TX-500 500mW FM Transmitter

Posted on Feb 4, 2014

The TX500 is a simple to build 500mW FM Transmitter. It consists of three blocks; modulator / oscillator, two stage 500mW VHF amplifier and LED based power meter. The TX500 allows to transmit audio signals to FM band at frequencies from 88 MHz to 108 MHz. Due to the very low power consumption of less than 100mA the circuit may be perfectly powered

TX-500 500mW FM Transmitter
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by using 9-12V battery or power supply if you prefer. The circuit has been divided into separate stages so that it is be better for everyone to understand how every part works independently. Notice that all of the transmitter blocks have been built using four low noise general purpose NPN transistors. These transistors should be easy to find. Most of the regular NPN transistors should work fine and these are just some of the examples: BC549 (low noise), 2n2222, BC109 (low noise), BC107, etc. Please be aware that audio power transistors like BD140, 2n3055, TIP3055, 2SC5200 and such will not work because they can not handle higher frequencies. Thanks to the fact that the regular NPN can work at 100MHz and above we can use them in all of the lower power blocks of FM transmitters. If you prefer you may also use RF transistors like BF199, MPSH10, BF240, etc. The first block of TX500 based around transistor Q1 - BC549 acts as a simple modulator / oscillator. Many of the low cost FM transmitters use very similar circuit to generate radio signals but this oscillator has been carefully improved to provide much better stability. First thing that does make a great difference is the use of a tunable coil for L1 as opposed to a regular air coil. The use of this kind of coil allows for precise tuning of a desired frequency which is especially important on crowded FM bands. An issue with an air based coils is that it is much more...

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