Using Multiple Systems
Because of the many advantages of wireless microphones, simultaneous use of four, six or even more systems at a location is quite common. Often, one or two wireless microphones are used initially, but as the benefits of wireless operation become more apparent the need for additional systems develops. The freedom, flexibility and enhanced presentation made possible by wireless microphones are almost irresistible, and it is natural for their use to increase over time.
However, as the number of wireless systems in operation at a location increases, so does the potential for problems. Unless some simple preventative measures are taken, it may eventually become difficult to achieve reliable and trouble-free operation. In particular, interference is a common problem when several systems are used unless the wireless frequencies are carefully chosen. Other problems such as reduced range, unexpected signal losses and erratic squelch operation can also occur.
The most common problem with using multiple wireless systems is interference, and the most common cause of interference is the use of wireless frequencies which are not compatible with each other. Wireless frequencies must be selected to work with local TV stations and with all other wireless systems in the area. Otherwise, serious interference problems are highly likely, especially when several wireless systems are used simultaneously.
Other types of interference can also occur when using multiple wireless systems. One is caused by using too many wireless systems in a particular frequency range or using wireless frequencies that are too close to each other. Another interference problem can occur when the antennas for two receivers touch or are parallel and close together. In these situations, a small amount of radio energy can leak from one receiver to the next, causing harmful interference under certain conditions. A similar problem can occur when receivers are stacked directly on top of each other. As a practical matter, the need to keep receiver antennas well separated makes it very difficult to use more than a few units on a tabletop. Audio cables and power cords also tend to compound the problem, especially if more than one row of receivers is needed.
As the number of wireless systems at a location increases, so does the amount of effort required to ensure reliable and trouble-free operation. It is also necessary to begin taking into account the arrangement and mounting of equipment, the positioning of antennas and other factors that can safely be ignored when only one or two systems are being used. Although there is no hard and fast dividing line, once more than four or five wireless systems are involved a new approach is probably appropriate.
Frequency selection is the critical first step in achieving success with multiple wireless systems. Unless all the frequencies used are compatible, the chances of satisfactory operation are very low. The TV channels in the area of use are also highly important and must be considered when selecting frequencies. It is common for a system to work well in one city and perform very poorly in another city because the local TV channels are different. For this reason, it is essential that TV channel data be available for all locations where the system will be used.
As the number of wireless systems used increases, the technique of simply spreading the receivers out over a tabletop (or some similar arrangement) eventually becomes impractical. The need to locate the receivers at some distance from the preferred antenna location or to arrange them so that they are easier to monitor may also arise. Mounting the receivers in travel cases for easy movement might be desirable in some instances. Once it becomes necessary to permanently mount the receivers in an equipment rack or travel case, several new issues arise.
The first question is what to do about the antennas. If the receiver antenna connectors are on its rear panel and the rack or case is metal or has a metal frame, the radio signal will be at least partially blocked and performance will almost certainly suffer greatly. Even if the antenna connectors are on the receiver front panel, they will be close to the metal structure and each other, resulting in at least some loss in performance. This arrangement also results in poor reception when the metal rack or case is between the transmitters and the receiving antennas.
The best solution to this problem is to use remote antennas; that is, antennas other than the whip antennas attached directly to the connectors on the receiver. This can be as simple as remounting the whip antennas on the top of the metal cabinet or using accessory antennas. It may also be desirable to have the antennas at some distance from the receivers. This allows the receivers to be positioned conveniently next to the mixer while keeping the antennas in a favorable location nearer the transmitters.
Remote antennas must be connected to the receivers with coaxial cables which are specifically designed to carry radio frequency (RF) signals. Even with good-quality coaxial cable, a significant amount of the RF energy is lost in the cable. This is especially true for small-diameter cables at UHF frequencies, where only 25 feet (8 m) of economy coaxial cable can reduce operating range by as much as 40%. Cables with better performance are available and should be used when cable lengths exceed about 15 feet (5 m) at UHF or 40 feet (12 m) at VHF. Antennas and Cables
includes more information on selecting and using coaxial cables for various wireless situations.
Many types of remote antennas are available for use with wireless receivers. These can be divided into two basic kinds: ones that work equally well in all directions (omnidirectional) and those which work best in one direction (directional). Directional antennas offer increased range in the preferred direction but reduced range in other directions, as compared to omnidirectional antennas. Directional antennas are useful when increased operating range is needed and the wireless transmitters will always be in one particular direction. They can also be used to make up for the losses in coaxial cables. Because of their large size, directional antennas are not usually practical in the VHF band. Antennas and Cables
includes more detailed information on the various types of antennas available and application recommendations.
In situations where the use of long coaxial cables cannot be avoided, RF preamplifiers can be used to overcome the inherent signal loss in the cable and the resulting decrease in operating range. Also known as boosters and RF line amplifiers, these devices must be located near the antenna to be effective. Because of the nature of RF, additional amplification at the receiver end of the cable will rarely improve range. Unfortunately, wireless systems using RF preamplifiers are usually somewhat less resistant to interference than systems with the antennas connected directly to the receivers. For this reason, preamplifiers should be avoided when possible by rearranging the system, installing a lower loss coaxial cable, or using directional antennas.
When a number of diversity receivers are used in a system with remote antennas, the amount of RF cabling and antennas required can become a problem. For example, four diversity receivers will need a total of eight antenna cables and eight remote antennas. Not only is this an installation problem, it can represent considerable additional expense. In this situation, an antenna splitter is usually a good solution. These devices divide the RF signal from the antenna into four separate signals that can feed four receivers. This allows use of only two antenna cables and two antennas for four diversity receivers. Because the loss in the antenna splitter adds to that of the coaxial cable, RF preamplifiers will usually be necessary unless the cables are very short.
When mounting wireless receivers in equipment racks or cases, it is a good idea to separate them from digital devices such as delays and processors. This is because digital equipment almost always has spurious outputs that can interfere with sensitive wireless receivers. Usually, just 12 inches (30 cm) or so of separation will prevent a problem. It is also a good idea not to mount wireless receivers directly above power amplifiers and other units which run hot. The extra heat will unnecessarily reduce the stability of the wireless equipment, reduce its performance and possibly shorten its life.
When feasible, it is good practice to leave space between wireless receivers when rack mounting them. This allows the receivers to run cooler and reduces the chances of spurious outputs from one receiver affecting another. This arrangement also increases antenna separation when front-mounted antennas are used. If there is not enough rack space available to allow for a blank panel between each unit, mounting in pairs so that there will be space either above or below each unit is suggested.
Large wireless systems can become moderately complicated, especially when remote antennas, RF preamplifiers and antenna splitters are used. Frequency selection can also present some challenges when a large number of wireless microphones are involved, the system will be used in several different cities, or wireless equipment from other manufacturers must be incorporated. If you are unsure how to proceed, contact Audio-Technica or your dealer for applications assistance.
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