I got a Garmin FM/AM/Shortwave radio for Christmas, and here and there I scan around and see what I can pick up. For a while I kept catching a channel out of the Netherlands, and a handful of other stations I can’t exactly place because they weren’t in English and I couldn’t listen to hear them say where they were broadcasting from .
The last week or so at night I have been picking up this station, broadcasting from New Zealand! That is probably one of the furthest places from here I have picked up stations from. It’s broadcasting on approximately 10MHz according to my little radio, which the site confirms is 9.8MHz.
Shortwave is really fascinating, both from a sociological and technical standpoint. On the human side, it allows people to broadcast all over the world, talk to people thousands of miles away, hold conversations, send data (albeit slowly – remember, you’re only at 10MHz, and bandwidth is a tiny fraction of this), and in emergency situations potentially act as the only form of working, reliable means of communication that can be used to save lives. I know what you’re thinking — the internet allows people from all over the world to connect already. Yes and no. First of all, shortwave has been going for much, much longer than the internet has been open to the public, so historically it was the first means of allowing individual people freedom to connect with people from all over the world. Besides equipment, it is completely free – you don’t get charged to use the airwaves. You have to get licensed, but there are no ongoing costs to use the frequency bands.
From a technical standpoint, one of my favorite aspects is the fact that it is in a special range of frequencies that make “ducting” and groundwaves possible.
Ducting
Shortwave is at a low enough frequency that it can reflect off the ionosphere, instead of traveling out into space, it bounces back towards the earth. This is one of the major mechanisms that allows, for example, stations in New Zealand to reach the US — like using a mirror to see around a corner, the reflections off the ionosphere allows signals to reach places beyond line of sight. This bouncing of the waves can happen multiple times, from the ground, to the sky, back to the ground, to the sky, …, and hence the name “ducting”, since it is like a waveguide. What’s really interesting is that this effect is modulated by the sun cycle and solar weather. During the daytime, the solar wind pushes the ionosphere down towards the earth, and this can cause the waveguide setup to either be in cutoff or can make it so that the reflections are too shallow to propagate very far beyond line of sight. But at night, the earth shields the sky from the solar wind, and the ionosphere is higher, allowing this phenomenon to take place.
The other phenomenon that can cause ducting is the change in refractive index in the air. As altitude increases, the refractive index decreases, and so waves travelling through this medium tend to get bent back towards the earth. Conversely there are also situations where the refractive index can increase with altitude, and the wave is bent further towards the sky into space. But both of these allow propagation of radio signal well beyond line of sight, or the horizon.
Ground Waves
It is possible to have, over a certain frequency range, propagating waves attach themselves to a ground system — in this case the surface of the earth — and propagate following the curvature of the earth instead of going in a straight line off into space as the horizon is reached. These can also be referred to as surface waves, or Zenneck waves.
Also, on top of the wave propagation aspect, it also allows people to learn hands on antenna and transmitter design and operation. It has created a whole Ham radio, or amateur radio culture that build and operate these systems.
