Last year, I was given a Zenith Trans-Oceanic multi-band radio as a birthday gift. AM/FM/Shortwave/ Longwave. The FM works fine, as does the standard AM band (medium wave), and I found a bit on shortwave. Now, I’d figured the longwave band worked simply as all the other AM bands work and it’s a matter of tuning circuitry (passive components) being switched.
I assembled a cheap signal generator kit to hopefully produce something the radio could pick up in the longwave band. For this radio, that band is from just under 150 KHz to just over 400 KHz. It seemed to work, but the function generator produces a simple signal. For AM, it’s be just the carrier wave – no modulation. This doesn’t work very well for AM. It’s not even really noticed unless there is some other signal it can mix with and generate an audio beat frequency. I did manage to work out that the radio was indeed working in that band, as I’d hear the ‘swoop’ as I adjusted frequency. And that was it. There is a way to amplitude modulate the signal, but the little source is just too unstable to be all that useful. Fortunately it was quite inexpensive.
Ah, but there some transmitters down in the “basement” of the radio spectrum, that are sending AM. Not entertainment, no. They are Non-Directional Beacons (NDB’s) at airports. They send simple short sequences of Morse code, but as tones for AM, keeping receivers simple (and letting pilots concentrate on things other than the radio). The local airport has such a beacon, and I’ve been listening to it for a while (an hour or so. I finally have a night off that is staying such). It’s dull, admittedly. Just a few letters of very slow Morse. It is perhaps 3 words-per-minute. But it’s not meant for the usual sense of communication. It’s “I am here, and I am this.” as it were. The code is slow so a pilot doesn’t need to learn and know Morse code, but can compare the sounds to marks on a chart. Folks who do know Morse have been known to confound flight instructors by not needing to check the chart to decode the signal.
While the beacons are not directional, the AM receiver is. I had set the radio on my desk at almost 90 degrees from how it needs to be oriented to properly pick up the signal. Oops. Also, with computers nearby, there’s a lot of local radio noise that must be dealt with. When I tried this last year, it might also have been that Later Spring and Summer weather is not kind to lower frequency bands. More storms, and lightning has significant energy in the lower bands. Storms need not be local to be troublesome.
There were never any longwave broadcast stations in the USA, but there were several in Europe. Most of those are now gone and what remain seem to be being phased out. They transmit for limited hours, and often at lower power than once upon a time. BBC Radio 4 will be ending its longwave transmissions sometime this year, if they have not stopped them already.