Antennas for Wireless Systems: A Practical Guide

This article is adapted from Wireless Systems Guide: Antenna Setup, a Shure educational publication available from shure.com. —ed.

We’re all familiar with transducers, such as speakers and microphones. Yet antennas, which also convert one form of energy to another — in this case, radio-frequency electric current to radiated electromagnetic waves (and vice versa) — are another form of transducer. And with any transducer, following certain guidelines helps ensure maximum performance.

When dealing with radio frequencies, considerations such as antenna size, orientation and proper cable selection are important factors. Without getting overly technical, we’d like to present a series of good practices regarding antenna usage in typical wireless audio applications. [Note: These recommendations only apply to pro wireless systems with detachable antennas. For entry-level systems with permanently affixed antennas, techniques such as antenna distribution and remote antenna mounting are simply not possible.]

And these recommendations to optimize wireless audio system performance are guidelines, rather than hard-and-fast rules that need to be followed to the letter. However, if a wireless system fails to operate as expected, it is often due to the disregard of several of these guidelines, compounding the negative effects. Rarely does a system fail to function if only a single recommendation is overlooked!

Omnidirectional Antennas

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 omnidirectional antennas, although unidirectional models can be an ideal problem solver in many pro applications — see Fig. 1, above.

The size of a 1/4-wave antenna is approximately one-quarter of the wavelength of the desired frequency, and the 1/2-wave as one-half the wavelength. Wavelength for radio signals can be calculated by dividing the speed of light by frequency. This “Wave Equation” can be expressed as:

C = F x L or C / F = L
where C = speed of light,
F = frequency and L = wavelength

C is 3 x 10⁸ meters/sec (approximately 186,000 miles/sec), and as we’re dealing in fairly high frequencies, we can also express this as:

L = 300 / f meters
where f = frequency in MHz

For example, a 200 MHz wave has a wavelength of approximately 6 feet (2 m). Therefore, a 1/2-wave receiver antenna would be about 3 feet (1 m) long, and a 1/4-wave antenna would be about 18 inches (45 cm). Note that antenna length typically needs to only be approximate, not exact.

For VHF applications, an antenna anywhere from 14 to 18 inches is perfectly appropriate as a 1/4-wave antenna. As 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 somewhat more important. For a system operating at 500 MHz, a 1/4-wave antenna should be about 6 inches. Using an antenna tuned for an 800 MHz system (about 3 inches in length) in the same situation would result in less than optimum pickup. Wideband omnidirectional antennas that cover almost the entire UHF band are also available for applications where receivers with different tuning ranges need to share a common antenna.

Also, 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. For proper reception, these antennas require a ground plane —
essentially 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. The chassis of the receiver (or distribution system) provides the necessary ground plane, so do not use a 1/4-wave antenna for remote antenna mounting.

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. Therefore, there is no compelling reason to “upgrade” to a 1/2-wave antenna, unless remote antenna placement is required for the application.

Unidirectional Antennas

A second type of antenna suitable for remote mounting is a unidirectional, such as yagi or log periodic antennas. Both types consist of a horizontal boom and multiple transverse elements. These can provide up to 10 dB more gain than a 1/4-wave antenna, and can also reject interfering sources from other directions by as much as 30 dB. However, yagi antennas are rarely used in wireless microphone applications due to their quite narrow bandwidth, usually just a single TV channel (6 MHz). The log periodic antenna achieves greater bandwidth by using multiple dipoles whose size and spacing vary in a logarithmic progression.

A longer boom and more elements result in greater bandwidth and directivity. Some unidirectional antennas have built-in amplifiers to compensate for losses due to long cable runs.

With regard to wireless microphone applications, unidirectional antennas are typically only employed in UHF systems. Directional antennas are somewhat frequency specific, so some care must be taken in selecting the proper antenna to cover the required frequencies. A directional VHF antenna is 3 to 5 feet wide (just like a roof-mounted TV antenna), which makes mounting a mechanically cumbersome task. Note that these antennas should be mounted with the transverse elements in the vertical direction, rather than horizontal as in a television application, because the transmitting antennas are usually also vertical. Unidirectional antennas are primarily used for long-range applications. In such cases, a minimum distance of 50 feet is recommended between transmitter and unidirectional antennas.

Antenna Placement

Most wireless receivers have their primary antenna inputs on the back of the receiver. The diversity receivers used in most pro applications have both an A antenna input and a B antenna input on the rear panel of the receiver. BNC connections are most often used for antenna inputs, although some older systems may have PL-259 connectors. Rack-mountable receivers often provide pre-cut holes on the rack ears to accommodate antenna connections for front-mounting the antennas. Short coaxial cables and bulkhead adapters with the proper connector type are all that is needed to bring the antennas to the front.

When deciding where to mount antennas, always try to maintain line of sight between the receiving and transmitting antennas. For example, if the back of the rack faces the performance area, then rear-mounting the antennas will provide better line of sight. If the front of the rack faces the performance area, then front-mounting may be better, unless a front door to the rack needs to be closed. Metal equipment racks will block RF from reaching the antennas mounted inside. Rear-mounted antennas may not work inside a metal equipment rack. If the receiver is not rack-mounted at all, then simply maintain a visible line of sight from the transmitting position to the receiving antennas.

Antenna Spacing

Antennas should be separated from each other by a minimum of one quarter wavelength — about 16 inches for VHF units and about 4 inches for UHF. This helps ensure adequate diversity performance. Diversity reception can be improved by separating the antennas further, but beyond one full wavelength the advantage becomes negligible. However, greater separation may be useful if it results in more strategic antenna location — for example, increasing separation to ensure line-of-sight with at least one of the antennas from any location in the room.

Antenna Height

Receiver antennas should be clear of obstructions, including human bodies, which can absorb RF. Therefore, placing the antennas higher than “crowd level” (five or six feet from the floor) is always recommended.

Antenna Orientation

Receiving antennas should be oriented in the same plane as the transmitting antenna. As the transmitting antenna is generally in the vertical position, receiving antennas should also be vertical. However, due to the dynamics of live performers, handheld transmitter antennas will often vary in position. As a compromise, antennas can be placed at approximately a 45-degree angle from vertical. Note: Never orient antennas horizontally.

Sometimes, when antennas are mounted on the back of the receivers, such as inside an equipment rack, there is not enough clearance for vertical orientation. In this situation, try front-mounting the antennas, or remote-mount them outside the rack. Antennas must always be kept clear of any metal surfaces by at least a few inches and not touch or cross other receiving antennas. Antenna distribution systems offer a solutions that can help avoid some of these problems.

Antenna Distribution

Proper antenna distribution is key to achieving optimum performance from multiple wireless systems operating in the same environment. Stacking or rack-mounting wireless receivers results in many closely spaced antennas, which degrades the performance of the wireless systems. Antennas spaced less than 1/4 wavelength apart can disrupt the pickup patterns of one another, resulting in erratic coverage. Additionally, closely spaced antennas can aggravate local oscillator bleed, a potential source of interference between closely spaced receivers. Finally, for remote antenna applications, antenna distribution is essential for keeping the number of remote antennas and coaxial cable runs to a minimum. Antenna distribution eliminates these issues by splitting the signal from a single pair of antennas to feed multiple receivers. Splitting can be accomplished by either passive or active means.

Passive Splitters (Using Two Receivers)

Passive splitters are inexpensive and do not require any power to operate. Using a passive splitter results in a signal loss of about 3 dB for every split. As a general rule, no more than 5 dB of loss is acceptable between the antennas and the receiver inputs. For this reason, passive splitters should only be used for a single split — i.e., splitting a single antenna to two receivers, as shown in Fig. 2. With passive splitters, an additional consideration is that some receivers provide a DC voltage on the antenna inputs for powering remote antenna amplifiers directly from the receiver. If two receivers are connected together with a passive splitter, each receiver will “see” the voltage from the other receiver at its antenna inputs. To avoid any potential damage, either use a splitter that incorporates circuitry to block the voltage, use an external DC blocker or defeat the voltage on at least one of the receivers.

Active Distribution (Three or More Receivers)

If distribution is needed for more than two systems, an active antenna distribution system, as shown in Fig. 3, is recommended. Active splitters require power to operate, but provide make-up gain to compensate for the additional losses resulting from multiple splits off the same antennas. A typical active system will have four to five antenna outputs. Many active antenna distribution systems will provide power distribution to the receivers as well. Multiple active distribution systems can be used together if more outputs are needed, but this must be done carefully. A theoretically perfect distribution system would provide unity gain from input to output. In practice, the antenna outputs of an active system may have as much as 1.5 to 2 dB of gain.

Over-amplification of the RF signal can cause unwanted side effects, such as aggravated intermodulation products and increased radio “noise.” To prevent these problems, avoid cascading antenna distribution systems more than two deep. A better method is to use a “master” antenna distribution system to split the signal to a second tier of “slave” distribution systems. All receivers are then connected to either the “master” or “slave” distribution systems. Connecting receivers in this manner keeps all the receivers closer to the pure antenna signal.

Pay attention to the frequency bandwidth specified for the antenna distribution system. These units are typically available in both wideband and narrowband varieties. Wideband refers to a device that will pass frequencies over a large range, typically several hundred MegaHertz. Narrowband devices may be limited to no more than 20 or 30 MHz. As these are active devices, frequencies outside the bandwidth of the distribution system are not passed on to the receivers.

Remote Antenna Considerations

As mentioned earlier, some installations require remote placement of 1/2-wave or directional antennas for better line-of-sight operation. Due to RF loss issues in coaxial cables, it is important to use the proper coaxial connection cabling, such as the 50-ohm low-loss cable typically used in pro wireless applications. The use of 75-ohm cable will result in additional losses due to an impedance mismatch, but such losses are typically less than 1 dB.

Manufacturer cable specs should list a cable’s attenuation (loss) at various frequencies in dB per 100 ft. You can use this value to calculate the expected loss at the receiver for the desired cable run. A loss of between 3 and 5 dB of signal strength is considered acceptable. If a cable run results in a loss of greater than 5 dB, active antenna amplifiers must be used to compensate to avoid poor RF performance. These active amplifiers may provide a selectable amount of gain. The appropriate gain setting is determined by the loss in the cable run. The amplifier is placed at the antenna, and often can be wall- or stand-mounted.

In extreme cases, two amplifiers can sometimes be connected inline to achieve longer lengths. Make certain the receiver or antenna distribution system can supply enough current to power multiple antenna amplifiers. And as with active distribution systems, note that antenna amplifiers are also band-specific, available as both narrow or wideband.

Each connection between two sections of cable may result in some additional signal loss, depending on the connector. To increase reliability, use one continuous length of cable from the antenna to the receiver. If antenna amplifiers are being used, mount the antenna directly on the input of the first amplifier, use one length of cable to go from the amplifier to the second antenna amplifier (if needed), and from the second antenna amplifier to the receiver.

Do not over-amplify the radio signal. More is certainly not better in this case. Excess amplification can overload the receiver’s front-end, causing drop-outs and RF “bleed” (one transmitter showing up on several receivers) on an antenna distribution system. Try to use only the amount of gain necessary to compensate for loss in the cable. Net gain should be less than 10 dB.

Antenna Combining

The converse of antenna distribution, antenna combining, can be employed in one of two ways. With wireless mic systems, multiple antennas can be combined to feed a single receiver (or multiple receivers with antenna distribution) to provide coverage across multiple rooms or in extremely large spaces.

For wireless IEM/personal monitor systems, antenna combining is typically used to reduce the number of transmitting antennas, where all the transmitters can share a common antenna. Also, several closely spaced, high-power transmitters can suffer from excessive intermodulation (a transmitter interaction that produces additional frequencies). In this case, a passive combiner should be used for combining two transmitters. For more than two, though, an active combiner is recommended. An active antenna combiner will typically accept between from four to eight transmitters.

Unlike active antenna distribution systems, which can be cascaded together for larger setups, active antenna combiners should never be “actively” cascaded. If more than one combiner is needed to combine all the transmitters together, a passive combiner should be used to connect two active combiners together. As always, be aware of any extra losses incurred with the passive combiners. Similar to active antenna distribution systems, active combiners also have a specified frequency bandwidth, so select the proper bandwidth for your application.