wireless transmitter

[audioguru]

Hi Tazmania,
1) 0.22uH is too high when you add about 10pF worth of wiring capacitance to its tuned circuit. Reduce the number of turns in its coil or spread-out the turns. Most toy FM transmitters use 0.1uH.
2) The transmitter has harmonics because it has only one tuned circuit. "Real" transmitters have many tuned circuits to attenuate harmonics.
3) Many people complain about picking-up their transmitter at many point on the dial of a cheap clock radio. I think that the radio's input stage is overloaded by the transmitter. Cheap radios don't have a tuned circuit ahead of its input RF amplifier like "real" radios have.
4) The inductor in series with its antenna is supposed to be part of an impedance matching circuit when you have coaxial cable to a low impedance antenna. If you have only a piece of wire for an antenna, the transmitter will probably work better without the extra coil.

[tazmania_kid]

I didn't use an air core coil, instead I used a normal inductor which I bought from supplier. Is this possible?

[audioguru]

Hi Tazmania,
The project instructs you how to simply make a suitable coil. Why did you buy an inductor with the wrong value?

[tazmania_kid]

As am new to circuit design, I thought, with coil value at 0.22uH, a 3-18pF capacitance change would in theory give you a frequency range of 80-196MHz. Too bad, it's not as easy as I think :(

[audioguru]

Too bad, it's not as easy as I think :(

Yeah, you must think of everything at these very high frequencies. Since stray capacitance has doubled the "theory" capacitance, buy or make a smaller inductor to match.

[tazmania_kid]

ok, I will try to make a coil myself and see what's the output :)

[Guest Alun]

The coil in series cancels out the capacitance of an antenna that's shorter than 1/4 wavelength.

You could reduce the harmonic output by adding a LC tuned circuit in parallel and series with the antenna this will help to filter out the harmonics.

[tazmania_kid]

Hi, anyone can explain to me the differences between the attached circuits in terms of construction and their performance?

The reason is: understanding more on these circuits will help me proceed with better and more complicated tx circuits.

Circuit 1:
http://www.geocities.com/tomzi.geo/4w_fm/4W_transmitter.htm

Circuit 2:
http://www.boondog.com//tutorials/rfTransmitter/rfTransmitter.htm

Circuit 3:
http://www.aaroncake.net/circuits/fmtrans.htm

[audioguru]

Hi Tas,
It's obvious that the 1st complicated 4W transmitter (same as the one on our site) will flatten a battery in no time and probably put you in jail! It has a strong signal by using two RF amplifier stages, and proper LC impedance matching circuits.

The 2nd circuit is a cheap little toy with a range of a whopping 25 feet!

The 3rd circuit isn't much better than the 2nd but has a mic preamp for greater sensitivity.

[Guest Alun]

Circuit 1:
Looks the same as one of the circuits on this site

Tr4 is an audio amplifier.
Tr1 is a hartly oscillator, coupled to the audio amp, the varying voltage on the base alters it's capacitance like a varicap tuning diode, thus altering the circuit's resonant frequency.
Tr2, 1st class C RF amplifier.
Tr3, 2nd class C RF amplifier.

Circuit 2:
Looks like a good tutorial but half the pictures and schematics are missing

Circuir 3:
A crappy 2 transistor transmitter drawn upside down, with no RF amplifier.

Here's a good transmitter circuit: http://sound.westhost.com/project54.htm

I know it looks like a simple 2 transistor transmitter it's actually a lot better.

Q1 is the oscillator and Q2 is an RF amplifier, there's no audio pre-amp - for that look at the bottom of the page there's also an option for pre-emphasis too. And I would recomend a 2N2222A for Q2 instead of the BC547 as it has a higher Ft.