LOW POWER FM TRANSMITTER

Posted on Feb 7, 2014

A low power fm pirate radio. The output power is around +35 dBm ( 3. 16 watts) over 50 ohms load and +24 volts power supply. The project consists in a hartley oscillator ( modulated VCO), and tree stages ( Class A, common emitter) of RF amplifiers. The first one, you can use a MAV-11 (from MiniCircuits) that have +12 dB of gain and a good 1 dB compression point of +17. 5 dBm, at my project

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I used a MWA-130 ( from Motorola) as driver stage. At second stage I used a 2N5109 (Motorola), but you can also use a 2N3866, and at final stage a 2N3553. With proper biasing and setting up the trimmers to maximum output power you can achieve around 2. 8 watts of EIRP using a 0 dBi antenna. A good irradiant system is also important, as well the coax cable that I recommend low loss cables if you are using a long cable distances. RG-58A haves a 67% of propagation velocity and 21. 65 dB/100m of insertion loss at 146 MHz. I also recommend you use a low pass filter at final stage to prevent harmonics products cause harmful interferences on the MARS/CAP systems. Here in Brazil, many pirate radios don`t use such filter causing many trouble at MARS service. At final you can found many schematics, and of couse a suggested low pass chebyshev filter and the respective response frequency curve. You will experience a low insertion loss within this filter, but not so expressive. The VCO can operate as free ( open loop) or synthesized (PLL) oscillator, you just must insert a jumper to close the loop for PLL or release it to free oscillator operation. The frequency deviation is controled by varactor diode that change the instrinsic capacitance within the voltage audio input causing a frequency deviation. Note: set it to ± 75 KHz to FM pattern broadcast operation. Another varactor BB809 control the operation frequency, 200 KHz step...

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