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Amateur Radio Repeaters
- a overview or summary about amateur radio or ham radio repeaters, typically used on the VHF and UHF bands to improve coverage for portable, mobile or other low power stations, and a description of how they operate and they can be used.
Amateur radio repeaters are now a very well establish part of the amateur radio scene. These amateur radio repeaters provide a very valuable serve to many stations, and in particular those that do not have particularly good locations or those amateur radio stations that are using only low power.
A repeater, whether an amateur radio repeater or even a commercial repeater is a station that receives a signal on one frequency and simultaneously re-transmits it on another. As the repeater is well sited and has a good coverage it means that any station which can transmit into the repeater will be able to be heard by any station which can hear the repeater. Effectively it gives the low power amateur radio station the same coverage as that of the repeater. In this way radio hams using small handheld transceivers, or those using mobile equipment in a motor vehicle are able to make many more contacts than they would otherwise be able to do.
Although in theory amateur radio repeaters can be used on any frequency, their use is generally limited to frequencies above 29MHz. There are some ham radio repeaters on the Ten Metre band in the HF portion of the spectrum, but most repeaters are to be found on the VHF and UHF amateur radio bands, where propagation usually limits distances achieved to the line of sight or a little more. On these frequencies good well placed antennas and high locations are of particular importance, and stations using low powers and small antennas from poor locations have difficulty making contacts.
Amateur radio repeater operation
Although ham radio repeaters are in essence only a relay station that receives a signal on one frequency and retransmits the audio on another, they have a number of automatically controlled functions to ensure that they operate efficiently and do not cause any unnecessary interference or radio spectrum pollution. This means that when operating through an amateur radio repeater it is necessary to have a basic understanding of their operation so that it can be used satisfactorily.
Although the actual operation of a repeater will vary from one repeater to the next, and dependent upon the country in which it is operated, and sometimes the band which is being used, in general the overall concepts are the same. However, before venturing on to use a particular amateur radio repeater, it is best to monitor the station first to understand exactly how it operates.
Initially when a repeater is not in use it will not radiate a signal. This will conserve power and also reduce possible interference. Amateur radio repeaters are also designed so that spurious signals and noise should also not cause the repeater to transmit. In order to "open" the repeater up there must be a signal on its receive or input frequency. This signal must be sufficiently strong for re-transmission and it must also have a tone to identify that the station being received at the repeater wants its transmission to be re-radiated. There are two methods of achieving this:
Both of these systems will be described in greater detail later on.
Once the repeater has been accessed, the audio from the incoming signal will be re-transmitted on the output frequency of the repeater. However, if the signal falls below the required level the repeater may stop the signal from being re-transmitted. Also many repeaters have a time out facility. This monitors the time a signal has been relayed and if a certain time has been exceeded then the repeater will go into a busy or time out mode and stop re-transmitting the signal. This facility is included on many amateur radio repeaters to discourage people from talking too long on the repeater, and thereby enabling more people to use the repeater.
When a transmission is complete the repeater will detect that the signal has disappeared from its input. After a short delay many amateur radio repeaters will transmit an audio Morse character as an invitation for the next station to transmit. This character is often "K" - the Morse code character that is used to invite people to transmit. Other letters may be used to indicate the CTCSS tone to be sued for access to the repeater. After the end of one transmission the timers are re-set and a new transmission can start. However, this time no tone burst is normally required to access the amateur radio repeater.
Once a contact has been completed and there are no further transmissions appearing on its input, the ham radio repeater will close down. Before any further transmission can be made it will have to be re-opened.
Ham radio repeater offsets
In order that any repeater can receive and transmit at the same time, it is not able it must use different frequencies for the transmit and receive channels. In addition to this the channels must be sufficiently spaced apart so that the transmitter does not impair the performance of the receiver. To achieve this there must be sufficient offset between the two channels used by the amateur radio repeater. In view of the way filters operate, spacing in terms of an absolute frequency difference can be less at lower frequencies than at higher frequencies. Also the width of the various amateur bands plays a role in determining what offset should be used.
It is also important that the receive / transmit offsets for amateur radio repeaters are standardised so that they are easier to use. Accordingly there are a variety of offsets standardised for the different amateur radio bands in different areas of the world. Some are given below, but it is worth checking any other standards with the local websites and data if they are not included.
* positive offsets are sometimes used, esp in N Calif
Repeater tone bursts and CTCSS
The use of a straight tone burst at the beginning of a transmission is the simplest way to access a repeater. It consists of a short tone (less than a second) at the beginning of the first transmission to open the repeater). The standard frequency generally adopted for this is 1750 Hz in Europe. A margin of around 25 Hz either way will normally access a repeater although it is wise to maintain the tone burst frequency more accurately than this.
With the rise in the number of repeaters, channels have to be re-used relatively frequently. As a result it is sometimes possible for a station to access more than one repeater at any given time. This is obviously not desirable and a system known as CTCSS (Continuous Tone Coded Squelch System) has been devised to overcome this problem.
CTCSS uses a sub-audible tone transmitted on the signal of the transmitter trying to access the repeater. The amateur radio repeater has a very sharp filter to detect whether the exact tone is present. It will also remove it before transmitting the required audio. Only when the correct CTCSS tone is present will the amateur radio repeater be enabled and it will allow audio to be re-radiated on the output channel signal.
Using amateur radio repeaters
Activity on amateur radio repeaters is often very high. This means that it is necessary to maintain high standards of operating and, in particular, not hold the repeater for too long. There are also a number of procedures that are special to repeater use.
The first is to note that CQ calls are not made through repeaters. Instead stations announce that they are "listening through" the repeater. This can be done quickly and it is quite sufficient to enable other stations to hear anyone who is calling and then to reply.
Once a contact has been set up it is quite possible that both stations find they can complete their contact without the use of the repeater. This is particularly true, for example, when two mobile stations are moving towards one another. If this is so then the repeater should be vacated to allow others to use it.
In addition to this, as repeaters are intended mainly for mobile or portable stations, fixed stations should only use them when absolutely necessary and priority should be given to mobile or portable stations.
When actually making a contact through a repeater it is necessary to be careful not to time out as many repeaters have a time-out mechanism. To prevent this happening transmissions should be kept to about a minute or two. Another point is to ensure that the repeater recognises that a transmission has ceased. If this is not done then the repeater may assume that there has been no change in transmission and is may time out.
Finally, even though all amateur radio repeaters follow the same basic rules, they will vary slightly according to the country they are in, the local conditions, and also the team who designed it. Therefore it is always wise to listen to any amateur radio repeater for a while before actually using it. If this is done few problems should be encountered.