Need help about a transmitter project

[grkan13]

Hello, I've found a schematics on web, i've tried to build on a breadboard but it didn't work. Then I thought the problem was testing with a breadboard and i did my own pcb tested over there but it didnt work again.

I'm not good about transmission, electromagnetic, but i have to do that project. Can you help me about analyzing schematic if something is wrong?


View attachment 60915

C1 is 1nf
C2 is 4pf
C3 is 10pf
C4 is 10pf

R1 is 5.1M Ohm
R2 is 8.2K Ohm
R3 is 75 Ohm
R4 is 27k Ohm

Q1 is 75.025 3rd overtone crystal

L1 should be around 0.05uH as i calculated
L2 is 8 turns on 1/8" diameter.

T1 is NE68119-T1-A but instead of that i used 2n3904
T2 is MMBTH10LT1G but instead of that i used 2n2222

 

[duffy]

Try replacing the T1 with a 2N2222. I have not had much luck getting the 2N3904 up into the RF range.

 

[colin55]

You will never get the circuit to work with 5M1 on the base and such a low impedance on the collector-emitter section. You only have 3v to play with.
The 2N3904 is ok for this stage.

 

[grkan13]

How can I improve the impedance between emitter-collector? Should I put a capacitor? If yes, how can i calculate the value?

And I've tried the below circuit. I've used 72.025Mhz designed for 3rd overtone. I've changed the capacitor(C3 on first circuit) to 10pf and inductor (L1 on first circuit) to 0.05uH. How can i calculate the value of L2?

View attachment 60943

 

[colin55]

How can i calculate the value of L2?

You don't have to. The dimentions are given. The vale of L2 is not important. It simply keeps the signal from C1

 

[grkan13]

Thanks colin55, i did make it work by your helps. Now I have one last question, is this inductors have an effect on transmitting power? Or how can I improve the transmitting power?

 

[audioguru]

If you double the supply voltage to 6V then the output power will be 4 times higher.
The antenna length should be 1/4 wavelength long for the most output power.

 

[grkan13]

So basically, these inductors values dont have any effect on power, I've already setup my antenna to 1/4 of wavelength.

 

[audioguru]

The 12 turns coil and its parallel 22pF capacitor are tuned to the 3rd harmonic of the crystal. The output power will peak when the tuning is accurate.

 

[grkan13]

For example I'm using 10pf and according to the calculations I need to use .05uH for 216Mhz. Can I just use .05uH SMD inductor? Or do i have to make them by hand?

 

[audioguru]

For example I'm using 10pf and according to the calculations I need to use .05uH for 216Mhz. Can I just use .05uH SMD inductor? Or do i have to make them by hand?

Your calculations do not include the extra capacitance caused by the wiring. You should use a trimmer capacitor so it can be tuned to the peak. Make-it-yourself coils are simple and work very well. We don't know if a SMD coil will work.

 

[colin55]

You have to have a "high-Q" coil. That's why you have to wind it yourself.

 

[grkan13]

Hello again,

I've tried to apply 5 volts but now its not beeping, its always giving a sound, but the range is much better now. I think 1st transistor is always on. What should I do to make it beeping again? Should I increase the value of resistor thats connected the base of 1st transistor?

 

[grkan13]

Hello again,

I've tried to apply 5 volts but now its not beeping, its always giving a sound, but the range is much better now. I think 1st transistor is always on. What should I do to make it beeping again? Should I increase the value of resistor thats connected the base of 1st transistor?

 

[audioguru]

The circuit does not have anything to produce a sound. It does not have a microphone and it does not have a beep oscillator.

 

[colin55]

The beep is produced by the RC-timing of the 200k and 4u. You have to adjust R and C until it beeps with the voltage you are applying.

 

[duffy]

grkan13 - what is the "sound"? Is it a continuous tone, or do you mean the silence from a carrier wave?

If you hear a tone, that's all you can expect. The circuit's producing an RF carrier wave and giving you an audio relaxation oscillator with that RC network on the base, which is pretty amazing if it can manage that - but there's nothing to interrupt the audio tone to produce individual "beeps".

 

[audioguru]

The Cochran transmitter was designed to be carried by a small bird so it is extremely small and light weight. The 200k resistor and 4uF capacitor turn on the transmitter for only a moment one time each minute so its small battery lasts for months.
I think a radio tuned to its frequency will have quieting (not a beep) when the transmitter turns on. But most of the time the radio will produce background noise.

"Lost model airplane" circuits work the same.