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I read a law for canada and it states:

Industry Canada permits transmitters that have an output lower than 100 µV/m at 30 meters (approximately 1 microwatt output).

Now what fools me are these transmitter designs and why I can't produce much output with them when transmitting at 300Mhz.

This circuit was featured under "An Improved Design" half-way down the page at this URL: http://www.talkingelectronics.com/projects/Spy%20Circuits/SpyCircuits-1.html

transmitter

I looked at it for ideas after building mine. I used the same concept but I changed some component values in an effort to increase transmitting distance.

my transmitter

But talkingelectronics.com claims I should use 330+ohms for the emitter resistor, but I think there's some math with the parts I circled in red in my circuit. I read on another section of this site that to find out the power a transistor was using, do this:

1. find voltage at base via voltage divider attached to base.
2. subtract voltage drop at base-emitter junction (700mV).
3. divide the result by emitter resistor.

The problem is I don't have a real voltage divider as far as DC is concerned, but AC wise, when I used an online calculator, the 470pF capacitor ended up having a 1 ohm resistance at 300Mhz and the base resistor is some sort of multiple of the emitter resistor?. or did I get this part wrong?

So my question is, to maximize the transmitting power possible without increasing the chances of blowing up any part, what values of the circled parts should I use?

For now I'm transmitting audio from the computer but later I'll make it a data transmitter.

I wanted to add that 5VDC is regulated and cannot be changed as this circuit is attached to another unrelated circuit.

Dave Tweed
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  • So you are not getting much output - so how do you use this circuit and what powers it? Do you have an earth connection? What does your antenna look like? How sure are you of the transmit frequency? – Andy aka Apr 26 '17 at 07:20
  • I'm using a straight 6-inch 22 gauge solid wire with 1 inch of the covering stripped off the end as my antenna and for 2.7pF my board actually has 4 switches with 1, 1.2, 1.5 and 1.8pF capacitor connections from collector to base. I turned on 1.2 and 1.5 to get 2.7pF. 300Mhz is a good approximation but I'd like to be able to transmit far at least at 300Mhz. I don't connect this circuit to actual earth, but all grounds are connected to -ve of the battery. 5VDC comes from a 7805 regulator connected to 6VDC wall supply which will later become a 9VDC battery. –  Apr 26 '17 at 15:12
  • Also, the earphone jack connection connects to any audio output source (in my case, earphone output of my laptop) and the sound travels through an audio extension cable from my computer to this circuit. It does work but I'm not getting any better than about 25 meters. –  Apr 26 '17 at 15:13

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