A Little Rig For Medium Wave
Posted: Tue Feb 09, 2021 5:05 pm
A friend of mine found this little board in his garage the other day. It's a little Medium Wave transmitter that he used to annoy his neighbours!
He got fed up with their choice of music at high volume (one of those religious efforts), so he replaced it with some pop oldies (more normal fare for MW).
The version we saw used a VFO, so that it could be tuned to a range of frequencies between 1420 and 1580 kHz, though building it for other frequencies would be easy. It was tuned with a transistor radio-type variable capacitor. The basic version was designed to work from a lead-acid battery and delivers roughly 2.5 Watts carrier and 10 Watts peak. The four coil cores are all easily available - a T50-2 for the oscillator, a T37-2 (or T50-2) for the output filter, and a couple of T37-43 for the interstage coupling and for the drain RFC. The original had a 2N3866 for the intermediate transistor, but this could easily be a BFY51 or a BC142 (or anything similar) without any changes. The output FET (IRF510) is widely available for <£1.
All the coils are wound with enamelled copper wire ("ECW"). A "turn" is one pass through the centre of the core. Only the VCO coil needs to be tapped - wind on the first 25 turns, then extend out a loop of the wire, and then continue winding a further 75 turns. It's a good idea to secure the wire to the core - "Copydex" rubber glue is good, because you can peel it off if you need to change anything. The original had the coils glued to the board, each with a large drop of rubber glue. The interstage coupling transformer has two windings - the enamelled copper wire needs to be doubled , and then twisted (about 6 - 8 twists per inch) before winding it on to the core for 12 turns. Remember to scrape the enamel off the wire where you're going to solder. Some ECW enamel will burn off when you apply a soldering iron, but most needs to be very carefully scraped off!
The original board also had the interconnections made using "lands" of PCB material superglued to the main sheet of PCB material. The layout isn't critical, but as with most RF constructions, it's sensible to keep lead lengths short. The output FET had a small TO220 heatsink attached to it, and the FET was isolated from the heatsink (the metal tab is RF live). At full output, it didn't run more than warm.
The modulator is a little unusual, using a TL431 "adjustable zener" to control the current through the driver stage. The modulation quality achieved using this method sounded OK. This is a minimum parts count design, so it's less than perfect, but the audio quality is remarkably good given how simple the circuit is.
The original was a free-running oscillator design, and drifted a bit in use. One of the reasons we saw this thing was to add a PLL to it, so I used a 3-chip CMOS circuit (4060, 40103 and 4046), and a passive loop filter, to hold it to the chosen frequency. I tried several different varicaps to tune the thing - the best being the BB212 (expensive!). I ended up using a couple of reverse-biased white LEDs which gave me a range of around 35kHz - more than enough for the PLL to hold the oscillator on to frequency.
If you're going to build this thing for lower down the band, scale the output filter appropriately. As drawn, it goes over at about 2MHz (around 1.33 times the carrier frequency). It's not hugely critical, but at 1.33 times the output frequency, there's little loss of wanted carrier, but the second and subsequent harmonics are well suppressed.
The rig matches into 50Ω and will give a blank carrier of about 2 - 2.5 Watts, and will peak at about 9 - 10 Watts. It's capable of about 92% mod with this circuit configuration.
It's far from perfect, but it's cheap, simple, and a good introduction to medium wave transmission. You're not going to get great coverage at night, but in the daytime it'll cover a small town.
Have fun!
He got fed up with their choice of music at high volume (one of those religious efforts), so he replaced it with some pop oldies (more normal fare for MW).
The version we saw used a VFO, so that it could be tuned to a range of frequencies between 1420 and 1580 kHz, though building it for other frequencies would be easy. It was tuned with a transistor radio-type variable capacitor. The basic version was designed to work from a lead-acid battery and delivers roughly 2.5 Watts carrier and 10 Watts peak. The four coil cores are all easily available - a T50-2 for the oscillator, a T37-2 (or T50-2) for the output filter, and a couple of T37-43 for the interstage coupling and for the drain RFC. The original had a 2N3866 for the intermediate transistor, but this could easily be a BFY51 or a BC142 (or anything similar) without any changes. The output FET (IRF510) is widely available for <£1.
All the coils are wound with enamelled copper wire ("ECW"). A "turn" is one pass through the centre of the core. Only the VCO coil needs to be tapped - wind on the first 25 turns, then extend out a loop of the wire, and then continue winding a further 75 turns. It's a good idea to secure the wire to the core - "Copydex" rubber glue is good, because you can peel it off if you need to change anything. The original had the coils glued to the board, each with a large drop of rubber glue. The interstage coupling transformer has two windings - the enamelled copper wire needs to be doubled , and then twisted (about 6 - 8 twists per inch) before winding it on to the core for 12 turns. Remember to scrape the enamel off the wire where you're going to solder. Some ECW enamel will burn off when you apply a soldering iron, but most needs to be very carefully scraped off!
The original board also had the interconnections made using "lands" of PCB material superglued to the main sheet of PCB material. The layout isn't critical, but as with most RF constructions, it's sensible to keep lead lengths short. The output FET had a small TO220 heatsink attached to it, and the FET was isolated from the heatsink (the metal tab is RF live). At full output, it didn't run more than warm.
The modulator is a little unusual, using a TL431 "adjustable zener" to control the current through the driver stage. The modulation quality achieved using this method sounded OK. This is a minimum parts count design, so it's less than perfect, but the audio quality is remarkably good given how simple the circuit is.
The original was a free-running oscillator design, and drifted a bit in use. One of the reasons we saw this thing was to add a PLL to it, so I used a 3-chip CMOS circuit (4060, 40103 and 4046), and a passive loop filter, to hold it to the chosen frequency. I tried several different varicaps to tune the thing - the best being the BB212 (expensive!). I ended up using a couple of reverse-biased white LEDs which gave me a range of around 35kHz - more than enough for the PLL to hold the oscillator on to frequency.
If you're going to build this thing for lower down the band, scale the output filter appropriately. As drawn, it goes over at about 2MHz (around 1.33 times the carrier frequency). It's not hugely critical, but at 1.33 times the output frequency, there's little loss of wanted carrier, but the second and subsequent harmonics are well suppressed.
The rig matches into 50Ω and will give a blank carrier of about 2 - 2.5 Watts, and will peak at about 9 - 10 Watts. It's capable of about 92% mod with this circuit configuration.
It's far from perfect, but it's cheap, simple, and a good introduction to medium wave transmission. You're not going to get great coverage at night, but in the daytime it'll cover a small town.
Have fun!