I built myself an FM transmitter with an enable option so I can interface it with a micro controller. The Enable line is set through the micro via a 1K resistor (not shown). Once set to enable, the FM transmitter starts sending whatever sound is injected to the input on the left (in this case sound from the computer earphone terminal via earphone extension plug).
With this design (excluding the circled parts and using test point 1 as a 6-inch wire antenna) I'm able to achieve a transmission of up to about 50 meters.
I then researched buffer stages and adopted an idea from http://talkingelectronics.com/projects/Spy%20Circuits/SpyCircuits-2.html by adding the circled components and using TP2 as an antenna instead of TP1. After testing again with the parts added, I get no audio signal at all, even if I have the transmitter and receiver within 10 centimeters of each other.
I'm trying to run this transmitter with a carrier between 300 and 500Mhz and I'd like to achieve a few hundred feet distance.
Rather than place an order for trimmer capacitors and waiting weeks for them to come in so I can ultra-fine-tune the band (which I didn't have to do when testing with the red components excluded from my design), is there anything else I can do to increase the range of my transmitter and still receive an audio signal?
Is capacitor or transistor wrong?
Two things that no one pointed out yet were the 10pF capacitor value (capacitor circled in red) and the add-on transistor (also circled in red).
I'm just wondering if a certain reactance amount in ohms between the first and second amplifier stage needs to be met for both stages to function.
Also, could I get away with a Pn2222 instead of a Pn3563, or maybe even use a set of diodes instead? I'm asking because 99% of the time when I make a circuit with Pn3563, the transistor internally blows up and I have to replace it (no I didn't short the circuit board). Also the maximum current limit for Pn3563 is 50mA and I think with the part values I use and the frequency of interest, I'm approaching and/or exceeding that limit.