The basic radio repeater was designed to allow us to communicate in efficiently reliably and economically. Advances in technology have increased the effectiveness by reducing noise and interference.
Technological advances in radio communications have led to more complex radio systems. Occasionally, radio systems engineers and manufacturers need to be reminded of what we are trying to accomplish-to communicate in an efficient, reliable and economical manner. The basic radio repeater was designed to accomplish this task. The repeater extends the range over which two or more hand-helds or mobile radios can communicate effectively. It combines a transmitter and a receiver on different frequency channels and rebroadcasts the receiver’s audio information through the transmitter. This works well if the repeater is located in an appropriate position-usually a high vantage point.
This method has created its own problems, however. With more and more repeaters battling for the optimum vantage point, the noise floor and interfering signals have increased dramatically over the last 25 years. Thus, advances in repeater technology must increase the effectiveness of the repeater by reducing the adverse effects of noise and interference.
These advances are driven by three factors: customer requirements, government regulations and manufacturing and design costs.
The customer demands a product at a reasonable price that will provide a solution to a technical requirement. A product with marginal specifications may work in the interim, but it is not necessarily the best solution.
On the other hand, a high-end communications system may be more sophisticated and more expensive than is required. Thus, the customer drives repeater technology as long as it meets his requirement-no more or no less.
In an effort to expand the limited channels available in the VHF and UHF land mobile bands, the government has regulated narrower channel assignments. This requires technological advances in transmitter frequency stability (1.5 parts per million in the UHF band), and narrower IF filter stages if the receiver is to achieve selectivity specifications similar to wideband receivers.
For some bands, the government stipulates that new repeater systems must be trunked to ensure spectrum efficiency on newly assigned channels. Manufacturers of repeaters have the unenviable task of supplying, at a competitive price, a product that meets the demands of the customer and fulfills the current regulations. This is not always an easy task, considering the complex circuitry required.
Technological advances have improved the reliability of repeaters. The mean time between failure has increased as the components used in the radios have become more reliable. As communications systems expand into remote areas, it is not the cost to repair a malfunctioning repeater, but the cost associated in accessing the site that concerns owners of repeater equipment. This can include expensive transportation costs such as snowcats or helicopters. Many manufacturers, to ensure reliable performance, will test the system in different environments before shipping. Advances have created modular repeaters with the added advantage of easy maintenance and upgrade. The modular repeater makes repair as easy as sliding in a new TX or RX card. Modules can be upgraded to new frequency bands or, in some cases, expanded to add more TX and RX modules. For the repeater owner, this means less down time and lower costs for future expansion.
As the air waves become more congested, the customer’s biggest requirement is basic-a repeater that will filter out the noise and interference and retransmit the original message. Repeater manufacturers have been addressing this problem from the first days of radio technology. Tone-controlled squelch not only prevented false keying of the radio, but it also provided user-defined channels over the same repeater frequency, making a more efficient radio system. But nothing can filter out unwanted signals better than a receiver can with superior selectivity and intermodulation specifications. To achieve high receiver specifications, one must look at the design advances of the receiver’s front-end, local oscillator (LO) and intermediate frequency (IF) stages. Most modern repeater front-ends will incorporate some type of bandpass filter. It may be a LC-style filter or a more selective helicalfilter, but it is the first step in filtering out unwanted signals. Both styles of front-end filter have their merits, but a helical filter can achieve a narrower bandpass and may better withstand a lightning strike at the site. Advances in front-end filters have created products that have tighter bandpass responses with simpler tuning procedures. In a pinch, a good front-end can be tuned by ear for a 12dB SINAD sensitivity. The best front-end filter available will not provide the expected benefits if the LO exhibits poor phase noise characteristics. Any phase noise produced by the local oscillator will directly affect the selectivity and intermodulation specifications of the receiver, by allowing unwanted signals to be mixed down to the first intermediate frequency (1st IF). To achieve a pure LO signal, manufacturers must keep the power and ground lines used in the LO circuitry clean and free of unwanted noise and power spikes. This can be a difficult task when, in the same housing, a repeater is usually equipped with a transmitter generating significant amounts of radio frequency (RF) power.
Therefore, the power lines are kept clean by regulating and filtering the input dc power to the repeater many times over. Any modern synthesizer will incorporate a microprocessor to control the synthesized LO. This creates a noisy ground bus if the digital ground is not kept separate from the analog ground used in the LO circuitry. Many processes achieve this result, but one of the best methods is to completely separate the circuit boards for the LO and microprocessor circuits.
Communication between the two circuits is achieved using inferred signals, similar to a TV remote control. The two ground signals are never connected, and the microprocessor noise is contained with its shielded circuitry. Crystal-controlled receivers have also advanced over the years. Although they may not have the flexibility of channel selection like the synthesized modes, they do produce clean LO signals if the doubled or tripled fundamental frequency is filtered correctly. This usually requires a helical filter stage before the mixer. A crystal-controlled receiver has the added benefit of low standby current, an advantage at remote sites where commercial power is not available.
The IF filter is the most important circuit in achieving a selective receiver. It would be easy for a manufacturer to narrow an IF filter section to a point where increased selectivity demonstrated superior results, but the cost is higher audio distortion. Narrowing the IF filter not only filters out adjacent channels but also clips the received audio. An optimal IF will be a happy medium between selectivity and audio distortion. If done correctly, a receiver can achieve a selectivity of greater than 90dB while still maintaining a distortion of less than 3%. Be certain that when a manufacturer states these figures, they can be achieved over the intended frequency and temperature range. Remember the repeater will have to work in more places than just a test bench.
The other half of the repeater system is the transmitter. Advances in the design of transmitter exciters and power amplifiers have created transmission that is clean and free of harmonics and spurious noise, while maintaining high stability. If the receiver on site is suffering from desense, the transmitter most likely caused it. This can happen when exciter output stages are not correctly tuned to the input of the power amplifier. The latest technology in transmitter design minimizes this occurrence. Generally speaking, modern designs are closer to the 50V input and output impedance over wider frequency and temperature ranges.
The receiver and the transmitter are naturally the most important pieces in a repeater system, but they are not the only vital parts. In a never-ending effort to get more use out of the limited land mobile spectrum, there have not only have been advances in transmitter and receiver technology, but also in the designs of signaling and trunking systems. The advent of tone- and digital-controlled squelch prevented spurious transmission from opening up the receiver’s squelch and also created channels on the same repeater system. This led to the development of analog trunked repeater systems, thereby expanding the efficiencies of spectrum usage. A group of users could access one of two or more channels in a trunked system, reducing the wait time for a free channel. This meant that many additional transmitters and receivers would be co-located for an analog trunked system.
This stresses the need for high-specification repeaters. Analog trunked systems are naturally migrating to digital trunked systems, with the added advantage of mixed voice and data transmissions. Keep in mind that conventional repeater systems are far from being phased out. Although there are definite advantages to both analog and digital trunked systems, additional features are not for everyone. In many applications, a conventional analog repeater is more than adequate, and a trunked system would be too complex and an inefficient use of the spectrum.
When shopping for a repeater, there is more to look at than just price and specifications. Look for items such as ease of tuning, reliability, expandability, warranty, delivery and customer service. Your repeater manufacturer should be able to offer more than just a product. The manufacturer should offer the solution to a custom application because your system is different from the one the manufacturer sold before. This requires a commitment to the customer or potential customer in the form of timely quotations, immediate technical support, the ability to offer application-specific repeaters and the commitment to make it right if the end product does not solve the customer’s application.
With all the technology available in today’s repeater systems, you need to only ask one question regarding your new repeater system: Are you sat-isfied with its performance? If not, then is the manufacturer prepared to correct the problems? Technological advances in repeater systems are only as good as the support offered to make them work.