FM transmitter, antenna and radiated power

[vinodstanur]

Hi,
I am just playing with an FM transmitter, I took a simple single transistor FM modulator circuit from online. It is basically the same common base configuration for 88 - 108 MHz oscillator and a mic coupled to the base of same transistor. I added two stages of amplification for the 91MHz signal generated by the above mentioned circuit. I am not having the circuit diagram, I will attach it tomorrow after drawing it. At the output of final stage amplifier I got a 91.9MHz 2.2v peak to peak signal on oscilloscope (10x) and when connecting a 0.7meter thick wire as antenna the peak to peak decreases to 1.2v, so it seems like the antenna wire is loading the circuit. I feel this is much better than the one transistor based circuit. I am getting a good range inside room on my mobile phone FM radio without headphone antenna, it means I will get a much better range outdoor with proper antenna on receiver for around one km or more.

How will I calculate the radiated power in my case?
How will I confirm the output impedance of my transmitter is 50 ohms? Do I really need to make it 50 ohms?
Also how will I calculate the antenna impedance for a wire antenna? I am connecting a quarter wave length wire by considering the centre frequency as 91MHz.
What kind of antenna I should use in this case for getting maximum range?
How will I again increase range other than increasing the supply voltage? What exactly I should take care for increasing range again?


I know it is illegal to broadcase FM at high power, but this is just for learning and not for prolonged period.

 

[Audioguru]

Your transmitter is messy, I am surprised it works. Usually a 100MHz transmitter circuit is neat and tidy.
The distance is increased when the antennas focus the radio beam. A Yagi antenna has good focussing.

 

[chuckey]

If your aerial is a quarter wavelength then its 75 ohms, fact of physics. As for you transmitter, its best to tune and "couple" it by monitoring the current the PA is taking. You loosely couple the aerial to the PA (use a 200 ohms resistor in place of your AE), then you tune for minimum PA current. Your then connect your aerial and the PA current should rise. readjust the tuning to dip the current again, then readjust the coupling until the current increases stop, back it off a bit.
Pout = V^2/75, you probe will detune the aerial at 100 MHZ, but its an order of magnitude, i..e. 1W or 10 mW?
Frank