The circuit is identical to the Wideband FM Transmitter but with a few small additions. TR1 (BC547) is an inverted Hartley oscillator which based upon an inductor fabricated on the PCB. This makes it megga-stable, and setable anywhere in the VHF FM band (76MHz to 119MHz) and the BB105 varicap makes it voltage tuneable over about 8MHz of that band. The inductor has one tapping for feedback and a second to feed an optional prescaler. TR2 is a buffer/amplifier and TR3 it the PA stage.
Mainly, the PCB; it has all the optional bits on it to make it into a FM pirate radio broadcast transmitter. It is NOT limited to just pirate radio, you can still use it for other purposes and frequencies. The noise floor is low enough to make a good NBFM transmitter. But let me assume you want to build a braodcast bands TX and it is legal in your country.
The Prototype
Now that is a bit of a laugh. I have already built loads of these on the same PCB so the use of the term "prototype" is a bit misleading. All functioned well and very close to each-other in specification. Output power of the last six units is about 25% higher than that observed from the first V7, probably because the first one had so much "hacking" around the PCB. Anyway, here is a picture of the final unit.
In these two pictures you can see the completed unit, plus a view of the PA stage. At the time of the photograph it was loaded with a 6v 0.1A bicycle lamp and fed with +15v DC supply. The lamp is 600mW lamp at 60 Ohms when fully lit. Here it is lit to well over half brilliance (lowers impedance). This particular unit is delivering +27dBm, according to my Hewlet Packard analyser. I think I am happy with that!
Download PCB
Continue reading: WBFM TX V7b
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