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Forgive me if this question is a duplicate, I looked around but did not see anything that quite fit.

The scenario: I found a box of old FM receivers, the kinds used (I think) for assistive listening or something like it, from Williams Sound, evidently. They had been sitting around for some years. I cleaned out the battery cases (the old AAs had leaked something awful) and voila, they work. But of course, there is no nearby source of 72-76 MHz signal.

In any case, for my students, I thought it might be a fun experiment to show that you can generate a radio signal with a spark gap (or something similar), and use the FM receivers to prove that it happens. I don't expect sound or anything like it; just to hear a bunch of clicks or a buzz would be fine.

We also have an old Wimshurst machine that could generate some sparks with a little TLC. (Needs some dust-off, the thing is pretty old). I wasn't sure if that could generate a signal in the right range. (I mean, you can only crank the thing so fast with your arms).

Anyhow, the point would be the show that sparks generate radio signals. Intuitively I would think that even though the receivers are set up for picking up FM signals (and they aren't analog, clearly) they should "hear" a spark gap transmitter provided it is oscillating at the relevant frequency, and even though any modulation on the signal will be amplitude (since the only thing I can vary is the current going in). I would expect to hear a "click" or "pop" in the receiver every time the spark goes off.

So the question is: is it feasible to make a spark gap transmitter in the relevant frequency range given the stuff I have (Picture a high school that hasn't got much in the way of equipment, but I have a lot of magnet wire, some breadboards, simple electronic components, and some stuff that clearly dates back to the analog era). Would such a thing be able to transmit in the 72 MHz range?

And if anyone has good ideas for setting up an experiment/demo like this I am all ears. I was just trying to find a good use for a bunch of receivers discovered in the back of a closet.

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    \$\begingroup\$ FM receivers are not good for listening for noisy signals, since their audio output is noise even with NO signals. I'd suggest a broadcast-band AM receiver. \$\endgroup\$
    – glen_geek
    Commented Feb 4, 2023 at 19:14
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    \$\begingroup\$ @glen_geek Ah I guess that's why people specify specifically an AM radio for hunting down noise. \$\endgroup\$
    – DKNguyen
    Commented Feb 4, 2023 at 19:40
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    \$\begingroup\$ Note that I heard that using spark gaps to transmit is highly illegal due to the broadband interference. \$\endgroup\$
    – DKNguyen
    Commented Feb 4, 2023 at 19:41
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    \$\begingroup\$ @DKNguyen only if you get caught :). Seriously, though, at those frequencies and for a signal just strong enough to be useful in a lab, you're probably going to be OK -- mostly because (at least in the US) the FCC isn't going to step in unless someone complains. \$\endgroup\$
    – TimWescott
    Commented Feb 4, 2023 at 21:09
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    \$\begingroup\$ Please don't demo spark gaps, nor encourage others to do so, nor teach kids to do so. If, for some reason, you feel you REALLY must screw over every RF user in the area by doing this, please speak to the FCC or your country's similar body first to get their permission, as they have big $teeth. (Similarly, please also avoid any experiments involving bouncing lasers off airplanes; boiling mercury; allowing kids to use overly powerful inductors or magnets; lockpicking; molotov creation; counterfeiting; etc. Kids already get in plenty of trouble without that kind of encouragement). \$\endgroup\$ Commented Feb 5, 2023 at 4:15

5 Answers 5

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Yes, but not as loud as in an AM receiver. This is why:

  1. Broadcast FM uses a wider bandwidth than broadcast AM. That reduces its susceptibility to electrical noise
  2. Broadcast FM works at a higher frequency than broadcast AM. The spectrum of electrical noise from a spark gap drops as frequency increases, so there is less of it in the FM band than in the AM one.
  3. FM receivers are specifically designed to see changes in frequency, not to see changes in signal level. Conversely, AM receivers are designed to see changes in signal level. Therefore, an AM receiver responds to each transition in the spark and an FM receiver does so only as a result of imperfections in its design.
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  • \$\begingroup\$ Why does using a wider bandwidth reduce its susceptibility to noise? I would expect the opposite. \$\endgroup\$
    – Barry
    Commented Feb 4, 2023 at 23:10
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    \$\begingroup\$ @Barry: averaging. If you have noise with a 1 kHz bandwidth, in AM (10 kHz bandwidth), the noise is 10 % of the signal; if FM (200 kHz bandwidth), the noise is 0.5 % of the signal. \$\endgroup\$ Commented Feb 4, 2023 at 23:17
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    \$\begingroup\$ but in the case of a spark gap, the noise is broad-spectrum so I'd have thought point 1. wouldn't help. Point 2 does, of course. \$\endgroup\$
    – CharlieB
    Commented Feb 5, 2023 at 13:59
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A spark-gap (without a tuning circuit) will generate a broad band array of radio interference and it's likely to produce some crackles in the FM radio's speaker at any unused tuning position. You could try this out with a power supply and a decent value (10 mH upwards) inductor to prove the concept. AM radios operate at a lower frequency and are better candidates BTW.

is it feasible to make a spark gap transmitter in the relevant frequency range given the stuff I have

If you want to focus the energy into a specific frequency band, then look up how circa 1910 - 1920 radio transmitters used a spark gap and a tuning coil. Keyword: Spark-gap transmitter: -

enter image description here

You should be able to pick values that work in the VHF region at short distances. Image from here.


\$\color{purple}{\text{Will an FM radio pick up an AM transmission?}}\$

Regarding whether an FM radio will receive AM is something that some people might deny but, it will and I've witnessed it. I've received an AM radio signal on an FM receiver; back in the 1970s, where I lived, some police channels overlapped into the top of the FM band (88 MHz to 108 MHz) and they were AM transmissions and were perfectly audible. So, as a budding eavesdropper/geek I used to tune in and be nosey.

And the reason why they can be received is that plenty of FM demodulators clip/limit the incoming broadcast signal and then use a frequency selective filter to turn the clipped FM waveform back into an AM signal and use a similar-to-AM-diode-detector. So, if the "rogue" AM signal were fairly low in amplitude it would pass through the limiter circuit fairly unhindered and, be AM demodulated in the FM discriminator.

So, providing you tune to an area of unused spectrum you will pick up crackles and maybe shot type noises.

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    \$\begingroup\$ +1 for schematic values in mics and jars! \$\endgroup\$
    – user16324
    Commented Feb 4, 2023 at 21:41
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    \$\begingroup\$ Thanks this helps. I was just thinking really of the old proof that sparks generate RF radiation, and a fun demo, and we're not talking a lot of power here. (I mean, it'a a single room). Your diagram is very helpful! \$\endgroup\$
    – Jesse
    Commented Feb 5, 2023 at 0:25
  • \$\begingroup\$ sneaky... so a British Mic is that the same as an American Joe? \$\endgroup\$
    – P2000
    Commented Feb 6, 2023 at 1:12
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FM radios aren't made to receive AM or pulsed RF. They are generally immune to the kind of interference that a spark gap transmitter would produce.

You might be able to gimmick up a spark gap transmitter with a circuit resonant at 72-76 MHz. It would be really difficult, though, and you wouldn't really hear much. I think a click with each discharge is about all you can expect. If the spark gap discharges with a high enough frequency, then you would hear the individual discharges merge into a buzz.

The Wimshurst machine won't do you any good. Each discharge of the Wimshurst machine makes a wide band discharge that you can hear on an AM radio. FM radios are immune to that kind of thing. When was the last time you heard a lightning bolt crash on an FM receiver? A lightning bolt is just a much larger version of the Wimshurst machine discharge.


I don't really think you could make a spark gap transmitter put out anything that an FM receiver could detect. It would be very, very difficult to make a resonance circuit for that range without a good way to test your circuit. You'd never know whether you aren't hearing anything because the FM radio can't detect it or if the transmitter isn't hitting the right frequency.

I know you can't detect a Wimshurst machine discharge on an FM radio. I just took my Wimshurst machine down to the kitchen by the radio and tried it out. AM picks it up just fine, FM doesn't. That wasn't really a surprise. FM radio's main claim to fame is that it is immune to that kind of thing.

Besides everything else, it is illegal in most places to operate a spark gap transmitter. They generate a lot of noise all across the radio frequency spectrum, interfering with licensed transmitters.

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One of the important selling points of the FM (quite a leap in complexity compared to AM) was exactly its great immunity to spark radio noise - pretty much an important property in a a world full of switching contacts and brushed electric motors.

The ideas to limit unwanted EM emissions and the actual regulation of the electromagnetic interference always lagged behind the EM noise.

One could pretty much hear the ignition circuit of a '70s or older car passing by in any AM band even when listening to a local station.

In this regard, FM is a blessing.

So in short, one could not expect to hear a spark noise in an FM receiver, unless either the receiver has grave flaws (unlikely) or the spark-transmitter is powerful enough to inject noise in the transmitter stages after the FM demodulator (illegal and easy to detect by the local enforcement agency).

On the other hand, in order to hear any distinct noise from an FM receiver, one needs a sane FM signal in the first place. Otherwise, the receiver will either trigger its no-signal noise suppression or will receive a great deal noise to start with.

Transmitting something in the 72-76MHz band itself requires a license in most of the developed world.

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I've managed to kill a cell phone signal at close range, around 1 ft, and antenna-tv/radio throughout the house.

Take a lightning globe, remove the glass globe. Connect the high-voltage wire to a screw mounted into a plastic cup, pointing at a screw on the other side of the cup. This lets you get the fine level of adjustment needed. You may need to take a piece of foil and attach it to the second screw; it seems to help sparks jump farther. For cell phones you will only notice the call drops.

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