What decides range isn't the transmitter's power. It's the frequency, and almost nobody knows that.
UNDERSTAND
A radio wave behaves differently depending on its frequency, and that difference matters more than anything else.
Low frequencies follow the ground and bend around obstacles. They carry far, cross hills and go through buildings. That's the domain of long wave and medium wave: a handful of transmitters cover a whole country. It's also why they're the channel chosen to broadcast safety information, and what the receiver described in Building a radio receiver picks up.
Intermediate frequencies, called shortwave, have a unique property: they bounce off the ionized layers of the upper atmosphere and come back down very far away. Modest power can be heard thousands of kilometers away, jumping over the terrain. The price is instability: propagation changes with the time of day, the season and solar activity. At night, they carry much farther than by day.
High frequencies travel in a straight line and bend around almost nothing. A hill, a building, the curvature of the Earth stops them. That's the domain of walkie-talkies and vehicle radios: real range is counted in kilometers, sometimes in hundreds of meters in a city, whatever power is advertised on the box.
The rule that sums it all up: the lower the frequency, the farther it carries and the better it gets around things; the higher it is, the more you need to be able to see each other.
ACT
- To receive general information, look for medium wave and long wave first. They work when everything else is down, and they cover huge areas.
- Listen at set times rather than all the time. It saves batteries and gives the day a structure.
- Improve reception before increasing power. A long wire attached to the antenna, a good ground, and height do far more than a more expensive set.
- Keep away from large metal masses and electronic devices, which produce a lot of electrical noise.
- To communicate over short distances, go up: gaining height increases range far more than gaining power, since the obstacle is the horizon.
- Write down what you hear, with the time and the frequency. One isolated piece of information is worth nothing; cross-checked over several days, it sketches a situation — see Recueillir de l'information.
- Save batteries. Volume at minimum, listening in time slots, and a reserve kept for a use you've decided on.
ADAPT
All you have is a car radio. It's an excellent receiver, with a decent antenna and a comfortable power supply. It draws little with the engine off, but keep an eye on the battery — see Les batteries.
You want to listen far away. Shortwave, at night, with a long wire and a good ground. It's the only simple way to hear what's being said at very long distances.
You have to coordinate a small group. Low-power sets are enough if you stay in sight of each other or close by. Agree in advance on a channel, a fallback channel, listening times and short messages. The protocol matters more than the equipment.
You want to stay discreet. Receiving is completely passive and can't be detected. Transmitting gives away your location. That asymmetry should govern everything: listen a lot, transmit little, briefly, and at an agreed time.
You have no set at all. A receiver can be built, and it needs no power: see Building a radio receiver.
Emergency frequencies. There are international frequencies reserved for distress, monitored around the clock at sea and in aviation. They're not to be used for anything else: transmitting on them without need keeps someone else from being heard. It's the only rule that really matters when you use a radio.