06-12-2016, 06:58 PM
I've been thinking of posting this for a few weeks now and thought it might complement Trevor's MW transmitter project.
In the mid 60s I had a couple of attempts at building one transistor FM modulators. My idea was not to attach an aerial to them but to rely on radiation from the coil, which ruled out a metal box. My attempts ended in failure because, although they worked, the drifted all over the place if you went near them or even moved them on the bench.
One day I came across a circuit which was designed to match a 50Ω load, so I thought I'd give that a try. I etched a small PCB for it and built it up. When I tested it - still no metal box! - I got a surprise. It was so stable, I couldn't believe it! I could pick up the PP3 that powered it and drag it around the bench and the only thing that happened was that you could hear the twanging as the air spaced coil vibrated in the process. Its stability was unshakeable - the only way to detune it was to place a finger in very close proximity to the coil! I suspect that using the coil in a pi output filter which matched the relatively low impedance output was the secret of its success. It radiated well and, like all of its successors, never had an metal box or an aerial fitted!
I should still have the original but I've hunted high and low for it for years and never found it but I did manage to un-earth a poor copy of the original circuit which I've redrawn for clarity.
In the original design it was intended to be fed from a 12V supply but a 9V battery supply was much simpler.
In fact, it radiated so well* with a 9V supply that you might want to drop it to 6V.
In subsequent versions I built, the 8 - 20ρF trimmer was replaced by a fixed 10ρf (or thereabouts) fixed capacitor and tuning accomplished by stretching or squeezing the coil as required.
If you are not using a PCB I would suggest using a single earth point or a hefty bus-bar - 16 swg tinned copper wire, for example.
* From the workshop at the rear of the building on the first floor it still provided an excellent signal at the front of the shop - a distance of about 40 feet!
In the mid 60s I had a couple of attempts at building one transistor FM modulators. My idea was not to attach an aerial to them but to rely on radiation from the coil, which ruled out a metal box. My attempts ended in failure because, although they worked, the drifted all over the place if you went near them or even moved them on the bench.
One day I came across a circuit which was designed to match a 50Ω load, so I thought I'd give that a try. I etched a small PCB for it and built it up. When I tested it - still no metal box! - I got a surprise. It was so stable, I couldn't believe it! I could pick up the PP3 that powered it and drag it around the bench and the only thing that happened was that you could hear the twanging as the air spaced coil vibrated in the process. Its stability was unshakeable - the only way to detune it was to place a finger in very close proximity to the coil! I suspect that using the coil in a pi output filter which matched the relatively low impedance output was the secret of its success. It radiated well and, like all of its successors, never had an metal box or an aerial fitted!
I should still have the original but I've hunted high and low for it for years and never found it but I did manage to un-earth a poor copy of the original circuit which I've redrawn for clarity.
In the original design it was intended to be fed from a 12V supply but a 9V battery supply was much simpler.
In fact, it radiated so well* with a 9V supply that you might want to drop it to 6V.
In subsequent versions I built, the 8 - 20ρF trimmer was replaced by a fixed 10ρf (or thereabouts) fixed capacitor and tuning accomplished by stretching or squeezing the coil as required.
If you are not using a PCB I would suggest using a single earth point or a hefty bus-bar - 16 swg tinned copper wire, for example.
* From the workshop at the rear of the building on the first floor it still provided an excellent signal at the front of the shop - a distance of about 40 feet!
