The chances are that many of you will have made an FM “bug” style transmitter, a simple one-transistor oscillator usually driven by a small electret microphone. It’s also relatively straightforward to do the same for AM, and if you take a look through AliExpress you’ll find some modules which do just that. [Doz Television Workshop] has one, and he’s treated us to a thorough run-down of its design before addressing some of its shortcomings.
An AM transmitter is simple enough, in this case an oscillator and buffer driving a class C power amplifier. The modulation is applied by a transistor in series with the power amp, driven from an audio amplifier. Some attention has gone into the design of this one, with a proper output filter and plenty of room for tweaking to achieve proper levels and modulation density. There are some problems though — The modulator transistor is mounted upside down for the heatsink, and the frequency stability leaves something to be desired. [Doz] fixes the heatsink mounting and incorporates a DDS frequency synthesizer with an Arduino for control.
More after the break…
The resulting transmitter is better, but there’s still a problem. The limitations of AM broadcasting demanded both limiting and pre-emphasis, which he applies in software through one of the more powerful Teensy boards. We have to admit we’d have tried to do the job the analogue way, but that’s merely preference.
This board looks to be a good solution for an AM radio collector wishing to use their sets in an age of declining AM transmission. It should be legal under Part 15 for Americans, but as he points out it’s not for Brits. We suspect such a low-powered device wouldn’t attract adverse attention though. The video is below the break.
If you don’t need so much quality, it’s possible to do the job in a much less elegant manner.
I never built one of these because where I live you’d need a Amateur Radio license to be allowed to do so. You’d also need that license to attach a proper antenna to a low power device that just had a piece of wire hanging out it’s side. And the local radio amateur community, when asked about how to do any of these things online wasn’t very friendly or helpful to outsiders.
So of course I still fiddled with that antenna, but the “you don’t even get to ask about that because you don’t have the fancy papers I do” kinda put me off ever wanting to get my license. Kind of a bummer. Today there is much more helpful tutorials around for things one may not “technically” be allowed to do, like electrical work. But since people still often do it, educating them (while mentioning the legal situation) is probably better than trying to gatekeep one’s sacred knowledge.
Should have maybe just tried to build a transmitter from one of the radio building books, but the scary stories of a dark van with an antenna on the roof immediately pulling up and fining my parents a lot of money scared kid me off.
Seems like a fun project, and I like the use of the Arduino controlled frequency generator board – solves all sorts of problems. As for the ham radio guys you met, those awful crumudgeons are out there, most of them are quite literally dying off, but for every one of them, there’s a dozen decent people. Just gotta find em. There are lots of clubs. The dicks have their club, and they whinge about digital modes during their meetings and how nobody wants to come to the club BBQ, and the nice guys have their clubs.
It’s true however, not many people will AT FIRST be too thrilled with the idea of you making a broadcast frequency transmitter, but anybody with half a brain cell is likely to come around once you explain that you’re not trying to broadcast to the entire room, let alone the city.
Depends where you live but in EU countries have automatic spectrum monitoring stations working 24/7 round the clock. Any attempt at interference with commercial or military frequencies is recorded for later manual analysis.
Recently some pranksters broadcasting emergency signal used to automatically stop trains got caught this way (saved RF monitoring data combined with CCTV recordings)
I don’t think that was technically possible back in the day.
It’s been technically possible for a good few decades, one of the UK radio ham vloggers had a video or two about the remote monitoring stations, might have been RingwayManchester?
+1
Here in Germany we have these spectrum monitoring stations, too.
They record a few hundreds MHz of spectrum in real-time.
The next station I know of is said to be near a technical college in my part of my city. It’s an open secret among hams here.
And it’s not just here since now, but since late 80s/early 90s.
Back then the data was written to high-bandwidth analog medium, I guess?
Nowadays they probably record from 100 KHz to 3 GHz or, I guess?
Here in Germany we have these vans, too.
Back in the 80s, it was yellow though and operated by our federal postal acency (the Deutsche Bundespost, DBP).
It was often to be informally being refered as “der Gilb” (the yellowing/the yellowed, Gilb refers to color yellow/gelb).
Picture: https://tinyurl.com/mr3x4p3z
The vans had high-end scanners, spectrum analyzers of all sorts and antennas mounted on the roof and were searching for illegal transmitters.
Once they showed up, the person responsible not seldomly did make the radio pirate a deal:
Pass an amateur radio exam within a given time and you’ll be forgiven or you get a fine and your equipment will be confiscated.
The confiscating thing was common.
Some illegal equipment was allowed to be possesed, but not to be used.
If it was found out to be used actually (antenna and mike connected), it could be confiscated.
That’s why CBers here got very creative hiding their linear amplifiers. ;)
Because, opening and searching cabinets etc. required a search warrant, it couldn’t be demanded so easily.
Ham radio transceivers were a special case, I vaguely remember.
Listening amateurs, SWLs, without a license were allowed to use them for receive, but mike/key had to be unplugged and the transmitter tubes (the finals) had to be physically removed.
That was in a time when the Yaesu FT-101/Sommerkamp FT-277 was still in wide use.
Nowadays, the vans (now white) still exist but the fiercy Deutsche Post is no more.
The people who are nowdays on duty in this vans are are from the federal network agency and they look for man made noise.
Hams and ordinary citizens can call them for help if they have radio noise in their neighborhood.
They also help hams in legal situations if they have stub born neighbors who refuse to stop using noise makers such as the infamous power line communications (PLC), plasma TVs or noisy solar panel chargers/regulators.
The citizen who is found out to be responsible for the interference must pay a bill for that, um, “investigation”, AFAIK.
There is not a jurisdiction on the planet where an amateur radio license allows you to broadcast on the AM broadcast band. It’s either forbidden, or low power devices are permitted/exempt, and a ham ticket has nothing to do with it.
About that license: I took the U.S. Technician, General and Extra class licenses all in one sitting and I passed them all with flying colors. I have never engaged with the amateur radio community. Never visited a club, never met a ham, never even talked on the radio – “engaged in a QSO” – the irony of which is not lost on me.
So you have convinced yourself that you cannot get into radio building without amateur radio community support, but that’s just nonsense. Every marginally decent library on earth, perhaps outside of North Korea, has the books you need, in case you didn’t have access to the internet, which I think you do, where all the information is available in abundance.
Getting into trouble for building transmitters requires a LOT more effort than getting a license, because unless you build and run a poorly built, seriously high power transmitter, persistently, willfully, that actively messes with commercial operations like cellular, or public safety (say GPS), NOTHING is going to happen, even if you build a modest pirate radio transmitter across the street from the spectrum regulators. So build that little transmitter and learn from it, and move on – license or not.
About amateur radio: if half a day of playing video games, watching a few movies, or doomscrolling on TikTok is a better use of your time than getting an amateur radio license: that’s FINE. Your life, your choices. But I object to the notion that the barrier of entry is high.
I also don’t have a license and been doing radio stuff for years now.
I stick by “Don’t disrupt other stuff” and use approved radios (usually Ti ‘s CC1101 or Hope SX transceivers) on ISM bands.
Probably helps that I am more interested in “stuff” like tpms, RF outlets, garage doors, weather stations and the like.
To be fair, the U.S. Technician class is perhaps the easiest class all around the globe.
Just a few questions of multiple choice on a paper. It’s doable to anyone with a minimum of common sense, average IQ level and basic understanding of electronics.
By comparison, passing one of our easiest CEPT license exams in Europe is closer to passing General exam in US.
We must be able to answer questions from learning material that spans a hundred pages worth of questions.
We normally spend months of preparation before we attend to the exams.
Our FCC counterparts aren’t present at Disney Land or local carnival, either. ;)
We rather must wait for an appointment in which exams are taken and then travel throughout country to that one office that’s open.
Alternatively, online exams are slowly becoming available, too. Vy73s
This ^^^
Great video and looks like a nice project. An AliExpress kit is a good way of lowering the threshold of starting with such a project, because low investment in time/money and it’s already working albeit suboptimal. It’s funny that you can expect things like heatsinks the wrong way.
Is there any chance the transistor might be flipped upside down for electrical reasons? Maybe the die paddle is connected to some sensitive node in the circuit (like the modulator output) and this was a cheat to reduce stray capacitance? Normally I would expect something like a ceramic thermal plate between the device and heatsink, adding extra thickness (and cost) beyond an ordinary thermal pad.
Yep, they got the transistor pinout wrong :-) Happens to the best of us. Fixed in next PCB rev!
The other possibility is that the heatsink was intended to be mounted on top of the transistor.
Looks a fun little kit, could make a nice little QRP top band transmitter