HFU HF Underground
Technical Topics => The RF Workbench => Topic started by: Mr.ArchVile on October 05, 2016, 0137 UTC
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So I've been experimenting, and have this relatively elementary question to ask.
Why did the circuit in the attachment change in distance?
Circuit #1 was what I had started with and had a range of about 100 feet
Circuit #2 is what I currently have and has a range of 1500 feet
I think it was because it was the increase in current due to the 120 ohm resistor being used, and also I was told that the removal of the 1k resistor allowed the transistor to use more power.
This goes into what I really want to ask, how do you determine the power of homebrew transmitter?
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Circuit 2 is better as you have used a choke which should have little resistance to DC allowing the full 12V to be present at the collector, using a 1K is going to lessen the volts and not what you want. The resistor in the emitter leg can be decreased to the point where the transistor just gets too hot (increase the current) and the capacitor across it (like the choke) will pass RF allowing greater AC feedback. I think that's it? You need to look at this basic config, 'common emitter amp'.
All explained rather well here, take time to read it all, it may look complicated and it's not. This is one of the most basic of electronic amplifiers and worth understanding!
http://www.electronics-tutorials.ws/amplifier/amp_2.html
To measure power you need a power meter but as they're calibrated to 50 Ohms and this circuit isn't so the meter wont work, you can measure the volts with a 'scope, multiply the peak to peak by 0.35 then divide by 50, oh you cant do that as your circuit isn't 50 ohms!
A simple but very good circuit and used in all L/C tuned bugs on VHF.
So to answer your question..... use a bigger transistor with more gain and more volts and a better antenna higher up.
What freq are you using BTW?
Str
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7.3828 Mhz.
I have it set by what I'd call a self contained crystal oscillator, small box with 4 legs.
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OK, have sent you a PM.