Saturday, 9 August 2014

Listening to ISEE3


1.0 Background


A few days ago I made a parent-y type tweet, which was more a badly hidden humblebrag over my kid saying "dish" before "mama." *shrug*



That tweet led to at least three more technically aimed questions over how it was we were planning on listening to ISEE-3. So this post is an attempt to answer those, and cement my own thoughts. It's not a new method - see Section 2.3.2 of this post for links to other people doing exactly this. See Section 2.3.1 if you're going "WTF is ISEE-3?"


Major Disclaimer

I am not the mad engineer behind all of this, I am the cheerleader (and now PR person). The things reported in this post come from the brain of the eminently brilliant [redacted due to request for mild anonymity], who has been kind enough to let me share his life with him. Errors in this post are entirely my own.

The enthusiasm for the ISEE-3 mission is also my own.


2.0 The Device

2.1 General Layout


2.2 Equipment

2.2.1 The Dish

Not as cool as The Dish.
  • 90cm offset fed "Sat TV" dish. 
  • Mounted on our Skywatcher EQ-6 equatorial mount that we temporarily borrowed from the optical telescope. 
  • Mount is driven on both R.A. and declination axes, but this is not strictly necessary. The dish has an approximately 2.5° beamwidth, which should allow for several hours of "tracking" with a stationary mount. 

2.2.2 The Feed - LHCP Helical Antenna

  • Homebrewed 5-turn helix feed, centre frequency 2271 MHz. Wound in a left-hand circular polarisation (LHCP), which is converted to RHCP once reflected in the dish. Deodorant can wrapped in a toilet roll tube comes out pretty close to the right diameter for winding copper wire.
  • See the links in 2.3.2 or google "S-band helix feed" or particularly "AO-40 feed" for lots of designs that will work. Helicals are very broadband and forgiving in construction tolerances. 

2.2.3 Low Noise Amplifier (LNA)

  • LNA4ALL, 25 Euros with shipping. 
  • Approximately 11 dB gain at 2.3 GHz. Noise figure ≤ 1 dB. 
  • Mounted directly after feed. 
  • Powered by 2x 9V PP3 batteries in parallel for approx. 1200 mAh. Just for simplicity and noise considerations. 

2.2.4 Downconverter

  • Modified MMDS downconverter for 2.2 - 2.4 GHz, 25 USD with shipping. 
  • Modified to be directly supplied with 12V (12V, 7Ah lead-acid battery), to avoid use of the bias tee (2.2.4a). 
  • Approx. 40 dB gain. Noise figure 1.4 dB. 
  • Local oscillator at 1998 MHz, 200 - 400 MHz after downconversion. 

2.2.5 Software Defined Radio (SDR)

  • Cheapy (10 USD) R820T RTL-SDR dongle. 

2.2.6 Laptop (Laptop)

  • Windows 7 running SDRsharp (SDR#), with plugin for auto-doppler correction, or HDSDR. 
  • No need for FFT integration expected over next 2 - 3 weeks of reception [August 9]. 

2.2.7 Others

[Not shown in block diagram but might be inserted, as and when, depending on what happens when we plug everything together.]
  • Attenuator between downconverter and R820T dongle, in case 40 dB gain + 11 dB from LNA might swamp the tuner (although doubtful). 
  • FM-trap (i.e. broadcast band notch filter/band-stop or VHF notch filter if there are strong TV stations present). 
  • 1/4 wavelength impedance matcher between LNA and downconverter to match 50Ω to 75Ω. Will use a 1/4λ 62Ω microstrip line, etched on PCB material. Possibly not necessary if LMR-195 length is kept very short. Probably. 


2.3 Notes

Image credit 

2.3.1 ISEE-3


ISEE-3 stands for International Sun/Earth Explorer 3, which nods to the spacecraft's function when it wasn't named ICE (International Cometary Explorer). In a nutshell it is a spacecraft which was abandoned by NASA, which has been subsequently hijacked by crowdfunded private citizens operating out of an old McDonalds. I don't think it gets cooler than that. You should look through all these fabulous linkages:

At the time of writing, ISEE-3 was on track to flyby the moon later on August 10th. Its signal should remain fairly strong for home-brewers for the next month or so.


2.3.2 The work of others

  • (Mis)appropriating standard TV dishes for use in antenna setups is not new. Howard Long has a very decent write-up on the exact shennanigans described in this post here. 
  • UHF-SATCOM gives options for suitable LNAs, as well as some more detail on the helical antenna design, for the lower S-band frequencies. Also look at that site's image gallery and scroll down to "S-Band receiver systems." 
  • Twitter user @PA4DAN has gone and done all this himself already - so go look at his photos for a non-Visio version. 
If I've missed something that really should be in this list, lemme know in the comments.
Posted by at 1 comment:
Subscribe to: Posts (Atom)