A Little Rig For Medium Wave

[Albert H]

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!

Little AM.jpg

[sinus trouble]

Interesting design Albert, Especially the modulation!

When i get some time? I will build something similar to this! I doubt i will go as far as a PLL?

The practicalities of a MW antenna in London is difficult to say the least.

I will most likely simulate something in LTSpice and if successfull? A Clad Proto may follow.

[Albert H]

An aerial isn't too difficult - even in London. The best is a tower-block sloper. If you get the angle right, you get very close to 50Ω, and it really shifts!

The PLL circuits for the little rig were really simple. One version we tried was a "huff & puff" stabiliser that used just two cheap CMOS ICs and a 48 MHz crystal oscillator module that came from Farnell for about £1. The H&P stabiliser is weird - the tuning goes in "jumps" up the band as you turn the tuning knob. When you stop turning the know, it stays where you leave it....

[Zozo]

That schematic looks awfully familiar in places, the method used for the series modulation was demonstrated by me and a fellow ham in Australia a few years ago now on our YT channels. However I went for the "Zener Clamp" for series rectified modulation using a power transistor as an Emitter follower, which translates in my brain as Series Plate Modulation being as I'm an old valve guy.

This allows you to have a greater forward power swing / PEP, as the negative parts of the AF signal are all rectified. A few months later he demonstrated his version using the TL431 as Emitter Series Modulation, which again translates as Cathode Modulation to me.

What really screams out the most in this schematic is the T37-43 Bifilar output on the driver stage, which gives active bias feedback and prevented envelope distortion at 100% mod. I don't clam that there's been plagiarism. In fact you may think your coming up with something new, only to find someone else had the same idea, or that what you've developed its pretty much "Textbook" in it's design.

The only other things in this design is the use of a 2n3866 which for MW has a pretty high "FT" I would always find transistors that would bottom out in frequency where the 2nd harmonic was. Also the original designs we both demonstrated used VXO's which has equal advantages / dis-advantages. However now that QuartzLab are back in business which will sell a little as 1 Crystal per order, the disadvantages seem insignificant.

You don't need a lot of power on Medium Wave or a massive antenna as most people think. Here in the UK we have 2 types of LPAM licenses, LTRL and RSL. However both have there restrictions regarding antenna configuration and frequencies. Firstly the Maximum power allowed for an LPAM is 1000mW PEP, Secondly the antenna cannot exceed 30 foot. Commercial LPAM antennas are provided by "Radica", they also provide the TX equipment too along with other interesting bits of kit to practically all the LPAM stations across the UK.

List of UK allocated LPAM frequencies .

LPAM4.png

The above picture shows what frequencies have be specifically reserved for use with LPAM stations here in the UK. There's also some other information on that picture which I will talk about in another topic soon.

[Albert H]

The reason for the 2N3866 was that it had plenty of gain at MW..... Actually not really - it was just to hand. The last one I made had a BFY51 as the second transistor, and worked just as well. I've also used a BD131 in that position too, so the transistor isn't particularly important, as long as it has enough gain at 1MHz, and can handle about a half Watt (peak).

The final stage is based on a "top band" mini- PA by Doug DeMaw. The circuit (and variants of it) has been around for as long as there's been power FETs!

It's a fun little rig, and I fired one into a shortened sloper using drainage pipes for the earth. The shortened radiator has a couple of inductors in it, and radiates surprisingly well and gives a good match. I put it on 666 kHz, and found that during the daytime I could hear it on my car radio pretty much all over town, and a couple of miles outside. I put a lowpass filter on the audio input, and gave the mod some pretty tight compression, and it sounded a whole lot better than Radio Caroline 18 kHz away with their "Super-Sock Mode Modulation"!