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.
The COVID-19 time offered a great opportunity to do a few good things. I revisited the crystal radio of my childhood days.
Yup, there is that old electronics saw that goes back to the thermionic tube/valve days,
“Amplifiers do, but oscillators don’t…”
In response to the tendancy of a circuit design to spontaneously oscillate.
(For those that do not know, if you connect the output of an amplifier to it’s input, there will be a path where at some frequency the total phase shift will be 360degrees and if that falls within the amplifer bandwidth it will “toot”. If you look at a single stage amp the active device will give 180degrees of phase shift, and the feedback circuit for gain setting or neutralisation will give another 180degrees at some frequency, if that is sufficiently within the active devices unity gain bandwidth then it won’t be “stable”. This is often seen with OpAmp circuits that are cascaded in some way and the LM386 can be viewed as an OpAmp with a power output stage…)
Would a RF transistor amplifier stage help?
Possibly but the problem is it would need some kind of rf filter most likely. Amplify it too much and it would be a huge mess of signals jumbled together.
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.
It was either Electronics Illustrated or Popular Electronics in maybe the 70s? I remember it as a simple crystal set tuned for vhf.
There was a circuit in an Elektor “sumner circuits” using just a single FET, that with a little modification would work on 76-108MHz, the problem being it was not designed for FM only AM envelope detectio, which is what is used in “air-band” comms. So the trick was to turn the circuit into a “Tuned Radio Frequency”(TRF) detector using the front end “tuned circuit” in an offset mode so the Wideband FM signal was on the “filter slope” so became AM and FM thus the AM component got “envelope detected” and you’ld hear the station (and sometimes several at the same time).
I built it as a kid! Great material.
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
So true. Especially if you use a Raspberry Pi. Then is is superiorer.
In fact if you use a Rapsberry Pi and some AI with a cloud connection and a blockchain it’s superiorerer.
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?
First, aviation radio is not FM so that answer fails.
” 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.”
Or you buy a $10 Radio Shack or whatever AM/FM radio and by tinkering with the box shaped components with the screw slot on top (might want to have a screwdriver handy) and bing bang boom Bob’s your father not your uncle and you’re basking in the mellow tones of ground to tower and tower to ground transmissions. You’re welcome.
Radio Shack stopped selling these well before they went out of business, but you could pick them up from any number of other places. A lot of “weather” radios also covered those bands too for obvious reasons.
“First, aviation radio is not FM so that answer fails”
When I wrote “There are small pocket receivers with air band that use headphones as antenna” I ment something like HRD767 which is a pocket FM/AM/airband. Would that work? Except more problems with tunning? I am asking seriously.
The root issue that gets you funny looks from airport personnel is that a superheterodyne receiver can splatter IF enough to cause interference with other signals/controls – in theory. Thus, someone somewhere made a big deal about it (but the people you run into the airport don’t know the details). All they know is “non-official radios bad.” The only time I’ve actually been stopped with one though, showing my little amateur license from the FCC was good enough.
Keep in mind that style of receiver is still a thing, I have a VX-7R in my car (introduced in 2002) and that’s designed around a superhet. So it’s still a valid concern.
The issue here is they know essentially nothing about the regulations or radios, so often enough anything electronic is assumed to be a problem regardless of antennas or the type of receiver.
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
Well that’s very cool! Thanks.
Now I have to figure out how to get Eagle 7.0 run on ARM MacOS..
send me an email or a message at the website and I’ll try to help
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.
Okay, but I’m just an old fart who follows the “No radios” rule on aircraft.
The rule isn’t “no radios” except to technically illiterate staff. You don’t have to get into arguments over it, but the “rule” only applies to certain types of devices, and very few receivers (certain heterodyne models). Even if you were transmitting only a handful of frequencies matter in any case.
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.
Re FM detection with an AM radio: slope detection.
https://en.m.wikipedia.org/wiki/Detector_(radio)#Slope_detection
This is essentially the non-digital version of the “Left ear Mozart, Right ear the Bible (Chinese)” Except it all ears all stations.
A ham friend of mine made one of those back in the 1980s.. I borrowed it a few times when I flew.. Worked really well to hear the transmissions from the pilots of our plane.
I would be tempted to add a filter, before the detector diode.
100pF and 10nH should cut off anything above about 150MHz.
I tried to take a fairly nice hand built prototype walkie talkie (handheld) radio battery charger in carry on a few years ago and the TSA did a full meltdown on it.
Total freakout. They thought a couple of large electrolytics were something else. Really didn’t like no labels or manufacturer branding.
Thought I was going to jail.
Took a little while but I made it on the plane. Bag flew separately.
Be careful bringing handmade circuits on a flight. Your mileage may vary.
Went to get on a plane, and my son had half a red candle with a great long bit of wick hanging out, in carry on. Looked exactly like cartoon dynamite.
Oh well, he has to learn what rubber gloves and truncheons are for sometime.
Why not use a jfet infinite impedance detector instead of the diode?
Such a thing might be too sensitive.
Dumb kinda related question. When I was a kid, my dad bought me a police scanner I could use to listen to ATC among other things from my house with (about 15 minutes from IAH airport straight as the crow flies)
Are those disallowed on planes? I don’t fly very often so I genuinely don’t know.
I lost mine in a move and have thought about looking for a new one in the past, but the digitalization of everything in recent years just made me decide to say to hell with it because I believed I probably would only get noise from digital devices rather than unencrypted analog RF.
Hearing that ATC is still analog and unencrypted now has my gears turning on what else is still analog.
It’s better to have a gain stage (transistor, BJT or JFET) before the detector for higher sensitivity. I griped why no schematic for the board and that comment was not accepted. Sigh.
The LM386 not sure if gain 20 or 200 or inbetween is being used.
I often wondered if something like this would be useful for listening to frequency hopping Comms as these are regarded as being quite secure. I know you would need close proximity etc.
As kids in boarding school in the late 1950’s, we would connect a germanium diode across the screw terminals of a high impedance WW2 headphone, with one end wired to our bedsprings, and the other to the heating pipes. This picked up the local BBC medium wave broadcast loud and clear.
Very nice and informative. Thank you 🙏
Is that anything like this old kit from Ramsey Electronics (RIP)? I had one at one point, but I seem to have lost it in a move…
https://web.archive.org/web/20150901024740/http://www.ramseyelectronics.com/Passive-Air-Band-Monitor-Kit/dp/B000E8ONGU?searchPage=1
This is essentially an aperiodic receiver (it’s a thing, look it up), and was quite common in analog counter-intelligence and signals intercept systems. When you wanted to monitor an entire band for an unknown emitter, filter out the weak (distant) signals, and record/DF on the strong (near) signals, it a the easy way to do it. Now there are better ways to do it, but the technique is still used.
The aperiodic receiver was also called a “crystal receiver” by US security services and was used as a “bug finder” and there is documentation as to it being used to find the “Great Seal Bug” or “Thing” designed by Leon Theremin and presented to the US ambassador, supposadly as a gift 9f friendship from Russian boy scouts.
It had an interesting front end attachment that was a tuned circuit, where the vanes of the variable capacitor could keep going round and round. Also connected to the shaft was an unstopped variable resistor and a geared down motor. The output of the variable resistor and the output of the crystal detector went to an oscilloscope in external timebase or “XY” mode.
It in effect made a crude spectrum analyser that just by changing the front end TRF coil (like was done on Grid Dip Oscillators) it could work frequency ranges in bands from 0.1Mhz to upto over 900Mhz.
The motor could be easily turned off and a manual knob used to “manually tune” it. By careful tuning the detector would detect both AM (envelope detection) and FM (slope detection). If you buy one of those pocket SA devices they have an AM output for “zero scan” and again by careful adjustment will do FM. And because they also have a “sweep oscillator” you can also get “carrier reinsertion) for SSB ISB and on theory “synchronous Direct Conversion”
Later detectors used two crystals to get better sensitivity and less loading on the tuned circuit.
Is this really out in the market and expect us low price and life time warranthy..