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A two-way radio repeater takes weak and low-performing signals and retransmits them at a higher power so they can cover longer distances, ranges, and terrains without degradation. Repeaters eliminate unwanted noise and interference, helping to clarify messages as they are strengthened and re-transmitted. When properly installed, radio repeaters provide reliable communication signals from one radio to another, almost completely eliminating dead zones.
By definition, a repeater is both a radio receiver and a radio transmitter, a device that receives an analog or digital signal and amplifies and transmits it further than it would otherwise be able to go. Repeaters are commonly used by emergency responders, commercial organizations and amateur radio operators (know as HAM radio operators) to extend frequency ranges from one receiver to another. The most basic repeater consists of a receiver on one frequency and a transmitter on another frequency, usually in the same radio band (i.e. UHF or VHF), as well as one or more antennas. They may also require amplifiers, isolators and other accessories.
A Brief History on Radio Repeaters
Military communication units have been using radio repeaters for many years to allow command posts to transmit both encrypted voice and data signals across hundreds of miles, while allowing for a forward moving regime. It was this type of technology that inspired the radio repeaters on the market today, which are far smaller, more durable and less expensive.
Military radio repeaters require vehicles to haul bulky and heavy equipment and provide a heavy-duty power source. They involve the installation of very tall antennas constructed by entire communication teams. Using a 50-ft. antenna, military radios can carry a signal for a maximum of 35 miles. At least a dozen repeater sites would be required to carry a message 500 miles.
Modern radio repeaters, on the contrary, are compact, user friendly, and easy to install. Many of them are smaller than a briefcase, weighing just a couple of pounds. Tough-to-cover areas are no longer a problem with a radio repeater because range and coverage limitations are eliminated. Some repeaters promise coverage for a hundred miles or more with the proper installation.
Types of Radio Repeaters
There are different types of radio repeaters, each designed for specific uses and situations. Repeaters strengthen and sustain both UHF (Ultra High Frequency) and VHF (Very High Frequency) signals, especially in rugged terrain and over water. Most repeaters available for businesses today use UHF frequencies.
A repeater uses two frequencies, a transmit freq and a receive freq. It receives signals on one frequency and re-broadcasts them on another frequency. For a two-way radio to work with a repeater, it requires a radio that enables the programming of separate transmit and receive frequencies that match the repeater.
Low-power repeaters are used for onsite communications, with antennas placed at a low level. These are generally used for areas as large as a small town or for a campus or building. These systems may only have 2 to 5 watts of transmitting power.
High-power repeaters are placed atop tall towers or hilltops to maximize coverage areas. These systems allow users with low-powered, two-way radios to communicate with each other across many miles. These systems may contain as much as 100 watts of transmitting power.
In a digital communication system, a repeater takes a transmitted regenerates it and sends it along to the next receiver station. A series of repeater sites make the extension of a signal over an incredibly long distance a possibility. Digital repeaters are able to eliminate the unwanted signal, a digital signal, even if it is faint or unclear, can be completely restored. Analog signals, however, are strengthened with amplifiers, which, unfortunately, often amplify noise as well as the audio information.
In communications systems, a repeater consists of a radio receiver, a transmitter, an amplifier, one or two antennas, and an isolator. The transmitter produces a signal on a different frequency than the received signal. This is called a offset, which is required to prevent the transmitted signal from disabling the receiver.
For more complex installations, an isolator in line with the antenna cable provides additional protection. An isolator is a one-way band-pass filter that reduces the ease of signals from nearby transmitters going up the antenna line and into the base station transmitter. This prevents the unwanted mixing of signals inside the base station transmitter which can generate interference. An isolator also reduces the transmission of undesired signals. Isolator circuitry may be built right into the lower power repeaters.
A repeater, when strategically located on top of a high building or a mountain, can greatly enhance the performance of a wireless network by allowing communications over distances that would otherwise be impossible to cover.
Some organizations now use all digital systems. Unlike analog signals, digital signals need to be more frequently repeated. Because digital signals scatter more quickly than analog signals, amplifiers are often needed. While analog repeaters are spaced at about 18,000 meter intervals, digital repeaters are usually placed in 3,000 to 6,000 meter intervals.
Installing a Radio Repeater
Installation of two-way radios can range from somewhat complex to incredibly easy, depending on the configuration. High-power repeaters usually require two antennas, one for receiving and one for transmitting. Installation of this type of repeater is complicated and not recommended for the average, untrained user. The typical repeater for shorter distances, however, requires only one repeater antenna, which transmits and receives to two-way radios, usually hand-held devices. These are relatively simple.
In two antenna installations, placement of the antennas is critical to prevent the receiving antenna from taking in energy from the transmitting antenna. The antennas are placed at different heights to minimize this interference.
To make installation easier, some repeaters have what's called a built-in or add-on duplexer that allows the unit to transmit and receive on the same antenna at the same time. Essentially, the duplexer contains circuits that isolate the transmitter from the receiver. This way, the transmitter’s radio frequency doesn't damage the receiver.
Regardless of the use of one or two antennas, a high-performance antenna is usually positioned at the coverage area’s tallest point with the repeater. When strategically located at the most elevated point of a communication site, height greatly enhances the overall operation and performance of a repeater’s signals. Antennas are preferably mounted with line-of-sight to all repeaters or other two-way radio users.
Before installing a repeater, a site survey and radio coverage test are vital to its proper performance. This type of testing and level of planning becomes helpful as it saves time, money and resources. Investing in these recommended practices on the front end can essentially eliminate poor equipment performance and ineffective communication later, and possibly when it matters most.
Repeater Site Survey
Conducting a radio coverage site survey and finding the right installation location is one of the most important steps in setting up a radio repeater. Positioning of the antenna is critical to the overall success of the radio communication that is depending on the repeater. Finding a prime spot is essential.
Choosing a place for a repeater should be relatively easy. Line of sight is very important in radio communication. Trees, electrical towers, hillsides and other dense structures or objects can impede signal transmission. That's not to say you must have line of sight. Just know that if you don't your range will be reduced.
Once a general area for the repeater has been identified, several options should be considered as potential antenna locations. This allows for revision to any plan negatively affected by unforeseen obstacles that might ultimately hinder signal performance.
Keep these criteria in mind when selecting the site for your repeater antenna:
Radio Coverage Test
Once a site has been decided upon, conducting an RF (radio frequency) coverage test is the next logical step in the repeater installation process. An RF coverage test eliminates the possibility of settling on a poor installation location and identifies adverse environmental conditions that might affect the repeater’s performance.
A coverage field test should be conducted at whatever site is selected as a repeater location. The objective of the field test is to replicate the quality and coverage of signals transmitted by the repeater from a given location. This should be completed before any permanent installations are implemented. It is always good to try several options to find the best location for an antenna to ensure maximum performance of the repeated signal.
Following these quick and easy steps for a standard coverage test:
Go to the planned repeater area with two people and pair of fully-charged handheld two-way radios. Before the test, make sure the radios are programmed exactly to the same specifications –bandwidths, frequencies, codes, etc..
If you’re planning to have an external antenna installation, you should try to duplicate, as much as possible, the antenna’s positioning to best replicate the antenna’s planned height. Typically, the higher the antenna the better, but this is not always the case. If needed, position a person on a ladder or raised element to more accurately replicate the height you intend to mount the antenna. Remember, you may have to try several heights and/or locations before finding the one that works best.
The antenna (not the repeater) will be in the center of the desired coverage area. One person should take one radio and go to the most likely antenna location. This person’s communication will represent the type of coverage you can expect if the repeater antenna were installed in that location. If coverage is inadequate, relocate to a different location and repeat the process until the desired range and coverage are optimized.
When evaluating sites where coverage is needed in multiple buildings, external mounting of the antenna may be required. If you are planning to use the repeater to cover such an area, a larger external antenna mounted as high as possible is recommended. This is usually true when setting up repeaters in office complexes, shopping centers and law enforcement compounds.Every communication configuration is different, and therefore, no specific set of instructions applies when it comes to determining where to locate an antenna for optimal coverage. In general, however, remember that the antenna acts as a pivot point for all radio communication operating on a given channel. The antenna must be at the area’s greatest vantage point, which will reduce potential obstructions and enhance the distance a radio signal can travel. This allows the signal to go from any place in the desired coverage area to the antenna.
Before considering an external installation of any antenna, a site survey should be conducted and then a coverage test. In this instance, one person should be positioned inside a centrally located building in the desired coverage area the highest possible elevation. The second person should walk the site, communicating from inside every building and at all outside areas where radio coverage is desired.
Always remember when handling radio equipment – safety is paramount. Use caution when installing and operating two-way radio repeaters.
Author: David Onslow