2001-11-25
FM Broadcast Band Transmitter
Building my AM broadcast loop antenna brought back old memories of being an FM broadcast pirate while I was at high school. So, figuring I've learnt a lot about VHF engineering since then, and having a evening to kill I assembled this half-watt transmitter out of whatever I had laying in the junkbox.
I am not going to provide the circuit diagram, at least not until I have it all stable and nicely engineered. I'll probably be besieged by requests for it, just as I am as a result of the page about my pirate days. Let's face it, a lot of us started of this way, and I am not shy about admitting that I was once a pirate. While licensing is preferable to anarchy, I don't mind seeing interested people building their own gear for any RF frequency, at any power level. Provided of course, they don't hurt anybody, or themselves in the process.
The design is a simple free-running VCO which runs at the transmit frequency of about 94.5 MHz. Coarse tuning is made via the red tuning slug of the VCO coil, fine tuning is available via the small pot also visible in the photo. The fine tuning voltage also effects the sound quality as it controls the bias on a tuning diode (a simple 1N4001), the wiper is also connected via a DC blocking cap to the input audio signal. The bandwidth is many MHz.
The VCO is almost a power oscillator, running at many milliwatts, about the limit for the 2SC710 used to implement it. The signal is then coupled into a two state driver and simple tank filter, followed by the C-class output stage consisting of a 2N4427, an RFC, and a pull-down resistor, (and not much else except decoupling of course). There is a simple PI filter on the output to suppress any harmonics, but there really should be a better filter, with notches at the harmonic points, the final stage is fairly dirty resulting in barely -36dBc of the first harmonic.
Stability is excellent, there are no spurious signals other than the harmonics. The shielded VCO coil makes for excellent stability as well. It is firmly soldered to the board so even mechanical abuse causes little drift. There is however quite a large thermal drift before equilibrium is achieved, this is to expected for a VCO running at such power levels, the 2SC710 is getting quite warm.
I've made a simple ground-plane antenna for it out of some copper wire and a panel mount BNC. Most of the time the unit is plugged into four 220 Ohm carbon resistors in parallel, but at least once I've hung the antenna from the roof of the office, plugged the audio line into the sound card turned on some copyright music and took off down the street with a Walkman to see how well it gets out. It gets out *very* well for such a primitive design, it was still pretty much 100% quieting when I decided I'd walked far enough and turned back.
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