The world of professional audio is full of transducers. A transducer is a device that converts one form of energy to another. The proliferation of wireless audio systems has introduced yet another category of transducer to professional audio, the antenna. The following tutorial on wireless microphone placement is adapted from Shure’s technical database.
The purpose of an antenna is to convert radio-frequency electric current to electromagnetic waves, which are then radiated into space. Attached to a receiving device, antennas can also work in the reverse fashion, converting the electromagnetic wave back to an electric current.
As with any transducer, following certain guidelines helps to ensure maximum performance. When dealing with radio frequencies in particular, considerations such as antenna size, orientation, and proper cable selection, are important factors not to be overlooked. This article presents a series of good practices for most typical wireless audio applications.
Types of antenna
The size of antenna is directly related to the wavelength of the frequency to be received. The most common types used in wireless audio systems are ¼-wave and 12/-wave omni-directional antennas, and unidirectional antennas. The size of a ¼-wave antenna is around one-quarter of the wavelength of the desired frequency and the ½-wave is one-half the wavelength.
For VHF applications, an antenna anywhere from 35 to 45cm is perfectly appropriate as a ¼-wave antenna. Since the UHF band covers much larger range of frequencies than VHF, ¼-wave antennas can range anywhere from 7 to 15cm in length, so using the proper length antennas is somewhat more important. For a system operating at 500 MHz, a ¼-wave antenna should be about 15cm. Using an antenna tuned for an 800 MHz system (about 7cm in length) in the same situation would result in less than optimum pickup.
¼-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. The chassis of the receiver provides the necessary ground plane. Do not use a ¼-wave antenna for remote antenna mounting.
A ½-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 ¼-wave antenna, in practice, this benefit is seldom realised. Therefore there is no compelling reason to upgrade to a ½-wave antenna unless remote antennas are required for the application.
The 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. They can provide up to 10 dB more gain than a ¼-wave antenna, and can also reject interfering sources from other directions by as much as 30 dB. Some unidirectional antennas have built-in amplifiers to compensate for losses due to long cable runs. Unidirectional antennas are primarily used for long-range applications and a minimum distance of 50 feet is recommended between transmitter and unidirectional antennas.
Antenna placement
When deciding where to mount antennas, always try to maintain line of sight between the receiving and transmitting antennas. Metal equipment racks will block RF from reaching the antennas mounted inside. Rear-mounted antennas may not work inside of a metal equipment rack! Antennas should be separated from each other by a minimum of one-quarter wavelength – about 40cm for VHF units and 10cm for UHF units. This helps ensure adequate diversity performance. Diversity reception can be improved by separating the antennas further, but beyond one full wavelength the advantage become negligible.
As far as height is concerned, receiver antennas should be positioned high enough to be clear of obstructions, including human bodies, which can absorb RF. Therefore placing the antennas high than “crowd level†(around 2m) is always recommended.
Receiving antennas should be oriented in the same plane as the transmitting antenna. Since the transmitting antenna is generally in the vertical position, receiving antennas should also be vertical. Never orient antennas horizontally.
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 is not only unsightly and a physical challenge, but actually degrades the performance of the wireless systems. Antennas spaced less than ¼ wavelength apart disrupt the pickup patterns of one another, resulting in erratic coverage. Additionally, closely spaced antennas can aggravate local oscillator bleed, which is a potential source of interference between closely spaced receivers. 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 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).
An additional consideration with passive splitters is the presence of DC voltage on the antenna inputs of some receivers. This voltage is usually present for powering remote antenna amplifiers directly off a 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. Depending on the design of the receiver, this may be a problem. To avoid any potential damage, either use a splitter than incorporates circuitry to block the voltage, use an external DC blocker, or defeat the voltage on at least one of the receivers.
If distribution is needed for more than two systems, an active antenna distribution system is recommended. Active splitters require power to operate, but provide make-up gain to compensate for the additional losses resulting from multiple splits of the same antennas. A typical system will have 4-5 antenna inputs.
Antenna remoting
As mentioned before, some installations require that the antennas be removed from the receiver chassis and placed in another location to ensure better line-of-site operation. Antennas can be placed outside of the rack on the microphone stands, wall brackets or any other suitable mounting device.
¼-wave antennas rely on the receiver chassis to maintain a ground plane, without which they lose their effectiveness. Therefore ½-wave antennas must be used when remote-installing antennas.
Because of RF loss issues in coaxial cables, it is important to use the proper low loss-coaxial cable. 50 ohm low loss cable is typically used in wireless microphone applications. Cable specifications from any manufacturer should list a cable’s attenuation (loss) at various frequencies in dB per 30m. 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 the cable run results in a loss of greater than 5dB, active antenna amplifiers muse be used to compensate in order 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 can usually be wall-mounted or stand-mounted.
Antenna combining
The converse of antenna distribution, antenna combining, can be employed in one of two ways. With wireless microphone systems, multiple antennas can be combined together to feed a single receiver (or multiple receivers with antenna distribution) to provide coverage across multiple rooms or extremely large spaces.
For multiple room coverage, use passive combiners. Since they do not require power and are typically compact, they can be located wherever necessary. A passive combiner will typically result in at least 3 dB of loss, so be sure to include this figure when calculating cable loss. Multiple combiners can be used in series, if more than two locations need to be covered, so long as enough amplification is provided to make up for whatever additional losses are incurred.
In most wireless microphone applications, there is rarely a single element that causes the whole system to fail, but rather an aggregation of bad practices that leads to poor performance. Following the guidance above should help you avoid those.