Antennas and Cables
Antennas connected directly to wireless receivers work well for most wireless applications. The whip antennas supplied with Audio-Technica receivers are reasonably efficient, convenient and inexpensive. There are situations, however, where the antennas need to be at some distance from the receiver. This need arises when receivers are mounted in metal racks and cabinets, when the preferred receiver location does not have good reception and in other similar circumstances. In these cases, other types of antennas and RF cables will be required. This section discusses using remote antennas, using antenna cables
and using antenna splitters.
Some Audio-Technica receivers have permanently-attached whip antennas and the use of remote antennas is not possible. In many situations it is feasible to locate the receivers themselves at the preferred antenna location and install longer cables for the audio. This approach also works well for other types of receivers and is often simpler and more effective than using remote antennas. The main drawback is that the adjustments and indicators on the receiver probably will not be readily accessible.
The use of remote antennas and coaxial RF cables can affect performance in unexpected ways. Coaxial cables have substantial loss at the frequencies used by wireless systems and can significantly reduce operating range. Certain types of antennas require special mounting arrangements and all antennas require clear space around them in order to operated properly. Achieving good results with remote antennas is not difficult, but does require a bit more care than just using antennas on the receivers.
Using Remote Antennas
A simple approach to remote antennas is to merely remove the whips from the receivers and connect them to lengths of RF cable. Unfortunately, this usually results in terrible performance. Whip antennas need to be mounted on a good-size metal surface to work properly. When they are attached to the receiver, the receiver case (or internal circuit boards) and the audio and power cables meet this need. The RF cable does not, and operating range can drop to a small fraction of what it should be.
Whips can be used as remote antennas if they are supplied with the necessary metal mounting surface. For example, if holes are drilled in the metal top of an equipment rack and RF "feedthru" connectors installed, good performance is possible. The whips are connected to the outside end of the RF connector and the RF cable to the inside end. It is important that the outside of the feedthru connectors be electrically connected to the metal top of the rack. A piece of sheet metal may also be used to mount the whip antennas. A size of roughly 36 inches by 36 inches (0.9 m by 0.9 m) is recommended for VHF frequencies, while 12 inches by 12 inches (30 cm by 30 cm) is adequate for UHF use.
There are other ways of mounting whip antennas so that they work well for remote use. The important thing is that the outside of the RF connectors on the cable and on the antenna have a good electrical connection to a proper-sized metal object. It is also important that the whips extend up and away from the metal object and are not held parallel to the metal. Any approach that is basically similar to the arrangement used on the wireless receiver should work reasonably well.
Many types of remote antennas are available. Except for high-performance directional antennas with gain, the performance is generally similar to ground planes. Directional antennas are sometimes used for special applications such as when operation at very long range is needed. Because such antennas are inconveniently large at VHF frequencies, they are normally only used outdoors or in very large spaces. UHF directional antennas are more reasonable in size and are more common.
Whatever type of antenna is used, it is very important that there be a clear path between the transmitter and the receiving antenna. Metal objects block RF signals and cause dropouts and noisy audio. For best results, there should be a clear path at least 6 feet (2 m) in diameter between the transmitter and receiving antennas at VHF; 2 feet (0.6 m) is adequate for UHF. The RF path should be completely clear of scaffolds, equipment cabinets, support structures, mesh screens, cable bundles and other metal objects. Mounting the antenna high above the floor, 8 feet (2.5 m) or higher, will improve range and help keep the path clear of obstructions.
Using Antenna Cables
Only coaxial cables designed specifically for RF should be used to connect remote antennas to wireless receivers. Other types of cables, even those that appear similar, will have extremely high losses and will usually reduce operating range to a tiny fraction of what it should be. RF cables should be in one continuous length; splices and mended spots will also significantly affect range. For the same reason, only the correct type of RF connectors should be used, and they must be carefully and correctly installed.
Even the best RF cables have considerable loss at wireless microphone frequencies. Since long cables will significantly reduce operating range, especially at UHF, RF cables should be kept as short as practical. The size and type of cable used should be matched to the frequency range and the required cable length. When the cables must be long, a higher-quality cable is required and a larger size with lower losses should be selected. RF cable losses are considerably higher at UHF frequencies as compared to VHF frequencies, and cable length and cable type are much more critical.
Even with the best cables, there is a maximum practical length of RF cable that can be used. Audio-Technica recommends that cable losses that reduce the normal operating range of the wireless by no more than 30% be considered the upper limit. To assist in cable selection, the table below shows the maximum recommended length of various types of common RF cables for VHF and UHF at a 30% and 50% reduction in range.
|Length for 30% Range Reduction||Length for 50% Range Reduction||Length for 30% Range Reduction||Length for 50% Range Reduction|
|Lo-cost RG-58||33' (10 m)||65' (20 m)||15' (4.5 m)||30' (9 m)|
|Quality RG-58||54' (16 m) ||107' (32 m)||24' (7 m)||48' (14 m)|
|Lo-cost RG-8||70' (21 m)||140' (42 m)||31' (9.5 m)||63' (19 m)|
|Quality RG-8||110' (33 m)||220' (66 m)||48' (14.5 m)||96' (29 m)|
|Foam RG-8 (Belden 9913)||165' (50 m)||330' (100 m)||75' (23 m)||150' (46 m)|
- RG-58 type cables are approximately 0.2 inch (5 mm) in diameter; RG-8 type cables are approximately 0.41 inch (10.5 mm) in diameter.
- The reduction of range listed in the table above is in comparison to the results that would be obtained with the same antenna connected directly to the receiver.
Audio-Technica does not recommend using cables with losses that reduce range by more than about 30% without taking steps to compensate for the excess cable loss. At UHF frequencies and for some VHF applications, it may be practical to overcome part or all of the cable loss by using a directional antenna with gain. It might also be possible to locate the antennas where they will receive more signal. Using a higher mounting location, moving the antennas closer to the transmitters or repositioning them to avoid objects that block the RF signals will all be helpful.
Using Antenna Splitters
When remote antennas are needed for several wireless receivers, the cost and complication of using separate antennas and cables for each receiver can become impractical. This is especially true when long cables are needed. The solution to this problem is to use an antenna splitter. An antenna splitter allows connecting several receivers (usually four) to one antenna, or two antennas for diversity receivers.
There are two general types of antenna splitters: active splitters and passive splitters. Passive RF signal splitters, also known as RF power dividers, have considerable loss. A typical device that can split one antenna signal into outputs for four receivers will reduce operating range to 40-45% of normal. This is obviously very undesirable, particularly if long RF cables are also required. Active splitters, on the other hand, have internal RF amplifiers to offset the inherent loss in the RF power dividers, avoiding the large penalty in range.
There is a drawback to active splitters, however. Because of the amplifiers, they are vulnerable to intermodulation problems when strong RF signals are present. Care should be taken to prevent overloading active splitters, especially if the antenna cables are short and the transmitters are close to the antennas. In this case, too much signal can cause almost as many problems as too little signal. When using active splitters, it is recommended that transmitters never be allowed to come closer than about 20 feet (6 m) from the receive antennas.
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