Wireless Systems and Antenna Placement
"Receiver antennas are one of the most misunderstood areas of wireless microphone operation."
Credit Shure's Product Support Group with that bold statement. And they ought to know. They're the ones rivaling Sherlock Holmes in the art of deductive reasoning as it relates to solving dropout mysteries. Many times the culprit turns out to be antenna placement.
Mistakes in antenna selection, placement, or cabling can cause short range, dead spots in the performance area or low signal strength at the receiver that leads to frequent dropouts.
Before we cover placement, let's review the types of antennas you're likely to encounter in a wireless system.
Omnidirectional Antenna Types
The size of an antenna is directly related to the wavelength of the frequency to be received. The most common types used in wireless audio systems are 1/4-wave and 1/2-wave omni-directional antennas, and unidirectional antennas.
¼-Wave. This antenna is one-quarter the size of the desired frequency's wavelength. (Example: A 200 MHz wave has a wavelength of about 6', so a ¼-wave antenna length would need to be approximately 18".) For VHF Applications, an antenna anywhere from 14–18 inches is perfectly appropriate as a 1/4-wave antenna. Since the UHF band covers a much larger range of frequencies than VHF, 1/4-wave antennas can range anywhere from 3 to 6 inches in length, so using the proper length antenna is important.
Tip: 1/4-wave antennas should only be used when they can be mounted directly to the wireless receiver or antenna distribution system; this also includes front-mounted antennas on the rack ears. These antennas require a ground plane for proper reception, which is a reflecting metal surface of approximately the same size as the antenna in at least one dimension. The base of the antenna must be electrically grounded to the receiver.
½ Wave. This antenna is one-half the size of the desired frequency's wavelength. (A 1/2-wave receiver antenna would be about 3 feet long for the 200 MHz wavelength in the example above.)
Tip: A 1/2-wave antenna does not require a ground plane, making it suitable for remote mounting in any location. While there is a theoretical gain of about 3 dB over a 1/4-wave antenna, in practice, this benefit is seldom realized. There isn't a compelling reason to "upgrade" to a 1/2-wave antenna unless remote antennas are required for the application.
Unidirectional Antenna Types
In wireless mic applications, unidirectional antennas are typically only employed in UHF systems. They tend to be frequency-specific, so some care must be taken in selecting the proper antenna to cover the required frequencies. A directional VHF antenna is 3–5 feet wide (just like a roof-mounted television antenna), which makes mounting a mechanically cumbersome task.
Unidirectional antennas are primarily used for long range applications. A minimum distance of 50 feet is recommended between transmitter and unidirectional antennas. Here are two types you might encounter:
The helical antenna is a unidirectional antenna that combines high gain with a wide frequency range. It has a somewhat higher gain than the traditional log periodic or "paddle" antenna (12dBi vs. 7dBi), but it has a narrower coverage angle, typically about 60 degrees compared with 120 degrees for the log periodic.
A unique characteristic of the helical antenna is that it doesn't have a "preferred" polarization angle. It works equally well with antennas that are oriented vertically or horizontally. For this reason, it has become popular as a wireless in-ear transmitter antenna.
The 'log periodic' or 'paddle style' antenna
The most common directional antenna is the log-periodic type, often called a "paddle" antenna because of its shape. In the direction of increased sensitivity, the antenna can deliver a stronger signal to the receiver, which can increase the effective range of the system. At the same time, the antenna can offer some rejection of interfering signals coming from other directions.
Tip: Keep in mind that most directional antennas are wideband devices. They are equally sensitive to all frequencies in their operating band. This means that a directional antenna aimed at an interfering source will increase the level of that interference as well as whatever desired signal may be in that same direction. Finally, too much antenna sensitivity (gain) may result in overload of the connected receivers. This can aggravate intermodulation and actually desensitize the receiver. For most applications omnidirectional antennas are still the norm.
10 Antenna Placement Tips for Optimum Wireless Performance
Modern diversity receivers offer better performance than single-antenna types, but you still need to put the right antennas in the right place to maximize the performance and reliability of the system. Here are some general rules to help you get the best results.
- Maintain line-of-sight between the transmitter and receiver antennas as much as possible, particularly for UHF systems. Avoid metal objects, walls, and large numbers of people between the receiving antenna and its associated transmitter. Ideally, this means that receiving antennas should be in the same room as the transmitters and elevated above the audience or other obstructions.
- Locate the receiver antenna so that it's at a reasonable distance from the transmitter. A minimum distance of about 16 feet is recommended to avoid potential intermodulation products in the receiver. The maximum distance is not constant but is limited by transmitter power, intervening objects, interference, and receiver sensitivity. It's better to have the antenna/receiver combination closer to the transmitter (and run a long audio cable) than to run a long antenna cable or to transmit over excessively long distances.
- Use the proper type of receiver antenna. A 1/4-wave antenna can be used if it's mounted directly to the receiver, to an antenna distribution device or to another panel, which acts as a ground-plane. If the antenna is to be located at a distance from the receiver, a 1/2-wave antenna is recommended. This type has somewhat increased sensitivity over the 1/4-wave and does not require a ground-plane. For installations requiring more distant antenna placement or in cases of strong interfering sources, it may be necessary to use a directional antenna suitably aimed. Telescoping antennas should be extended to their proper length.
- Select the correctly tuned receiver antenna(s). Most antennas have a finite bandwidth making them suitable for receivers operating only within a certain frequency band. When antenna distribution systems are used, receivers should be grouped with antennas of the appropriate frequency band as much as possible.
- Locate diversity receiver antennas a suitable distance apart. For diversity reception, the minimum separation for significant benefit is one-quarter wavelength. The effect improves somewhat up to a separation of about one wavelength. Diversity performance does not change substantially beyond this separation distance. However, in some large area applications, overall coverage may be improved by further separation. In these cases one or both antennas may be located to provide a shorter average distance to the transmitter(s) throughout the operating area.
- Locate receiver antennas away from any suspected sources of interference. These include other receiver and transmitter antennas as well as sources mentioned earlier-digital equipment, AC power equipment, etc.
- Mount receiver antennas away from metal objects. Ideally, antennas should be in the open or perpendicular to metal structures such as racks, grids, metal studs, etc. They should be at least one-quarter wavelength from any parallel metal structure. All antennas in a multiple system setup should be at least one-quarter wavelength apart.
- Orient receiver antennas properly. A non-diversity or single antenna receiver should generally have its antenna vertical. Diversity receivers, which have two antennas, can benefit from having antennas angled 45 degrees apart.
- Use the proper antenna cable for remotely locating receiver antennas. A minimum length of the appropriate low-loss cable equipped with suitable connectors will give the best results. Because of increasing losses at higher frequencies, UHF systems may require special cables.
- Use an antenna distribution system when possible. This will minimize the overall number of antennas and may reduce interference problems with multiple receivers. For two receivers, a passive splitter may be used. For three or more receivers active splitters are strongly recommended. Verify proper antenna tuning as mentioned above. Antenna amplifiers may be required for UHF systems with long cable runs from the equipment to the antennas.
For a more comprehensive discussion on the subject, Download Shure's free booklet Wireless Systems Guide - Antenna Setup.