Electronic Varactor Tuner Transmitter Kit

This past Sunday I put together a Varactor Tuned FM Transmitter / Mike kit# 80-065 by Philmore Datak. This kit is advertised as the best of the miniature FM transmitters, transmits to FM radio frequencies. The Cadillac of FM transmitters and wireless mikes. This started as a backyard picnic table project until the heavy rain came than it was a scramble into the garage to finish up.

This kit features a varactor (tuning diode) tuned Colpitts oscillator and an RF amplifier stage that offers solid performance in the 88-108 MHz broadcast band. Varactor tuning is free from stray capacitance and drift. Recommended voltage is 9 to 12VDC. Per the instructions typical range is from 50 to 100 feet depending on antenna and other conditions.

Parts identification and assembly went smoothly. One resistor lays flat and all other resistors are mounted standing up on end to fit in on the small PC board. The electret mike is mounted directly on board. Kit contains 33 components. Board size is 3/4" x 2-3/4".

How it works: The circuit is broken up into three stages, audio, Colpitts oscillator, and RF amplifier. Audio is picked up by the electret mike and converted to a voltage signal and amplified by a PN2222A transistor. This acts as a small signal linear amplifier. This signal is coupled out to the Colpitts oscillator to provide DC isolation. The advantage of the Colpitts is there are no coils to wind or mess with. The kit instructions give a decent description of theory of operation and a piece by piece run down of the Colpitts oscillator. The RF amplifier uses biasing components (resistors and capacitors) to configure a PN918 to provide amplification of the oscillator signal.

Tuning and alignment went very smoothly. Per instructions I just centered the 10K pot, attached 9V battery, and did a smoke / heat test. Everything checked out, turned on a FM radio with a digital display and found my signal near 92MHz. I put the transmitter in another room and was able to pick up sounds from the radio speaker with antenna wire supplied. But to get the most out of this circuit you will have to play around and tweak it. Using the formula AL= 234/FM Hz to make a antenna that will be compatible with the frequency your listing on. Just divide 234 by the frequency your circuit is broadcasting, cut a wire to that length, should improve your distance and signal.

Overall a very easy kit to put together, just take your time soldering. Street price is around $15.99. I still have to attached a proper antenna cut to size and maybe increase voltage to 12 volts and see how we do. Range on package is 140 feet. With its small size this kit can be hidden just about anywhere, James Bond type stuff. Remember, this is an educational kit only, do not get in trouble.