CK111 Variable FREQ. MINI FM XMTR with mic.

Price: $14.90

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This powerful little FM transmitter comes with a sensitive microphone and will transmit to any FM radio up to 100 feet. The frequency is variable within the FM radio band of 88MHz to 108 MHz. Runs on 4 AA 1.5V batteries. Comes with all parts and battery holder. Complete instructions included. Measures 1.5cm x 6cm

The CK111 comes with excellent documentation. It shows all components pictorially for ease in identification and even listed the color bands on each resistor used. It has basic tips on kit building, step by step instructions on what to solder in next and a complete circuit diagram. The PCB and all components were of high quality. The capacitors are very easy to identify.

The CK111's main tuning coil is 6 turns of copper wire wound around a small ferrite slug. It comes pre-built, even with the tap on the third turn. All you need to do is put it in and solder it. As the coil is correctly designed, you can immediately tune to the recommended 90 MHz without any fuss. You can get a range of about 30. The kit uses up only 25mA at 6V.

The suggested antenna is a piece of wire a quarter of a wavelength long , about 83 cm ( supplied with the kit) The Electret mike is very sensitive and it is very easy to overmodulate the transmitter by speaking close to the microphone. This justifies Can Kit's decision not to use an Audio stage. Looks like an Audio Limiter or Automatic Gain Control (AGC) is totally necessary for a bug so that it can pick up whispers at 3 meters, without overmodulating when someone speaks 10 cm away from the Mic.

As the kit does not have pre-emphasis, the radio receiver unnecessarily de-emphasizes all the high frequency content leading to a slightly boomy bass voice. On top of that, the Electret mike is fairly directional so if you do not speak directly into it (when used as a room bug, for example), it catches a rebound from a wall which makes the voice boom even more. Besides this boominess, the Audio was very clean with no noticeable distortion.

The Frequency of operation shifts about 100 KHz initially till circuit reaches Thermal equilibrium, after which it is quite stable. We have not yet run its battery down but I am sure its frequency will change slightly as the batteries go weak. If you change the antenna characteristics a bit by folding up the antenna wire, the transmitted frequency can shift by as much as 400 KHz.

All in all, this is an excellent kit for a beginner who will be amazed when he firsts conquers the air waves.

Some of this info was obtained at:

http://members.tripod.com/~transmitters/begin.htm