Solid State Tesla Coil AM Radio Transmitter
(try saying that really fast!)
Everybody that has played with or designed a Tesla Coil knows that it is tuned circuit, eg an inductor and a capacitor, which resonate at a specific freqeuncy, determined by the winds in the inductor (secondary in a TC) and the capacitance (topload in this case). But, did you also know that a Tesla Coil is a very good radio transmitter?
The technique is quite straight forward, produce a continues wave at a desired drive frequency suitable for your secondary, modulate this drive frequency with an audio signal to produce an amplitude modulated signal (both of the above can easily be done with the TL494 PWM IC) and feed this into the primary windings.
The drive frequency doesn't have to be in the receivable band on an AM radio, we can use an effect in radio known as harmonics. Multiples of the drive frequency will appear higher up in the frequency spectrum. Eg. Drive= 182Khz 2nd harmonic=364Khz, 3rd harmonic=546Khz. The 3rd harmonic of the frequency will ALWAYS be the strongest. 546Khz is within the AM band on a common radio receiver, so this is adequate. There will also be harmonics at regular intervals on the odd numbers much higher up (so bear this in mind, as you may also be broadcasting up in the megahertz!).
I won't go into circuit details, because they are identical to the SSTC driver setup and Plasma Sonic Speaker driver.
Things to note:
- No additional antenna has been used on my setup. All rf signals proporgate from the resonator itself.
- The drive frequency is tuned for resonance, 182Khz. Although I have found that if you go slightly out of resonance, the range is not severly affected.
- Power levels used have been 12V at 600mA (7.2W). The radiated power level is approximately 6W due to heat loses etc. The field is quite strong at this power level and easily illuminates the fluro tube on to of the coil.
- Modulation level (controlled by the potentiometer in the bottom left of the picture) setting is crucial for best range and "intelligable radio reception". I have it set to about 1/2 modulation, about 5K.
- Audio volume input to the PWM controller is crucial for drive power, so this will need to be experimented for your setup. I use an mp3 player with a headphone output jack, volume set to about 3/4 full.
- FCC limits! Please be aware that you are broadcasting in a private radio frequency (I don't know how it can be called a private radio frequency as radio is everywhere, naturally, but anyway). Limit your runtimes to under 10 minutes, so as not to disturb people's receptions or be traceable.
Range currently achieved:
With the above power levels and a perfectly tuned system, I have achieved about 2km.
Not amazing I hear you say, well consider that the transmitter is setup on a workbench, indoors with the windows/doors closed and being situated at sea level, and with NO additional antenna. I think this is pretty good from a resonator that is only 40cm high!
I have also run this setup on a 44V supply, with a similar tuning setup, but range was not greatly increased. Power level was about 25W. It is still unknown as to why no improvements in range appeared.
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© Oliver Hunt 2006-2011