How Repeaters Work
March 25, 2015 - Michael Martens - http://www.jpole-antenna.com/
What’s the most popular band and operation mode in amateur radio? If you guessed 2 meter VHF FM, you would be correct. As new hams, our first radio is usually a small handheld model. This is a great way to get started in ham radio and connect with other local hams. But your handheld radio would only be a fraction as useful if it wasn’t for repeaters.Why the need for a repeater
A repeater is combination of a sensitive receiver and high power transmitter located in an advantageous spot to simultaneously receive and transmit a radio signal. Repeaters are often sited on mountains or very high towers to increase their range. Now, instead of having to use high power transmitters and large antennas, small handheld radios have unprecedented range. Repeaters increase our range and make it possible to use small handheld radios to communicate.
Repeaters allow us to do some amazing things with our radios. A handheld on simplex has a range of maybe a mile or so. A repeater can extend that 20 fold or more. Amateur radio clubs use their repeaters to provide health and welfare communication support for local events. Emergency communications groups use repeaters to stay in touch while storm spotting. Now, with using internet linking technologies, we can even talk to other hams half-way around the globe on our local repeater. The repeater, in essence is the glue the holds a local ham radio group together.Types of repeaters
Repeaters systems break down into three types: single site, multi-site receive, and linked systems. These repeaters can run either analog or digital communication modes. Analog has been the mainstay of repeater systems for decades, but the digital modes are fast catching on in both commercial and amateur radio systems. Commercial and public safety repeaters may use APCO P25 or Motorola’s MotoTRBO or another type of digital modulation, but amateur digital systems can run the gamut of P25, MotoTRBO, Icom D-Star, Yaesu Fusion, and others. For the sake of this article we’ll concentrate on analog systems, but a digital repeater is similar in operation to analog.
Single Site repeater systems are monolithic in their operation. One receiver and one transmitter operate in concert to extend the range of a radio signal. Single site systems are the most prevalent and simplest of the repeaters available.
Remote receive repeater system
The downside of single site repeaters can be their limited coverage in difficult terrain or urban environments. There may be locations were the transmitter give excellent coverage, but the receiver has a hard time picking up a weak signal. Adding remote receivers to a repeater helps pick up these weak signals and relay them to the main site for retransmission. When there are multiple receivers listening, a device called a voter will analyze all the incoming signals and select the strongest signal for retransmission. A remote receive repeater can have any number of voters to strengthen the receive coverage of the repeater.
Linked repeater systems
A linked repeater system is, in essence, a network of repeaters retransmitting the same signals. Repeaters can be linked either over the air, or via Voice over IP (VOIP) technology. Linked systems can cover a state or region, or have global connections using VOIP systems like D-Star, Echolink, or IRLP.Parts of a repeater
Repeaters are complex systems with many components working in concert. But once you start breaking it down, the individual parts are quite simple. Recently I toured the repeater site of the Wisconsin Valley Radio Association. This is a large, multi-site receive repeater system that serves a large geographic area in North Central Wisconsin. Let’s take a look at their system and build a repeater from the ground up. First off, we’ll need are a couple of radios.
Receivers / Transmitters
The WVRA’s repeaters are housed in three cabinets. This first has the T/R radios, controllers, power supplies, and amplifers.
Every repeater has a receiver and a transmitter. Repeaters operate in full duplex, that is, they are simultaneously transmit the signal they receive. The receiver listens for a signal on one frequency and the transmitter simultaneously transmits it on another. Typically these radios are commercial grade equipment designed for heavy duty cycles. A repeater radio doesn’t need to be fancy, just reliable.
In building our repeater, it would be too simple to take these two radios and connect the output of one into he mic jack of the other. Unfortunately this won’t work for a variety of reasons that will become apparent as we look at some of the other equipment needed.
The controller is the brains of the repeater system. It provides the functionality required of a repeater system. The circuitry in the controller provides the all important FCC identification over the air, listens to the receiver for a signal to break the squelch*, decodes the CTCSS sub-audible access tone, and simultaneously keys the transmitter to retransmit the signal. It also has timers and filters to help prevent spurious signals from inadvertantly activating the repeater, or excessively long transmissions from locking up the system. The controller also manages the remote links and voter system (although most voters are separate from the controller).
At some repeater sites, the controller and radios may be all in one package, or as separate devices. There is no real advantage of one style over the other, other than the size and convenience of the all-in-one package.
*Repeaters don’t use the squelch circuitry to silence the noise on the receive channel due a lack of sensitivity and slow response time to acquire a signal. Instead, the receiver’s discriminator circuit is tapped and that signal is sent to the controller. The controller activates when the microvolt level of the signal rises above the noise floor, and a CTCSS tone code is present. When this happens, the controller keys the transmitter and relays the signal out.
In addition to the receiver at the main site, the WVRA’s VHF repeater has multiple remote receive sites to fill in the holes over the rolling terrain of North Central Wisconsin. In this cabinet, the voter system (above) takes the signals received from the link sites (radios below) and selects the best signal to send to the controller for retransmission.
Our repeater now has two radios and a controller to run the show. Can we get on the air with this? Yes we can if we want to run two antennas and two sets of feed line. Some repeaters will run split-site like this, with the receive antenna mounted high on the tower and the transmit antenna at a lower level. But feed line is expensive and tower space at a premium. So repeaters use more devices to combine the signals onto one transmission system.
Amplifiers, Duplexers, Filters, and Combiners
The second cabinet holds the duplexers, filters, and combiners. There are six repeaters and three data systems at this site, but only four runs of coax going up the tower. All that RF requires quite a bit of duplexing and combining to make everything work.
As the signal leaves the transmitter it heads to a power amplifier to increase to increase the power from the 25 or so watts leaving the radio to 200 – 250 watts or more. Why so much power? Because the next set of components will eat that power up. Feed line (especially the extremely low loss stuff repeaters use) is expensive and the tower may only let us put up one antenna. In order to use that same antenna and feed line for receive and transmit (remember, both are happening simultaneously) we need to add a duplexer to the mix.
The duplexer is a device that allows for bi-directional communication over a single path. Duplexers will consist of four tuned cavities. One set of cavities allows the receive frequency to pass and stops the transmit frequency, and the other set allows the transmit frequency to pass and stops the receive frequency. This series of filters split the two frequencies for our receive and transmit radios and also combine those two frequency so they can travel up a single piece of feed line.
Additional filters may be added to the system to eliminate spurious emissions and harmonics or to combine a third UHF signal onto the feed line system. Finally a set of combiners may be used to run our link receive frequencies up the same set of feed line as our main transmitter. All these devices, and the feed line absorb power, so our 250 watt signal, by the time it reaches the antenna at the top of the tower, may only be 50 watts.
The WVRA is located on a broadcast tower, You can see two sets of two folded dipole antennas, one at 500 feet and another at 750 feet. This is for two UHF, one 220, and one VHF repeater system.
Finally, the antenna. Its job is to receive signals and radiate RF energy. The choice of an antenna is dependent on the range and coverage needs of the repeater. Durability is also a top consideration, especially if you are installing the repeater at a commercial site. The antenna pictured is at 750 feet on a 1000 foot tower and consists of a 4 bay folded dipole design with about 7-9db of gain. So our 250 watt transmitter, cut down to 50 watts by the time it reaches the antenna, will end up having about 200-300 watts of effective radiated power due to the antenna gain.Conclusion
Repeaters not only extend the range and coverage of our handheld and mobile radios, but they also are part of the local amateur radio community. Most repeaters are sponsored by clubs or groups of amateurs, and this ‘over the air’ meeting place is the link that often binds the organization together. If you frequently use a local repeater, please consider supporting the sponsoring organization. Equipment and maintenance is expensive, and every dollar helps keeps a repeater on the air.