Radio waves work by radiating energy through space (it doesn’t need air, it just needs space). Radio stations work by radiating a specific frequency of radio waves. For instance, if your radio station operates at 590 on the AM band, it means that the station is radiating energy at a frequency of 590 kHz through space. In order to receive the signal from that particular radio station you need two things: an antenna, and a receiver that allow you to isolate that frequency from all of the other frequencies available.
How does an antenna work?
An antenna is made specifically for a particular frequency or frequency range. In the case of the AM radio, it is specifically designed to receive best somewhere right around the middle of the AM band. It’s total effective length is some fraction of the wavelength of the frequency. For instance, the wavelength of the 590 AM station we discussed earlier is 508 meters long. So, at the speed of light, in the time that the wave starts at zero, go to full positive amplitude, back through zero, to full negative amplitude, then back to zero, the total distance it would have travelled is 508 meters, or 1668 feet. This means that to receive that frequency, you need an antenna that is effectively that long, or some exact fraction of that. Fractional sized antennas are typically 1/4 wave, 1/2 wave, 5/8 wave or full wave of the wavelength, depending upon the frequency in question.
For our 590 AM station, a 1/4 wave antenna would have a length of wire of around 417 feet. Now, that’s obviously not the actual length of the antenna in your AM radio. Typically those can use an even smaller fraction (1/8 or 1/16 wave), and that’s only and effective length, meaning that other components (a loading coil or inductor) are actually making up the difference via electrical mechanisms such as ‘phase delay’. It’s not entirely relevant to this article, so I won’t get into detail, but suffice it to say that an antenna is a length of wire very suited to producing an electrical current when exposed to an alternating electromagnetic field (a radio wave). As an end-note to this paragraph, a typical single-coil guitar pickup has between 3000 and 5000 feet of wire on it, however it’s configured in a coil form with a ferrous core, which makes it an inductor, which means on the whole, it’s a very effective antenna.
What’s the receiver do?
Now that we understand that the antenna is the actual thing that detects a particular signal, what is it that the receiver does with this? The thing to understand is that although the antenna is “tuned” to the frequency to receive, an antenna by itself is very indescriminate. The antenna will receive a very wide range of frequencies, but it’s range is centered on what’s known as it ‘resonant frequency’. The resonant frequency is the one that it’s particular lenght is tuned for. So, how do I get just that 590 AM or 590 kHz frequency without anything else? You use a receiver, which filters out the adjacent frequencies using various mechanisms. The major component of this sort of filtering is what’s known as a bandpass filter. The bandpass filter can be a single filter mechanism, or it can be a combination of a high-pass filter and a low-pass filter, where the resulting overlap is the desired detection frequency. Basically, that’s all a reciever does – filters out everything above and everything below the desired frequency. Mind you, that’s a very simplifed description, but it covers just enough so that we can move forward.
And now the guitar part…
So, when you apply what you’ve just learned to your guitar, it’s now easier to see why you’re getting all that noise. In the way that your guitar has been built, it’s a very effective, albeit weak, wideband receiver. Given that the strongest radiated signal in your home comes from either your wiring or some other source of electrical device, that’s what you’re most likely to hear when you amplify it.
Some of the sources of that hum in your home or at a gig can be things like light switches, flourescent lighting, electrical motors (in refridgerators, air conditioners, sump pumps, etc), or possibly even from the transformer or wiring out on the street. All of these things will radiate energy and noise (known technically as EMI or RFI) in exactly the same way that a radio station transmits a signal, except the the energy radiated is not typically on a narrow band like the 590 kHz station. It’s more than likely across a relatively wide band comparitively, along with various “harmonic” frequencies. Harmonic frequencies are just like the ones in music, in that frequencies that are multiples of the original frequency can resonate along with the primary frequency. All of these things contribute to that “60-cycle hum” you hear so much about, and answers the question “how my Guit is like a radio”!