playing around and testing & demonstrating how satellite transponders work the design could be modified to be more light weight and flown on a UAV for a more realistic feel, or as a hobbyist SIGINT drone downlinking the 2.4GHz goodies on 10GHz to it's sinister masters.
Details

Like the description says, the idea is to make a simple/cheap transponder, or at least attempting to. Thankfully many of the parts this project is going to use can be re-used as is for other things. 

My aim is to do this as cheaply as possible and if possible in such way that no obscure surplus is needed for it to be cheap. 

So the receive antenna is going to be the humble cantenna. The receive preamp is going to either be some cheap MMIC or a homebrew deal using either BFP405 or BFU760f low cost bipolar transistors. 

The front end filter is going to be either a retuned surplus unit or cascaded SAW filters.

The first mixer is going to be some cheap mixer from my junkbox, possibly a MAX2671 because they are cheap. The 1st LO is going to either be a phaselocked surplus DRO osc illator or some low cost VCO. 

IF filtering to set the transponder passband is going to be a 608MHz TV saw filter or some homebrew LC lumped element deal.

Upconverting mixer is to be a GaAs mixer from a TVRO LNB. The LNB will also be the post-mixer filter, buffer, final amplifier and transmit antenna.

The really hard part is making a good microwave LO for upconverting the UHF band IF to 10.450GHz. It needs to be stable and it needs to be clean. Neither of these are cheap on the open market so I'll have to make my own. 

The exact IF frequency and bandwidth will be decided later on.

I'll make some pretty pictures soon enough to lay out the transponder block diagram. 

Components
  • 1 × BFP405 RF, IF, RFID, ZigBee Semiconductors and ICs / Transistors and FETs
  • 1 × Cavity filter - 2.4GHz filter, 2.4, GHz, microwave, pain-in-the-ass
  • 2 × coffee can coffee,can,antenna pup
  • 2 × SMA connector connector, SMA, microwave

Project logs
  • Image reject filters

    a month ago • 0 comments

    To deal with out-of-band interference and stuff at the image frequency (RF + LO in this case) I wanted to use a nice filter in front. The cheapo way of going about this is to use SAW or ceramic filters made for the 2.4GHz wifi band, these are plenty wide, usable and most importantly: Easy to get and cheap.

    Instead I went with the harder way and used surplus cavity filters from my junk box. These are the diplex filters from a scrapped 2.0GHz/2.2GHz Nokia QPSK data link. Most folks I have met say that retuning cavity filters with many poles to be annoyingly hard , with the difficulty rising exponentially with the added poles. These ware 8pole filters. But surprisingly it took me less than 30minutes and no pain at all to retune both sides of a filter with a VNA and I didn't know about the Dishal method back then. I just fiddled with the tuning screws and the logic which the they worked was easy to figure out. But then again, I had a VNA at my disposal. 

    No pictures of the tuning procedure. 

    But in my limited wisdom I didn't mark the filters in any way. I peaked one on the 2.2GHZ satcom band, and the other with one side on 2.3GHz slize of the 13cm ham band and the other side on the lower edge of the 2.4GHz ISM band, which overlaps the hamband. 

    I have been blessed with a Rhode&Schwarz CMU200 instrument, in addition to being a GSM/EDGE test set, it also has a signal generator that covers these bands in question. As the filters ware tuned I just had to sweep them and find the one that has the least loss on 2.3GHz. 

    I could have done it without it, sure. But I would have had to use a manually tuned VCO, an isolator after it and various buffer amps, a frequency counter ( mine is a 44eur ebay thing) that covers 2.3GHz and some sort of detector, in this case likely some random microwave diode pressed to do the job as I just want to see what filter passes the most signal, not exactly how many dBm it is. 

    Or that was the plan anyway. In practice I had to use the old diode detector because my CMD57 was not sensitive enough and fast enough at 2.4GHz.

    Here's the filter under test. Too bad that the markings don't mean anything anymore.

    Here the homebrew diode detector can be seen. Under the heat shrink and soldered directly to the SMA connector are two 100ohm resistors forming a 50ohm termination for the incomming signal, a microwave diode, a 100nF cap from the diode output to grounf for shunting RF away from the DC measurement signal. And that's it. 

    The filters:

    The frequency:

    Reading on the meter:

    And the measured center frequency marked on the case:

    In the following weeks I should again have access to vector network analyzers so retuning the filters closer to 2.4GHz should be doable. 

    These LED's designating the current function of the connectors on the CMU200 are a nice touch.

  • Test Cantennas

    a month ago • 0 comments

    Homebrew flux <3 -Fast edit/comment, not gona resize the pix at this point, It looked fine in the editor!

    My hombrew worked far better than anything else for soldering the SMA connectors to the cans.

    So I built two simple cantennas for 2.3GHz to act as test antennas for stuff down the road. I used the calculator here: http://www.changpuak.ch/electronics/cantenna.php to get the dimensions and then got to work.

    The SMA with the probe got a 4mm hole and the press-fit one got 5.5mm. The sondes ware cut to 33mm long. Returnloss measurement on a later date will reveal how well it went. 

    The connectors: 

    I just soldered the a piece of stripped housewire to the connector center pin with the aim of later on trimming it to exact length or adding tuning screws. 

    Here's the sondes. Especially the one with teflon is going to be fun, as I didn't really simulate anything. Which is not really a good way to do thing on microwaves. 

    I measured the SMA connectors with my calipers in advance of the drilling. 

    The soon to be antennas with the detailed notes on building them:

    To drill the holes without a backing would deform the thin cans, so I supported it with a piece of firewood with a suitable curve: 

    And the pilot holes are done!

    I sort of forgot to take any pictures prior to soldering in the connectors. I sanded around the full sized holes with some sand paper I found in my pile of sandpaper. 

    Now about the soldering. This what my first attempts looked like this with  my fluxpen and the flux in my solder (proper leaded 60/40 stuff!). The solder didn't stick and flowed inside the damn thing. 

    Frustrated I tried the other one. But instead of the flux pen I used my homebrew flux (IPA with as much as possible resin dissolved) with a twist, I light it on fire. unfortunately I was juggling various bits during that so I have no pictures about it currently. Maybe next time. But it sure did the trick! The solder flowed nicely around the connector and it was a joy to solder it in. 

    Encouraged, I did the same with the first one. It worked, but not as well. I suspect that my attempt at preheating the can with a small butane torch created some crap on top of the steel as scratching it up and applying the fireflux(tm) treatment made the solder stick. 

    I washed off the remains of the flux with some biltema electornics cleaner. It's nifty stuff, basicly a 500ml can of pressurised IPA, extremely handy. 

    So many projects, so little time. https://i.imgur.com/zDkAWYn.jpg

  • Progress!

    a month ago • 0 comments

    So while there has not bee any significant published progress here. Work behind the scenes is still going on. 

    The 2.3/2.4GHz RX antennas have been built, the RX image filter has been measured.

    This is what you get for not documenting things properly the first time! I tuned the damn things for <0.8dB loss in the pass band and forgot to mark what filter is on what freq. The things are former diplex filters from a 2.0/2.2GHz QPSK datalink. More on them later.

    The RF AGC has been changed to IF AGC due to it being easier, even if the IF is outside the BGA2031 speck it should still likely work. And at low IF frequencies I can use dualgate mosfets which I have. 

    The exact IF frequency and IF bandwidth are still open. I'd much prefer to use something like 200kHz wide, but all the commonly available filters for that bw are 10.7MHz ceramic units. Using a 10.7MHz IF would add even more stages to the system and increase it in complexity. So it is still possible that I'll just use a 608MHz SAW filter fro ma satellite TV receiver. But having the thing +5MHz wide puts additional strain on designing a very linear PA and running it far away from saturation. Neither of which are fun at 10GHz. 

    More talk later. 

    -2FTG

View all 4 project logs

Discussions

Adam Fabio wrote 3 months ago null point

Low cost satellite operations are key to moving ahead with the private space race! Thanks for entering The Hackaday Prize! I'm loving this so far - anything that gets me in contact with a satellite on the cheap is a win in my book! Keep the updates flowing!

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