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3 tx's, 3 results.

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Well-Known Member
audioguru said:
MStechca's transmitter has its tank as its emitter to ground resistor. Therefore the antenna can't have a voltage swing that is much higher than the supply voltage which results in a high RF power output as occurs when the antenna is connected to the transistor's collector.

Also, the tank is trying to be a high impedance parallel-tuned circuit, but it is in a low impedance emitter circuit so the frequency stability of the oscillator is probably very poor.
I agree that he can get more voltage swing from a collector tank. I also did a little research on short wire antennas, and found that they look like small capacitances, which means that more tank voltage means more radiated power (true in any case, but really important for short antennas).
I have been simulating a (center) tapped inductor in the tank (in the collector circuit), and can get, as you would expect, more voltage across the tank than is possible with an untapped inductor. A side benefit seems to be lower harmonic generation. It appears the inductors can be either mutually coupled or not. If they are not, you need the same total inductance. If they are coupled, the self-inductance can be less, because the coupling adds to the inductance.
I don't think this will work for a properly tuned antenna, because it will look like a low resistance load, killing the Q of the tank.


New Member
The class C stage also eliminates most of the incidental AM that you get from the oscillator/modulator, which you will also get from your one-transistor hardhead transmitter.
I think it is best that I transmit AM, because according to a number of you guys, a superregen receives AM, and can receive FM because of slope-detection.

I found that I get more range when I connect an antenna to ground. :wink:

the frequency stability of the oscillator is probably very poor.
Stability is great. The only problem I am having is that when the RESET pin is low, dead air is not transmitted (which is what I expect).

...killing the Q of the tank.
looks like I'm on the right track. A low Q = Higher bandwidth.
and I bet it is true that if a transmitter has a bigger bandwidth than the receiver, the receiver will receive it across several channels!


Well-Known Member
Most Helpful Member
You are on the wrong track:

1) You have the extremely low output impedance of the 555 oscillator connected to (shorting) the very high collector impedance of the transistor, so the RF frequency changes when the 555 is reset.

2) Also, you have the very low emitter impedance connected to (shorting) the very high tank impedance, so the tank's Q is ruined causing its RF frequency to vary at random.


New Member
I changed it.

Now instead of a coupling capacitor, I coupled the circuit with an opto-coupler. (chip# 4N37).

I took the 555's output and connected it to the internal LED (in the 4N37) in an inverted fashion for maximum output. The 555 likes to produce more power that way.

then I used the collector and emitter pins of the NPN in the 4N37. I connected the collector to +ve, and emitter to the output (where the other pin of the coupling capacitor was, before I made this change)

It seems to work alot better, but the range seems to be reduced. I wonder if my battery/ies are dying.
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