Radio relais are located in exposed locations and are used to transmit the received signal again in order to cover a larger transmission area. Such stations are typically installed on hills or mountains. But it can also be other towering positions in the landscape.
A frequency repeater, or relais for short, is an automatically operating radio station that receives a signal on frequency "X" and sends it on frequency "Y" at the same time. Accordingly, in order to be able to talk alternately via such a station, a radio device is required that receives on the frequency "Y" when listening and transmits on the frequency "X" when speaking. The distance between the two frequencies is called “Ablage” in German and “shift” in English. If a frequency converter is in a favorable position, it can have a large coverage area. In the case of an emergency call, you can hope that someone will “pick up” the radio message and react accordingly.
So that a relais does not continuously send noise, its receiver has a squelch, which only opens when a signal is actually received. As has been shown, this measure alone is often not enough. Intermittent interference can open the squelch. As an additional protection, many converters require a code to be sent from the received signal. If this is correct, the squelch is opened. A widely used analog system for this is CTCSS, a similar digital system is abbreviated to DCS.
In order to increase the range of the radio link, a so-called “simplex repeater” or “parrot repeater” can be placed between two stations. Technically, this is probably the simplest solution. In principle, it is a radio device that stores the received message on a chip and then sends it out again. The participants take turns talking and listening to the memory chip. This happens on one and the same radio channel. After getting used to it, you get to grips with the "parrot" very quickly. This option can be of particular interest for PMR radio, as they only work on one radio channel.
Some converters are also connected to a gateway and thus reach a telephone line or the Internet.
The terms repeater and transponder are not clearly delimited from one another. Sometimes a converter that transmits on a different frequency band than it receives (e.g. 145 MHz to 431 MHz) is called a transponder or crossband repeater.
Relay stations attached to balloons were also used from time to time.