Heterodyne

I just had some first-hand experience with some very wideband heterodyne mixing in my radio shack. (For those not in the know, a heterodyne is where you mix a signal at one frequency with a signal at another frequency to derive a new pair of frequencies that are mathematically related.)

My 2-meter radio was tuned to 146.940MHz, a local repeater W5KA, squelched down to silence. I was assembling a 10-meter dipole inside my apartment and working on trimming the ends for resonance. As I swept my MFJ-259 antenna analyzer around 28.718MHz, the squelch on my 2-meter radio briefly popped open with some random noise. As I scanned around and centered on that frequency, what I heard was piano music, classical piano music. Huh!

My apartment is line-of-sight with the range of broadcast towers over the river in Westlake, so it’s highly likely that what I was hearing was bleed-over from one of those stations. Sure enough, the local classical music station, KMFA, broadcasts at 89.5MHz on the commercial FM dial. So I was getting some heterodyning.

146.940 – (28.718 * 2) = 89.504

Wowza!

So why that station? Why not the others, which broadcast at higher power? Might be that my 10m dipole had hit a resonant spot and pumped enough of the signal from the analyzer into the nearby J-pole antenna connected to the 2m radio, raising the noise floor enough to open the squelch.

Also, why does the mixing frequency 28.718MHz have to be doubled? Is there a harmonic coming out of the antenna analyzer? Is it a triangle wave instead of a sine? Is KMFA itself sending a harmonic at 118.218MHz? (That’s in the restricted Air Band!) Is it because the 2m J-Pole was perpendicular to the center point of one leg of the 10m dipole? Is that where the doubling comes into play?

Radio is perplexing, and I love these mysteries.

Really, heterodyne mixing is used all the time in almost every radio device. It’s an important tool used to convert one frequency into another; you simply generate a constant modulated signal at one frequency (the “intermediate frequency”, or IF), and then use a variable mixing frequency to generate a heterodyne signal at your desired transmission frequency before sending it to the amplifier for transmission.

It’s all magic. Magic!