# How to make a multivibrator into a transmitter?

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#### beamin

##### Member
I want to make a simple transmitter beacon using parts off hand.

I guess the limiting factor is the transistor.
This is what I have:
2n4401
2n3904
pn2222
2n5426
8050
and their pnp cousins
pn4393
J113
tip120
mpsa13
as well as
2n3055
tip31c
tip41c

I also have standard ceramic disk, resistors and diodes, but no crystals.

I can find schematic, but cant figure out what frequencies they make. I have an SDR that goes down to almost DC so that will be my oscilloscope.

#### slicerwizard

##### Member
Frequency is a function of the RC components in the circuit, namely the capacitors that are charging up and the resistors that the charging current is flowing through. The lower C is, the higher the frequency. The lower R is, same thing.

Typically, the two capacitors have the same values. Same with the two resistors that connect to the transistor bases. Oscillation frequency = 1 / (1.38RC)

There is a table listed here: Astable Multivibrator and Astable Oscillator Circuit

It shows that 1nF capacitors combined with 1k resistors will oscillate at 714 kHz. Reduce R (within reason) or C to increase. Be aware that there is rapid switching in play, so there are likely harmonics being created.

I would try connecting a wire to one of the collector leads as an antenna and wrapping that wire around the SDR's antenna to get good coupling. It's not much of a transmitter, but it'll get you started.

#### beamin

##### Member
Thats definitely a start. But which transistor to use? What am I looking for in the data sheet?

#### beamin

##### Member
It transmits at 652400, and pulls between 0.001-0.000 amps at 10v on the power supply.

Pretty weak but it works.

#### beamin

##### Member
Would this transmit better if you turned the square wave into a sine wave? How would you do that could you use an op amp that would turn on and off?

Also the signal is strongest at 12.4V (pulls 14mA). above this voltage it draws more current but the signal gets weaker. That doesnt make sense unless the extra energy is going into a unseen harmonic(s)?

#### slicerwizard

##### Member
Thats definitely a start. But which transistor to use? What am I looking for in the data sheet?
Current gain bandwidth product. Your 3904's are rated at 300 MHz, while your TIP41's hit only 3 MHz.

Would this transmit better if you turned the square wave into a sine wave? How would you do that could you use an op amp that would turn on and off?
I would think that you would get more signal out if you fed the pulses into a resonant tank circuit, but now you're moving beyond a simple guaranteed to oscillate multivibrator and into finicky tuned circuits.

Also the signal is strongest at 12.4V (pulls 14mA). above this voltage it draws more current but the signal gets weaker. That doesnt make sense unless the extra energy is going into a unseen harmonic(s)?
Possibly some sort of resonance artifact. To move forward, you'd probably have to switch gears and build a true RF oscillator. Try starting here: Hartley Oscillator and Hartley Oscillator Theory

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