As the name suggests, these are intended to receive (on the 'input channel') VHF or UHF signals from portable and mobile stations, and re-transmit (relay) them on a different frequency (the 'output channel') within the same amateur band. Input and output frequencies are spaced by 600kHz at 144MHz and 1.6MHz at 433MHz.
Repeaters are unmanned and entirely automatic in operation. They are situated at the top of a hill or where the antenna can be positioned as high as possible. Thus the rather limited range of portable and mobile equipment may be increased from 10-15km to something like 50km. This is indicated in Fig 11.2. This increase in range is at no cost to the amateur in respect of extra power or improved antennas.
Continuous operation of the repeater transmitter is undesirable and therefore means of remotely turning the repeater on and off is included; this is generally known as 'accessing' the repeater. In UK repeaters accessing is achieved by transmitting a 1750 ±25Hz tone (the 'toneburst'), which is approximately 0.5s long, at the beginning of each period of use of the repeater. (Some repeaters do not require a toneburst, being carrier operated, ie they are activated by the presence of a user's carrier on the input channel).
More and more in the UK the toneburst is being supplemented or replaced, by the sub-audible tone method of access. As the name implies, the tone is sub-audible, and continues throughout the transmission. Each region of the UK has its own tone specified, which avoids users 'opening up' repeaters outside the local area, unless that is the intention. By this method, a repeater user can thus 'target' a single repeater, though there may be several on the same frequency within range.
The tone switches on the transmitter and starts an internal clock which will turn the repeater transmitter off after typically one ortwominutes,before which the users carrier must be dropped or the repeater will time out. This will then require it to be re-accessed. This is known as 'time-out' and is intended to keep user transmissions short.
Repeaters operate only with frequency-modulated signals which must be of the correct frequency and deviation. The frequency and deviation are monitored continuously and if these parameters fall at any time below the standard required for valid access the repeater transmitter may be shut down.
When an 'over' is finished and the incoming signal disappears, the repeater will, after a short delay, indicate its readiness for another input transmission by transmitting either a 'K' or 'T' in morse code.
In the UK there are approximately 75 repeaters on the 144MHz band and 150 on 432MHz, and all use FM. A much smaller number of repeaters use other modes and frequencies. The current list can be obtained from the RSGB.
Recently there has been a move to link repeaters Internationally, via the Internet and a variety of methods are on trial around the UK. This makes it possible for a someone in (for instance) Sheffield (England), to communicate with a station in Brisbane (Australia) using nothing more than a simple handheld transceiver at each end of the link, their own local repeaters, with an Internet link in between.
Fig 11.2. The improved range of communication between mobile stations when using a repeater. Under normal circumstances car A would not be able to contact car B, due to the hill between them. Even where line-of-site is possible, communication may still prove difficult between moving mobile stations. A well placed repeater greatly improves both the range, and range of reliable communication between stations.
P11.1. A typical amateur repeater system. On the left, the grey 'drainpipes' are the cavity filters which prevent the receiver being swamped by the signal from the transmitter. Both transmitter and receiver have to operate at one and the same time and without the careful filtering, the receiver would not be able to 'hear' your signal on the input.
A number of satellites specifically designed for amateur radios have been launched since 1961. These have been called Oscar1, Oscar 2 etc (from Orbiting Satellite Carrying Amateur Radio).
The satellites are 'transponders' - they will accept CW, SSB or RTTY signals over a band of frequencies and retransmit them in another amateur band. Current satellites have 432 to 144MHz, 144 to 432MHz, 144 to 28MHz and 1269 to 435MHz transponders.
Accessing a satellite does not require an access tone as the satellite monitors incoming signals continuously and its transmitter is permanently operational.
Obviously the satellite will only receive sufficient signal when it is in direct line-of-sight to the transmitter. This happens periodically when the satellite, which is rotating round the earth, appears over the horizon.
The power required to use the Oscar series transponders is 80-100W ERP. This power level is usually achieved with a low-power transmitter (10-15W output) followed by an antenna with a gain of 10dB. The antenna must be pointed in the direction of the satellite to achieve this gain and so it must move in both azimuth and elevation in order to track the satellite properly. This can lead to complex antenna systems if best results are desired, although simple fixed antennas are capable of satisfactory operation if correctly designed. Information on the current amateur radio satellite scene can be obtained from the RSGB.
A brief comparison of the modes of operation of repeaters and satellites may be useful. A repeater accepts FM signals at a single frequency and re-transmits them in the same band. An access tone (1750Hz) is often required (sometimes only for the initial transmission of a contact) and the input signal must be of the correct frequency and deviation.
A satellite transponds a band of frequencies from one amateur band to another and will accept any form of modulation (CW, SSB and RTTY are preferred).
A repeater extends the user's range by a relatively small factor and the final range is still essentially local under most conditions. However, a satellite permits coverage of very long distances on VHF and UHF.
