All what you should know about repeaters in amateur radio

A repeater is a combination of a receiver and a transmitter that receives signals and retransmits it so as to cover a longer distance. Usually repeaters are located at high elevations so that two stations located at lower elevations can communicate with each other through the repeater at longer distances. Most of the repeaters are for VHF and UHF frequencies, while there are some repeaters for 10m and and some for higher frequencies. In my region, all the repeaters are for VHF while there are a few UHF repeaters in other regions of the country. Usual communication on VHF, UHF and higher frequencies are limited by line-of-sight.

Hence repeaters are needed for long distance communications on this frequencies. As an example, I am able to communicate easily through VU2MJJ repeater all the way from Kasaragod about 180 km North to Kottayam, which is about 200 km to the South. Without the repeater, maximum distance I can contact is usually about 20 km, though sometimes it might be possible to work up to about 40 km. VU2MJJ repeater is at about 30 km from my location, situated on hill so that people in my region are able to access it even with a small hand held radio. So with the help of VU2MJJ repeater, someone in my region using a handheld radio, could contact even in a radius of 200 km of the repeater.

That is about terrestrial repeaters. Repeaters can also be onboard satellites. The crossband repeater on International Space Station is located at an altitude of about 400 km and it is possible to contact over thousands of kilometers using it. I have personally contacted one near New Delhi, at a distance of around 2000 km.

If you think of the repeater onboard the Geostationary Amateur Radio Satellite Qatar Oscar 100, the range is much more as it is located at around 36,000 km altitude. Person from Brazil can contact someone in Thailand as the footprint of the satellite is quite large. But these require much higher setup and not just a handheld radio as in case of local VHF repeaters.

Coming to 10m repeaters which work on HF, one person in Germany has worked large number of countries with a 3W radio and a vertical antenna, sitting on the balcony of his home as he had a 10m repeater nearby. There is also one VU ham who has been able to access 10m repeater in Australia and have a few contacts through it, with a directional antenna. But most amateur radio operators are confined to terrestrial VHF or UHF repeaters on those bands. https://www.repeaterbook.com/ gives a global list of amateur radio repeaters in a region wise manner, which is easy to locate. Frequencies and other details needed for access are also available on that site.

A terrestrial repeater works on two frequencies, mostly on the same band. Satellite repeaters are are usually crossband, like the amateur radio on international space station, which receives in VHF and transmits on UHF. These are called the uplink and downlink frequencies. In case of a typical terrestrial repeater like VU2MJJ, in my region, it receives on 144.850 MHz and transmits on 145.450 MHz, the difference in frequencies is known as a negative shift of -600 kHz. Some repeaters may have a positive shift like VU2RFF which has a positive shift of 600 kHz. UHF repeaters and higher band repeaters naturally have higher shifts.

Crossband repeater on International Space Station has uplink on 145.990 MHz and downlink on 437.800 MHz. Qatar Oscar 100 has uplink on 2.4 GHz and downlink on 10 GHz band. The difference in frequencies is needed because the repeater receiver and transmitter work at the same time. So in order to prevent the transmitted signal from overloading the receiver, significant difference in frequency is needed.

Even with a shift between transmit and receive frequencies, repeaters need special equipment for prevention of receiver overloading and whenever possible, two antennas separated by a physical distance, for transmit and receive sections. Duplexers with large cavity resonators are used to prevent overloading of the receiver side of the repeater in case of VHF, UHF and 10m repeaters. In case of cross band repeaters, much smaller circuits known as diplexers are enough to prevent overloading of the receiver from the transmitted signal.