2010-06-11
Non-Emissive Air-Band Receiver (Implementation & Use)
I threw this receiver together the day before I left for the United States for a months holiday. It is anything but physically elegant, I half expected it to be confiscated by TSA so I did not spend a lot of time on making it pretty.
The circuit is only superficially different to Charles Wenzel's original design. The major departure from Charles' implementation is the use of the main tank inductor as a loop antenna. A stiff piece of ECW is used to create a 50 mm diameter loop, which is both the receiver antenna and the tuned circuit providing the receiver selectivity. In this way it is somewhat similar to my VHF Wavemeter, but the detector circuit is an AF amplifier and the cross-section of the loop is larger to intercept more RF.
Construction centred around the physical bulk of the polyvaricon tuning capacitor, the loop antenna, and the 9 volt battery power supply. The small centre-off switch selects between bands. One band tunes 32.7 - 126 MHz, the other a narrow range of interest in the Air Band, 121 - 133 MHz. In the centre "off" position the loop acts as a short-range magnetic probe for RF with sensitivity increasing rapidly with frequency (the loop inductance is quite small) - in this mode the device is essentially Charles' Amazing All Band Receiver.
The receiver selectivity is so poor tuning can be quite imprecise. This is actually a benefit in practical use, allowing sloppy tuning while searching for signals. In practice the only signals you hear on board an aircraft are transmissions from the aircraft which hop around in frequency. I did on occasion hear other aircraft on the ground or the tower, but the signals were quite weak, the receiver is not very sensitive. That said, on the broad tuning band I can hear FM and TV stations quite clearly at home - a high signal area.
In-Flight Experience
On the international flight from Australia to the US I heard very little. Once over the pacific VHF is basically not used until shortly before landfall in the US. I did hear the push-back, taxi, and take-off clearance acknowledgements, and once in the air a few other transmissions before leaving Sydney's controlled airspace. Similarly on arrival in the US I heard the pilot contact the approach controller, get talked down through the busy LAX airspace and once landed heard the instructions to wait for towing to the gate. Sydney security didn't even bat an eyelid about the device, it just went straight through the carry on x-ray scanner. On the aircraft the crew and passengers ignored it, but I didn't exactly wave it around nor try to hide it either.
The domestic flight from LAX to Boston was more interesting. Over land the pilot is continously contacting different airspace controllers and moves around in frequency quite a lot. We had to fly around a massive storm system and deep into Canadian airspace, changing flight level many times, it is quite interesting hearing the pilot request course and level changes to avoid storms or take advantage of jet streams. It is an extra window on the whole flight experience. Once upon a time there was an in-flight entertainment channel for this, but AFAIK no airline has this option any longer? Generally tuning coarsely to the "top" and "bottom" ends of the narrow range was sufficient to keep up with the pilot, even if you didn't hear what frequency he was acknowledging the QSY to. If the signal became weak (indicating the TX frequency was at the other end of the band from where I was tuned) you just racked the polyvaricon to the other end of the band. Yep, the selectivity is that hopeless you can hear the transmissions (weakly) even if completely mis-tuned by 10 MHz. Once again security couldn't care less about the unit. I had purchased a small (Pelican 1030) hard-shell case for the device in LA, bright yellow like a Civil Defence radiation meter. One humorous moment was hearing the pilot argue with the tower about how much fuel he was wasting while delayed on take-off.
On the flight back from Boston to LAX TSA did flag the device. I aroused their suspicion somewhat because I had a 800 ml dewer of drinking water with me that I forgot to tip out until right at the check-point. I also suspect the staff at Boston are somewhat more on the ball since the September 11 hijackers took off from there. The TSA lady asked me a few questions, I said I was an amateur radio operator, etc. When asked why I built such a device I answered simply "I am a geek." Apparently a good answer, I was allowed to continue on my way with my receiver. The in-flight reception was a little poorer this time, some broadband RFI from the entertainment system was more audible and the air band signal was a bit weaker in the particular seat I was in - still quite usable though.
The international flight from LAX to SYD was quite similar to the initial flight. No security dramas and no traffic once out over the pacific. The periodic data-bursts I could hear on all flights was somewhat louder on this flight. I assume this is ACARS? Oddly enough the voice transmissions were much weaker, perhaps they use different antennas and I was near a node in the RF pattern inside the airframe that the voice TRX antenna excites?
So, in summary a bit of geeky fun and no security dramas despite a quite conspicuous looking, clearly home-made device.
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Parent article: Non-Emissive Airband Receiver.