There are many ways to build a radio receiver, but most have a few things in common, such as oscillators, tuned circuits, detectors, mixers, and amplifiers. Put those together in the right order and you’ve got a receiver ready to tune in whatever you want to listen to. But if you don’t really care about tuning and want to hear everything all at once, that greatly simplifies the job and leaves you with something like this homebrew all-band receiver.
Granted, dispensing with everything but a detector and an audio amplifier will seriously limit any receiver’s capabilities. But that wasn’t really a design concern for [Ido Roseman], who was in search of a simple and unobtrusive way to monitor air traffic control conversations while flying. True, there are commercially available radios that tune the aviation bands, and there are plenty of software-defined radio (SDR) options, but air travel authorities and fellow travelers alike may take a dim view of an antenna sticking out of a pocket.
So [Ido] did a little digging and found a dead-simple circuit that can receive signals from the medium-wave bands up into the VHF range without regard for modulation. The basic circuit is a Schottky diode detector between an antenna and a high-gain audio amplifier driving high-impedance headphones; [Ido] built a variation that also has an LM386 amplifier stage to allow the use of regular earbuds, which along with a simple 3D-printed case aids in the receiver’s stealth.
With only a short piece of wire as an antenna, reception is limited to nearby powerful transmitters, but that makes it suitable for getting at least the pilot side of ATC conversations. It works surprisingly well — [Ido] included a few clips that are perfectly understandable, even if the receiver also captured things like cell phones chirping and what sounds like random sferics. It seems like a fun circuit to play with, although with our luck we’d probably not try to take it on a plane.
LM386 does this sorta on its own sometimes
Yeah, during one of the COVID lockdowns I built myself a Paesano (Pete Juliano design) and was thilled when I ‘buzz tested’ the LM386 amplifier and heard radio stations, they’re an interestingly bad chip.
Would a RF transistor amplifier stage help?
Forrest Mims had a similar circuit in his “Circuit Scrapbook” (The best Mims publication). Wenzel’s Techlib is a great resource for curious minds. I’ve been using his lightning detector design for ages. Nice to see others inspired by him too.
yeah – this project reminds me of something similar that I believe i saw decades ago (source forgotten; maybe Popular Electronics or similar), intended for listening to the ATC transmissions of the plane you’re on. It may have used a germanium diode and a nice beige crystal earphone (remember those??).
Anyway, nice to see the concept updated with a biased schottky diode.
Is this just basically an rtl-sdr?
no its just an R using your abbreviation, and recieve only at that. Closer to an untuneable foxhole radio from WWII
Not at all. An RTL-SDR is tunable and selective, far superior to rhis
I will master this one day. One day for sure…
” there are commercially available radios that tune the aviation bands, (…), but air travel authorities and fellow travelers alike may take a dim view of an antenna sticking out of a pocket.”
There are small pocket receivers with air band that use headphones as antenna. They don’t come with extra satisfaction of well executed design (which I really like), but I think it’s easier to explain “commercially available FM radio” than “it’s only receiver – I know because I built it myself” to airport security.
On the other hand I have never listened to air band – maybe those radios have air band only in user’s manual?
Please somebody do a jlcpb assembly service compliant zip file so we can order it mostly built :)
I designed a PCB for a version of this a few years ago. It was one of the first PCBs that I designed. It was before JLC offered PCB assembly, but could probably be pretty easily converted to that.
https://github.com/ademuri/all-band-receiver
Isn’t listening to “all frequencies” listening to static?
I recall the old movie; “The World, the Flesh, and the Devil”, where the main character (believing he may be the last remaining human) goes to a NYC radio station and starts broadcasting an ‘I am here’ message on “all frequencies”.
He’d be better off broadcasting at one frequency.
If all frequencies had the same power, then yes it would be just static. But here, the detector has such low gain that only the strongest frequencies get detected. In other words you’ll only hear the airplane’s transmitter and maybe some of the cellphones around you. This works in part because you’re 6 miles/10km from the surface and therefore in a relatively RF quiet environment. Doesn’t hurt that you’re also in a metal tube that is relatively continuous below you and providing some shielding.
Disclaimer: I am not a ham or RF engineer. I know just enough about RF to be dangerous.
In regards to antennas, why not use a headphone conductor to perform this task?
This is one of mods he is about to do.
Maybe bias the diode for better detection? Or perhaps use an op-amp to completely eliminate the forward voltage drop? You’ve got power there for the LM386 anyway, so why not use it to improve sensitivity?
The diode is biase with the 10 MegaOhm resistor
Don’t really want to improve the sensitivity. This works in part because it is insensitive and will only pick the relatively high signal strength from the aircraft radio; hence why you’ll only hear the airplane’s side of conversations. Increasing the sensitively would cause it to start picking up multiple frequencies or even bands until it was just static.
i’ve long wanted to play with an untuned radio like this. it seems neat to have a detector for the strongest / nearest rf source. i think i understand why AM signals are comprehensible through such a device. i’m not as clear on FM signals?? i guess it’s aliasing into the audible spectrum?
it took me a couple reads of the article before i understood why they want to hide it. at first, i thought it was a pilot who wanted to monitor more frequencies than just the one/two the real radio is tuned to. took a while to accept they’re a passenger :)
maybe i’m just dreaming but i thought airlines generally pipe the atc communications into one of the channels of the armrest headphone jacks?? is that even still a thing? i remember when the headphones used to be stethoscopes, and you could hold your ear up to the hole in the armrest if you didn’t want to buy the headphones.
This is essentially the non-digital version of the “Left ear Mozart, Right ear the Bible (Chinese)” Except it all ears all stations.