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Radio Repeater Fundamentals
Long before cellphones and other wireless-enabled devices arrived on the scene two-way land mobile radio was king of the road. Repeaters made LMR really cover some ground.
While having undergone a fair amount of revision, the piece below was originally written some time back and the illustrations are pretty crude. Still, it sort of gets the idea across.
There are many different things called repeaters used in various fields of endeavor. There are mechanical devices, electronic and telecommunication products that might be called repeaters. This report will deal specifically with electronic devices used in two-way radio communication to extend the range of mobile and hand-held transceivers (a transceiver being a combination of transmitter and receiver in one unit) that are commonly known as repeaters.
What exactly, then, is a two-way radio repeater?
In the US, by definition, the Federal Communications Commission (FCC) calls these repeater devices "mobile relays" (or MRs) because this describes just the task they are intended to perform. A repeater will allow two or more transceivers, usually in a vehicle or hand-held, to communicate with each other over greater distances than they could strictly by themselves.
Fixed location transceivers, also known as base stations, can also communicate through repeaters. If a base station has the capability of controlling the operating state of a given repeater it is designated as a control station for that repeater.
How does a repeater work?
A repeater automatically receives and simultaneously retransmits the signals from a transceiver using it (this is the relay part of the term mobile relay). For this function to be of much use the repeater and/or it’s transmitting and receiving antenna(s) must be positioned so as to be physically higher than the surrounding terrain and other things such as trees that may impede a radio signal (see figure 1).
Why does the repeater and/or it’s antenna system need to be placed in an elevated location?
Repeaters are allowed by the FCC only to be operated at relatively high radio frequencies. Every radio frequency follows a certain behavior. The higher the a radio signal is in frequency (usually measured in millions of cycles per second, or MHz), the more that radio signal tends to follow a straight path from it’s transmitting antenna and not follow the earth’s curvature.
Note: This is most easily illustrated by the behavior of the two US terrestrial radio broadcast bands.
On the AM band, which occupies a band of frequencies centered around 1 MHz (1000 kHz), radio signals very much tend to follow the curvature of the Earth as they radiate out from their transmitter. That's the reason one can receive a reasonably powerful AM radio station over relatively long distances during the day. (During the night time hours AM radio signals may be received over even greater distances but this is due to a phenomenon known as ionospheric refraction, or skip, in which radio signals actually "bounce" off of a layer of the Earth's ionosphere.)
On the FM band, conversely, reception of a radio signal tends to be lost in a relatively short distance when compared to a signal of similar power on the AM band. This is because the frequencies used in the FM band are centered more or less around 100 MHz. This frequency range is high enough that the signals within it do not follow the Earth's curvature to any great extent. (Also, except for some occasional atmospheric bending of FM band signals there is usually no skip effect to extend a given signal's coverage.)
Frequencies above the shortwave band, from 30 MHz and up, are desirable for land mobile two-way communication because they tend to be less prone to both natural and man made interference than the lower shortwave frequencies. Thus it is from 30 MHz and above that the FCC allows the general use of repeaters (an exception being that of the amateur radio service where the frequency threshold is very slightly lower).
A point of interest: As circuit design and electronics production techniques have improved over the years, allowing radio equipment to easily operate at increasingly higher frequencies, the frequency range used in two-way communication has increased correspondingly. Two-way radio communication and other wireless devices operate on frequencies that not too many years ago were reserved only for point-to-point microwave links.
As previously indicated, at the relatively high frequencies which repeater systems are permitted to operate, radio energy travels in what is termed "line of sight" from it’s source. This means that the radio energy basically travels from its source out to the horizon then, in many cases, out in to space (what little of it survives atmospheric losses). If the source of radio energy, safely assumed to be a transmitting antenna, is very close to the ground the distance out to the horizon is very short. On the other hand, if the source of radio energy is fairly high above the ground the distance out to the horizon is much greater.
It is important to note that a given antenna's receiving performance usually closely follows its transmitting performance, which is to say that radio reception range will be extended by placing an antenna in a physically high position as well as transmitting range.
The goal, then, of the repeater system operator is to secure a place to put the repeater equipment and/or it’s associated antenna(s) in the highest spot physically or monetarily possible. The physical aspect depends on the availability of a tall hill or mountain in the repeater service area or, alternatively, a very tall building or radio tower at or near the top of which the repeater and/or it’s antennas may be placed. The monitory aspect of repeater placement deals with the fact that, due to the popularity of wireless communication services, good places for repeaters tend to command high rents or sale prices because the operators of wireless communication systems are willing to pay large sums of money so they may locate their system equipment at these sites. A site that’s good for a two-way radio repeater is typically also good for a cellular telephone installation as well, for example.
What equipment is needed to make a repeater system?
The heart of a basic repeater system contains two parts; a radio receiver and a radio transmitter (more elaborate repeater systems may make use of multiple receivers and transmitters, but for simplicity that type of system will not be considered at this time). Additionally, electronic equipment is needed to control when the transmitter is energized to prevent it from operating needlessly.
Also, the repeater needs an antenna system, feed line to the antenna(s) and associated equipment. Sometimes the repeater will use one antenna for the transmitter and another antenna for the receiver. These are usually separated vertically with the repeater equipment ideally (but often not) positioned between the two. It is also possible to use the same antenna for both the transmitter and receiver, in which case a piece of special equipment known as a duplexer is needed to keep the energy from the transmitter from interfering with the receiver.
There are also cases in which a repeater's transmitter and receiver are physically separated a considerable distance from one another to facilitate greater antenna separation and are linked together via a dedicated telephone connection or yet other transmitters and receivers.
How does all of this fit together?
Let’s say a mobile unit on one side of town needs to communicate with a hand-held unit on the other side of town. They could try to do so directly, with one unit transmitting on a particular frequency while the other listens on that same frequency (this is known as simplex operation). Chances are, if the town is of a fairly large size, one unit will either hear the other unit very poorly or not at all. If the repeater (remember, it’s also known as a mobile relay) station is located in a high spot between the two units they can then communicate through that repeater and increase their chances of hearing each other greatly.
To do this the two units must each transmit on one given frequency and receive on another given frequency (in the case of push-to-talk transceivers, where the receiver is muted during transmit, this is known as semi-duplex operation). The repeater receiver is set to the radio frequency being transmitted by the units using the system. The repeater transmitter is set to the radio frequency being received by the units using the system. The repeater system’s receiver picks up the signal of whichever unit is transmitting at the time and this signal is fed to the repeater system’s transmitter. The repeater’s transmitter re-transmits the signal its receiver is picking up, but the key is that the transmitter is operated at a slightly different radio frequency than the receiver. The repeater’s transmitter and receiver must operate at least a certain minimum value of frequency away from each other or the transmitter would interfere with the receiver. (Even with the best filtering equipment inserted in the antenna system, there is still a practical minimum frequency separation requirement between the repeater’s receiver and transmitter.) See figure 2 for an illustration of the repeater’s use of two frequencies.
The above drawings illustrate a simple, straight forward repeater setup. As mentioned above, a repeater system can also be arranged so it’s transmitter is at one site and it’s receiver resides at another, the two being linked by yet more radio equipment or hard wiring (usually via the local telephone network). This would be known as a "split site" repeater. Repeaters can also be arranged as to have multiple receivers located at various points in its coverage area with additional electronic equipment that causes the best signal among the different receivers to be retransmitted.
Repeaters can also make use of multiple transmitters placed at various locations within the system’s coverage area, all retransmitting the same signal at the same time. These are known as simulcast systems and the different transmitters' output frequency must be precisely matched to one another. To further complicate the picture, there are numerous repeater systems that operate in what is known as "trunking" mode that spread two-way conversations over multiple repeaters on multiple frequencies in a sort of first-come-first-served rotation. These are advanced repeater systems well beyond the scope of this paper.
To sum up:
A radio repeater system can be anywhere from very basic to highly advanced, but the purpose of any of them is to facilitate and extend the range of two-way communication between mobile or portable users of a given service area.
Some further items of note:
In the United States the small, relatively inexpensive UHF hand held transceivers (walkie-talkies)sold in blister packs in many retail outlets are designed and intended to be operated in a direct transceiver-to-transceiver manner on the same frequency (known as simplex operation). While good for short range communication, they are by intent not able to be operated in a split frequency, semi-duplex mode and thus can not extend their reach by operating via a repeater.
Repeaters are typically used by commercial and public safety personnel and hand held transceivers meant to operate through these systems are of a more robust specification than the store-bought blister pack walkie-talkies. These repeater systems must be licensed by the FCC and users of a given repeater are usually covered by that license.
However, PROPERLY LICENSED United States radio amateurs are permitted by the FCC to own and/or use repeaters operating within their assigned frequency bands. Amateur radio transceivers are available through certain retail outlets that can access repeater systems on frequencies from 29 MHz (the upper end of the ten meter band) to 1200 MHz. Repeater operation has even been known to occur in the 2400 MHz amateur radio band, though equipment to do so is rather hard to come by.
A great deal of information regarding amateur radio may be found on the website of the American Radio Relay League, or ARRL. The ARRL is known as "The National Association for Amateur Radio" and invests a great deal of effort in facilitating the amateur radio hobby in the US.
By DFH (amateur radio service call-sign N8NTO) from a time long ago. As of January, 2007 this report has been revised from its original form. As a somewhat humorous aside, this piece started out pretty much as a rewriting from memory of a freshman college English paper this author produced. The course instructor expressed concern that the submitted assignment may have been a recycled ROTC report. This author had to assure said instructor that, no, the paper was written specifically for her class and this author had never been an ROTC member. One can see where an officer candidate might want to know about repeaters, though.
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