The need for combiners has long been known and their recognized importance continues to grow at an accelerated rate. More and more land mobile radio systems are being equipped for simultaneous operation on several frequencies from a common site, and a combiner can eliminate the need for separate antennas for each radio system. In addition to reducing the number of antennas, better performance can usually be realized if the highest antenna site is selected and used with the optimum combiner. A single “master” antenna and its transmission line can be shared by two or more transmitters, receivers, or simplex base stations by connecting them to the antenna through a combiner.
Sharing of a single antenna is not limited to a single system operator. When several base stations, operated by different users, are located at the same site, they can often share a common antenna, depending upon the frequencies used. Most radio systems that operate at a common site and utilize independent antennas and transmission lines will require multiple interference protective devices. These usually are ferrite isolators for reducing transmitter intermodulation to an acceptable level, bandpass or band-reject cavity filters (installed
between transmitters and antenna) for reduction of transmitter noise, and bandpass or band-reject cavity filters for protection against receiver desensitization from transmitter carrier frequencies. These devices introduce losses to transmitter power and received signal strength. If interference-free radio systems are to be achieved, these losses can approach those of a combiner and yet not afford an optimum “RF clean”
antenna site.
Combiner Requirements
A combiner that enables the use of a common antenna by two or more transmitters should cause a minimum of insertion loss (transmitter power loss) and should provide a high degree of isolation between the transmitters. This ensures that potential transmitter-produced intermodulation frequencies are minimized.
Transmitter intermodulation is the primary factor that must be considered when two or more transmitters are combined into a common antenna. In addition to the above, a combiner that enables a number of transmitters and receivers to use a common antenna must also ensure that any receiver desensitization caused by the transmitters and any transmitter noise at the various receiver frequencies are reduced to an acceptable level.
When the transmitters and receivers share a common antenna through a combiner, the only practical method of protection for transmitter noise and receiver desensitization is by use of resonant cavity filters between the transmitters and receivers.
Should the frequencies be separated by a reasonable amount, a simple cavity-filter-combiner configuration can be used for two or more systems — the radio manufacturer’s duplex operation curves can provide the proper isolation required for any given frequency separation. If the transmitter frequencies are extremely close, the hybrid/ferrite isolator combiner is normally used.
Cavity Type Combiners
The cavity-type combiner is one of the most common combiners used to couple transmitters and/or receivers into a single antenna. This type of combiner is generally more economical and affords less insertion loss than hybrid/ferrite combiners. It is normally used when the channels to be combined have a frequency separation of at least 150 KHz in the low band, 500 KHz in the 150 MHz band, and 1 MHz in the 450 MHz band. Also, as mentioned previously, if the systems to be combined contain receivers, cavities must be used since hybrid/ferrite combiners have unidirectional devices (isolators) as components.
The cavity-type combiners can be composed of all bandpass cavities, all notch (band-reject) cavities, or a combination of bandpass and notch cavities. All Bandpass Combiners The bandpass combiner is used when a fixed number of stations with a relatively wide frequency separation are combined into a single antenna. Figure 2-1 shows a block diagram of a bandpass combiner. Two transmitters and two receivers are combined into a single antenna through the use of bandpass cavities and a five-way connector. The bandpass cavities in the transmitter lines protect the receivers from transmitter sideband noise radiation by attenuating the output of the transmitters at the receiver frequencies. Those in the transmitter lines also mutually isolate the transmitters, thereby reducing the possibility of transmitter intermodulation. The bandpass cavities in the receiver lines protect the receivers from receiver desensitization by attenuating the transmitter carriers
before they reach the receivers.
The number of cavities in each combiner system is dependent upon the frequency separation between the systems. In Figure 2-1, if the frequencies were closer together, four or possibly five bandpass cavity filters would be needed to mutually isolate the systems. The length of the interconnect transmission line from the cavity to the five-port junction is electrically an odd quarter-wave length (including the electrical length of
the cavity coupling loop). This is so the other three frequencies are presented a high impedance (open circuit) at the fiveport junction. Consequently, very little coupling loss is added to the insertion loss of the triple bandpass cavity for each frequency. The number of systems in this type of combiner is not limited to four. The limitation as to the number of systems depends upon the frequency separation between the systems, the bandwidth of the antenna, and the maximum insertion loss that can be tolerated. The all-bandpass combiner is best used when the frequency separation between systems is at least 500 KHz in the low band, 1 MHz in the 150 MHz band, 2 MHz in the 450 MHz band, and approximately 5 MHz in the 806-960 MHz frequency band.
The main advantage of an all-bandpass cavity combiner is the added protection the receivers obtain from the carriers of other transmitters in the area, as well as from those in the combiner. Likewise, the bandpass cavities in the combiner provide added protection against transmitter noise to other receivers in the area and to those in the combiner. Compared to hybrid-type combiners, the bandpass-type generally has lower insertion loss per channel. The disadvantages include its relatively large physical size, its inability to operate satisfactorily at very close frequency spacings, and the fact that the combiner is not readily expandable to accommodate more systems.
Notch Filter Combiner
The all-notch filter combiner is used normally when the frequency separation is too close for band pass filters, yet is still wide enough that ferrite isolators are not needed. This type of combiner is sometimes used, especially when only two systems are combined. When two closely-spaced systems are to be combined, the simplest device to use is a standard bandreject type duplexer. The standard band-reject duplexer can combine two transmitters or two simplex systems to a common antenna. In addition, the normal duplex transmitter and receiver can be combined to a common antenna.
Selective Cavities
As the demand for land mobile radio services steadily increases, the problems caused by frequency congestion, receiver desensitization and intermodulation, grow rapidly. The selective cavity will help in solving these problems. In this section, we will take a look at selective cavities, how they work and how they can be used. What is a Selective Cavity and What Does it Do? A selective cavity is a rather simple device that serves as a filter for radio frequencies. It has the ability to let a narrow band of frequencies pass through while frequencies outside this narrow band are attenuated. Stated differently, the unwanted and unselected frequencies are rejected and filtered out while the desired frequency is passed through with only slight attenuation. The narrow band of frequencies that pass through the cavity are within a few thousand hertz of the cavity’s resonant frequency. The selective cavity, with this “filtering action,” is important in the land mobile radio services as more and more new stations are crowded into the same area. As new stations go on the air, they can, and frequently do, cause interference to other stations that exist in the immediate area. In addition, these new stations can receive interference from the existing area stations.
The two most common forms of interference are receiver degradation (which consists of receiver desensitization and/or transmitter noise), and intermodulation. Without going into lengthy discussions of these two problems, let it be said that receiver desensitization occurs when a nearby transmitter “overpowers” a receiver. Intermodulation occurs when two or more nearby transmitters “mix” within the RF stages of a receiver and generate new frequencies, with one of the new frequencies being the same as the receiver frequency. In both cases, a selective cavity can be used to help solve the problem. In the case of transmitter noise, the cavity can be used at the transmitter to reduce transmitter noise sidebands. More specifically, it will increase receiver selectivity and make the receiver less sensitive to nearby transmitters. In the case of intermodulation, the cavity will filter out the unwanted transmissions, thus keeping them from reaching the receiver.
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