Pirate Nation Radio - 1/2 Watt FM Amplifier
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Description: Hello home builders, this time we build the FRB (Free Radio Berkeley) 1 watt amplifier.
This design is a 2 stage amplifier that has about 16db of gain, suitable for an input of 5 to 20 MW.
The transistors are a 2N4401 and a 2N4427.
At 13.7 vdc the most I could get out of the unit was 1/2 watt, you could probably raise the
dc voltage to 16 volts and get more output but then a much better heatsink would be required.
But for the sake of this project, we'll call it a 1/2 watt amplifier. So let it loaf and run cool.
Construction: The unit is constructed with copper clad board, a large piece is used as the construction surface. I took the pc layout diagram provided and traced it onto the board, then I used a dremel tool to remove the copper between the traces (you could make a conventional pcb also) and mounted into the case. A 1 amp 3ag fuse is part of the dc power cord assembly. I used BNC coax connectors for input and output, however SO-239 or F type connectors would be fine too. This unit has no swr protection and requires a 50 ohm antenna, do not operate the amplifier without a proper load. All of the coil, choke and parts information is at the link above or Click Here Do not omit the low pass filter, the info is here 
1 watt amplifier, pcb layout shown actual size Tune up and use: In order to make the amplifier operate properly, it needs to be tuned. To start, set all the trimmer capacitors to middle position, connect the rf out to a wattmeter/dummy antenna, apply power and drive from the exciter then tune for maximum output. Limit your drive to 25 MW maximum. If you can measure the amplifier transistor's current, then adjust for minimum current at maximum rf out. 
Completed 1/2 watt amplifier, note the low pass filter on the right. - DJ Pirate Joe |
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