Hi everyone. I don't have an oscilliscope yet (im planning 2 buy 1), so until then, i have to rely on very crude methods of trying to find my base frequency (trial n eror). Neway, in a good 9 volt transmitter, what kind of harmonics should I get? Should they be really crackly? or clear? Also, how far away should i be able to hear them? Lets say that my transmitter transmits 1/4 a mile. Thankee

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Hi Zach,
If your 9V transmitter is for the 88MHz to 108MHZ FM broadcast band, then it will have an output tuned coil/capacitor that will filter any 200MHZ, 300MHz, 400MHz etc. harmonics down to nearly zero output.
Harmonics of the carrier frequency are not received on your radio. They just cause interference on the frequency of each harmonic.
When you talk of "crackly harmonics" then you probably mean distorted audio, which would be caused by overloading the input of the transmitter with an audio signal voltage that is too high.

 

there is no coil to filter out the harmonics. I made the circuit meself. Here it is. I get static ALL over the dial though, but havent found the base frequency yet, wudup wit that?

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What you have is an AM transmitter but the third stage may be distorting the modulation. I think you would do better to attach the antenna to the coil. What is the diameter, length and number of turns of the oscillator coil?

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5 mm length, 4 mm diameter, and 7 turns. I found this transmitter on a website where the guy that posted it had built it and said he got 700 METERS out of it. Its supposed to be an FM transmitter, howcome its putting out AM, how do i stop this, and WHY DID IT WORK FOR THE OTHER GUY!?!?!? :evil:. I am getting so fed up with trying to build transmitters, but ive promised my friends that i will make a functioning one, and im a man of my word, lol :lol:.

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Hi Zach,
I saw your 3-transistor transmitter on the web, too. But the original author used a high-frequency transistor for the 3rd one, not an audio transistor like your BC557 that has no gain at 100MHz.
Your BC557 has about 3 times as much DC current gain as his RF transistor, so its 47K resistor's value is too low and causes it to saturate all the time. If you change the 47K resistor to 150K ohms, then the 3rd stage won't cause so much static all over the dial.
If I recall correctly, the original circuit had a high-Q tuned LC network at the output of the 3rd stage. That allowed it to produce a huge voltage swing that was much more than its supply voltage.

Of course, these simple FM transmitter circuits produce both AM and FM. Your FM radio ignores the AM component and the FM part of it will work fine.

 

The frequency should be 103 - 104 mHz if my calculations are correct. Audioguru's suggestions are excellent, if you follow his advice it should work.

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That makes sense. I am actually using 2n3904s for the first 2 stages, and a 293906 for the last stage. Does changing the 47k to a 150k still apply?

How do I do this?? Oh yeah, and one last thing, what coil should the lead going to the PNP be attached 2? Thanx so much

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Hi Zach,
Was the schematic that you posted from the author's original design? I doubt that it can transmit 700 meters.
Your 2N3906 is slighly better than the BC557 at 100MHz, but still needs its 47K resistor changed to 150K. If you use the author's PCB, then watch out for the transistors' pins, your 2N3904 and 2N3906 have a mirror pinout to the BC547 and BC557. Yours go EBC while his go CBE.

If you re-design the circuit to include a tuned LC network at its output, it would be difficult to tune both it and the oscillator.

 

hmm, ic. yes, the schematic is original. why did this work for the other guy!!!??? It always seems to work for someone else and not for me. ah well :roll: , im cursed. What is the purpose of the Cx capacitor? I'm already aware of the pin layout on the 2n3906 but thanx. Lastly, what coil should i put the lead that goes to the 2n3906 on? Thanx again

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I tried the 150k (actualy 147 k cuz i only got 100k n 47k, but im sure that should make no difference) and now i get a bunch of crackly in less places. Should I up the value again? Did i mention that the range is pretty pathetic? Only 3 or 4 meters.

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Hi Zach,
The circuit is very simple and since it operates at a Very High Frequency, depends more on its wiring layout than anything else, due to stray capacitance and inductance in the wiring.
People have reported building a quantity of circuits that are all identical, but some work well and some not.
Since the 3rd transistor operates at a low current and does not have a tuned load, it won't add any range to the transmission, but will isolate the antenna from the oscillator so that it doesn't change its frequency when you get near its antenna.

You can't just simply add a tuned load to the 3rd transistor. Then it will have an output exactly the same as the 2nd transistor. You need to change its bias to Class-C, and to operate it at a high peak current that will melt the little 2N3906. So it should be a power VHF transistor with a heatsink as well. Then the output power will be high but illegal.

Cx filters the supply voltage so that it doesn't swing all over the place at audio frequencies, especially when the battery is getting low and its internal resistance is higher. The 10nF capacitor that is in parallel with it does the same thing, but at VHF frequencies. Since the 10nF cap operates at VHF, it must be a ceramic type like most other capacitors must be.

 

well, thanx. im hopefully going to be getiing an oscilliscope soon, so i guess that i will just have to wait until then to try and fix this thing. so i dont really need this third stage then, do I? Cuz it doesnt really matter to me if the frequency changes when i get close to the antenna and stuff. Thanx

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Hi Zach,
An oscilloscope won't show much on this circuit. As soon as its probe touches the circuit it will stop transmitting. You need a Field Strength Meter.
See this 4W very illegal "pirate" FM transmitter project that uses a heatsinked VHF power transistor on its output. It has a double-sidded "ground-plane" circuit board. It has many coils to align and it eats batteries like you wouldn't believe! Also check the link at its bottom:
http://www.electronics-lab.com/proje...001/index.html

 

to that circuit above, i say up that voltage and replace all parts for the higher voltage rating, for even higher output...hmmm looks tasty

Good find Audio!

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