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These are usually a one transistor LC oscillator on the FM broadcast band. The microphone is audio amplified and slightly amplitude modulates the base bias or the collector voltage. This produces both AM and FM.
Here is one of my favourite designs taken from the web. It is very stable and VERY sensitive on the mic pick-up (increase the val of c15 for more sensitivity). It can be made very small as you can see from the picture. It uses 2 common 1N4007 diodes to act as varactor diodes.
It was designed to operate from a 9v batt but will work all the way up to 15v or so. The RF output was measured at 10mW on a spectrum analyzer with a 9v supply and increasing to about 100mW with a 15v supply. Be careful when operating from a power supply if it is not well regulated as you will introduce "hum" onto your signal.
Make the antenna wire about 32" long
The inductor L3 is about 5-6 turns of 22 gauge wire on a 0.1" former (like drill bit etc.) L1 can be common moulded type 0.2-1.5uH (Value not critical)
The transistors are CATV RF transistors from ON semi or Fairchild. You can try other devices but these really work well.
Is it possible to somehow secure the transmission?
I mean if someone gets the signal on the radio
will get just noise and I can use a filter to get the real voice?
The simple way to secure is to use narrow FM modulation wth quartz stabilization - but you will need find appropriate receiver . When received on normal FM receiver narrow modulation is detected as carrier . Note that narrow modulaiton will increase distance due to low noise input on receiver
Or use different freq and just add mxier to your receiver's receiver's input to shift signal from band you use to the normal FM receiver band .
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