Shure Wireless Systems Guide: Antenna Setup
Optimizing Performance Through Proper Antenna Management
By Gino Sigismondi and Crispin Tapia
Introduction
The world of professional audio relies on transducers, devices that convert one form of energy to another. For microphones and loudspeakers, this means converting sound waves to electrical impulses and vice versa. Wireless audio systems introduce another crucial transducer: the antenna. According to the ARRL (American Radio Relay League) Antenna Book, an antenna's purpose is to convert radio-frequency electric current into electromagnetic waves for radiation into space. Conversely, when attached to a receiving device, an antenna converts electromagnetic waves back into an electric current, a principle similar to how a loudspeaker can function as a microphone.
Following specific guidelines is essential for maximizing antenna performance. For radio frequencies, factors like antenna size, orientation, and cable selection are critical. This guide outlines best practices for typical wireless audio applications, focusing on professional systems with detachable antennas. Entry-level systems with fixed antennas may not support antenna distribution or remote mounting.
These recommendations serve as useful guidelines, not absolute rules. While a system might still function if one guideline is overlooked, adhering to them collectively significantly improves performance and reduces the likelihood of issues.
Section One: Antenna Types
The physical size of an antenna is directly related to the wavelength of the frequency it is designed to receive or transmit. The most common types used in wireless audio systems are 1/4-wave and 1/2-wave omni-directional antennas, along with unidirectional antennas.
Omnidirectional Antennas
A 1/4-wave antenna's length is approximately one-quarter of the desired frequency's wavelength, while a 1/2-wave antenna is half the wavelength. Wavelength (L) can be calculated using the speed of light (C) and frequency (F) with the formula: C = L x F. For example, a 200 MHz wave has a wavelength of approximately 6 feet (2 m). Thus, a 1/2-wave receiver antenna would be about 3 feet (1 m) long, and a 1/4-wave antenna about 18 inches (45 cm). Antenna length only needs to be approximate. For VHF, 14-18 inches (35-45 cm) is suitable for a 1/4-wave antenna. For UHF, which has a broader frequency range, 1/4-wave antennas can range from 3 to 6 inches (7-15 cm). For a 500 MHz system, a 1/4-wave antenna should be about 6 inches (15 cm). Using an 800 MHz antenna (approx. 3 inches) would result in suboptimal performance. Wideband omnidirectional antennas covering the entire UHF band are available for systems requiring receivers with different tuning ranges to share a single antenna.
The Wave Equation: C = L x F, where C = speed of light (3x108 m/s or 186,000 miles/s). L = 300/f meters, where f is frequency in MHz. A table shows sample wavelengths in meters for frequencies from 100 MHz to 900 MHz.
Diagram Description: The Wave Equation diagram illustrates the relationship between wavelength and frequency, including a formula and a table of wavelengths for various frequencies.
1/4-wave antennas are intended for direct mounting to wireless receivers or antenna distribution systems, including front-mounted rack ears. They require a ground plane – a metal surface approximately the same size as the antenna in at least one dimension – for proper reception. The antenna base must be electrically grounded to the receiver chassis, which serves as the ground plane. 1/4-wave antennas should not be used for remote antenna mounting.
A 1/2-wave antenna does not require a ground plane, making it suitable for remote mounting. While it theoretically offers about 3 dB gain over a 1/4-wave antenna, this is seldom realized in practice. Therefore, 1/2-wave antennas are primarily chosen when remote antenna placement is necessary.
Diagram Description: An illustration shows a 1/4-wave antenna and a 1/2-wave antenna, noting their use in the UHF range.
Unidirectional Antennas
Unidirectional antennas, such as Yagi or log-periodic antennas, are also suitable for remote mounting. They feature a horizontal boom with multiple transverse elements and can provide up to 10 dB more gain than a 1/4-wave antenna, while rejecting interfering sources by up to 30 dB. Yagi antennas are typically used for their narrow bandwidth (around a single TV channel, 6 MHz). Log-periodic antennas offer wider bandwidth due to their varying dipole sizes and spacing. Longer booms and more elements increase bandwidth and directivity. Some unidirectional antennas include built-in amplifiers to compensate for losses over long cable runs.
In wireless microphone applications, unidirectional antennas are generally used for UHF systems and are frequency-specific. Directional VHF antennas can be 3-5 feet (1-2 m) wide, making them mechanically challenging to mount. Their transverse elements should be oriented vertically, as transmitting antennas are typically vertical. Unidirectional antennas are best suited for long-range applications, with a recommended minimum distance of 50 feet (15 m) between the transmitter and antenna.
Section Two: Antenna Placement
Most wireless receivers feature antenna inputs on the rear panel. Diversity receivers typically have both 'A' and 'B' antenna inputs, often using BNC connectors, though older VHF systems might use PL-259 connectors. Rack-mountable receivers often have pre-cut holes for front-mounting antennas. Short coaxial cables and bulkhead adapters are needed to bring antennas to the front.
When mounting antennas, always strive for a clear line of sight between the receiving and transmitting antennas. Rear-mounting is ideal if the rack faces the performance area. If the front faces the area, front-mounting might be better, provided rack doors don't obstruct them. Metal equipment racks can block RF signals, potentially hindering rear-mounted antennas. If the receiver is not rack-mounted, ensure the antennas are directly visible from the transmitting position.
Antenna Spacing
To ensure adequate diversity performance, antennas should be separated by at least one-quarter wavelength. For VHF, this is about 16 inches (40 cm), and for UHF, about 4 inches (10 cm). Greater separation can improve diversity but offers diminishing returns beyond one full wavelength. Increased separation can also be beneficial for maintaining line-of-sight from various room locations.
Diagram Description: The 'Adequate spacing' diagram illustrates optimal antenna separation, recommending a minimum of greater than 1/4 wavelength and ideally greater than 1 wavelength, with specific measurements for VHF (16 inches) and UHF (4 inches).
Antenna Height
Receiver antennas should be kept clear of obstructions, including human bodies, which absorb RF signals. Placing antennas higher than typical crowd level (5-6 feet or 2 m from the floor) is recommended.
Diagram Description: An illustration depicts proper and improper antenna and receiver placement relative to a crowd, with a checkmark indicating the correct setup.
Antenna Orientation
Receiving antennas should match the orientation of the transmitting antenna, typically vertical. For handheld transmitters used by performers, antenna position can vary. A compromise is to angle antennas at approximately 45 degrees from vertical. Never orient antennas horizontally, which can occur when rack-mounted receivers have limited clearance. If vertical orientation is not possible, use front-mounting adapters or remote-mount antennas. Antennas must be kept clear of metal surfaces by several inches and should not touch or cross other receiving antennas. Antenna distribution systems can help mitigate these placement challenges.
Antenna Distribution
Proper antenna distribution is crucial for optimal performance in multi-system environments. Stacking or rack-mounting receivers can lead to closely spaced antennas, degrading system performance and causing erratic coverage due to disrupted pickup patterns. Close spacing can also exacerbate local oscillator bleed, causing interference between receivers. For remote antenna applications, distribution systems minimize the number of antennas and cable runs by splitting a single antenna pair to feed multiple receivers. This can be achieved through passive or active means.
Passive Splitters (2 receivers)
Passive splitters are cost-effective and require no power. Each split introduces approximately 3 dB of signal loss. It is generally advised to limit loss between antennas and receiver inputs to 5 dB. Therefore, passive splitters are best suited for splitting a single antenna to two receivers. A consideration is the DC voltage present on some receiver antenna inputs, often used to power remote antenna amplifiers. When using a passive splitter, each receiver may 'see' the voltage from the other, which can cause issues depending on receiver design. To prevent potential damage, use a splitter with voltage-blocking circuitry, an external DC blocker, or disable the voltage on one receiver.
Diagram Description: A diagram shows two antennas connected via passive splitters to two receivers.
Active Antenna Distribution (3 or more receivers)
For systems with more than two receivers, active antenna distribution systems are recommended. Active splitters provide make-up gain to compensate for losses from multiple splits. A typical active system offers 4-5 antenna outputs and may also provide power to receivers. Multiple active distribution systems can be linked, but caution is advised. Ideally, a distribution system provides unity gain; however, active systems may introduce 1.5-2 dB of gain, potentially causing intermodulation products or increased noise if over-amplified. It is recommended not to cascade more than two distribution systems deep. A 'master/slave' configuration, where a master system splits the signal to slave systems, is a better approach to keep receivers closer to the pure antenna signal.
Pay attention to the specified frequency bandwidth of distribution systems, which can be wideband (covering several hundred MHz) or narrowband (limited to 20-30 MHz). Frequencies outside the specified bandwidth will not pass to the receivers.
Diagram Description: A diagram illustrates an active antenna distribution system connecting four receivers.
Summary:
- 2 receivers typically use a passive antenna splitter.
- 4-5 receivers typically use active antenna distribution systems.
- More than 5 receivers may require multiple active systems connected in a 'master/slave' arrangement.
Antenna Remoting
Antennas may need to be relocated from the receiver chassis for better line-of-sight. Antennas can be mounted on stands, wall brackets, or other suitable devices. When remoting antennas, 1/2-wave or directional antennas must be used, as 1/4-wave antennas rely on the receiver chassis for their ground plane. Directional antennas are also designed for remote mounting.
Due to RF loss in coaxial cables, using the correct low-loss coaxial cable is important. 50-ohm low-loss cable is standard for wireless microphone applications. While 75-ohm cable can be used, it may introduce slight loss due to impedance mismatch, typically less than 1 dB.
Active antenna amplifiers are used to compensate for cable loss exceeding 5 dB. These amplifiers can offer selectable gain levels. Power for these amplifiers is supplied by the receiver's antenna inputs or the distribution system. It's important to consult receiver specifications regarding voltage availability. The amplifier should be placed at the antenna. Connecting amplifiers in-line is possible for longer cable runs, but ensure the receiver or distribution system can supply sufficient current. Antenna amplifiers, like distribution systems, are often band-specific.
Diagram Description: An illustration shows a remote antenna amplifier mounted on a wall or stand.
Hint: Avoid over-amplifying the radio signal, as this can overload the receiver's front-end, causing drop-outs and RF 'bleed'. Use only the gain necessary to compensate for cable loss, ideally keeping net gain below 10 dB.
Summary:
- Always use 1/2-wave or directional antennas for remote mounting.
- Use the proper low-loss cable for the installation.
- Use required antenna amplifiers to compensate for cable loss.
Antenna Combining
Antenna combining is the inverse of antenna distribution. It allows multiple antennas to feed a single receiver (or multiple receivers with distribution) for broader coverage, or enables multiple transmitters to share a single antenna, particularly for wireless personal monitor systems.
Multi-room Antenna Setups
For multi-room coverage, passive combiners are recommended. They are compact and require no power. Passive combiners typically introduce at least 3 dB of loss, which must be factored into cable loss calculations. Multiple combiners can be used in series, provided sufficient amplification compensates for cumulative losses. For systems needing more amplifier power than receivers can supply, additional bias 'Tee' power adapters are necessary to inject bias voltage into the antenna cable.
To minimize potential phase cancellation and signal dropout, it's important to keep multiple antennas feeding a common receiver input as isolated as possible.
Diagram Description: A diagram shows a multi-room setup with three separate rooms, six antennas, passive combiners, and RF amplifiers connected to an equipment rack.
Diagram Description: Another diagram illustrates a multi-room setup within a single room divided by airwalls, connecting four antennas via passive combiners.
Antenna Combining for Personal Monitor Transmitters
Antenna combining is vital for optimal RF performance with personal monitor transmitters, as multiple high-power transmitters can cause intermodulation issues. For combining two transmitters, a passive combiner is suitable. For more than two, an active combiner (typically accepting 4-8 transmitters) is recommended. Unlike active distribution systems, active combiners should not be cascaded. If more than one combiner is needed, a passive combiner should connect two active combiners. Be mindful of any additional losses introduced by passive combiners.
Active combiners, like active distribution systems, have specified frequency bandwidths. Ensure the selected bandwidth matches the transmitter frequencies.
Diagram Description: A diagram illustrates active antenna combining for personal monitor transmitters.
Diagrams
Refer to pages 7 and 8 for rear connection details.
2 receivers
Diagram Description: Shows two receivers connected via two passive splitters, each receiving signals from an 'A' antenna and a 'B' antenna.
3-4 receivers
Diagram Description: Illustrates an active antenna distribution system with 4 outputs, connecting to receivers.
5-8 receivers
Diagram Description: Depicts two active antenna distribution systems, each with 4 outputs, connected to receivers, along with two passive splitters. A note indicates that for 5 receivers, only one active splitter is required.
9-12 receivers
Diagram Description: Shows three active antenna distribution systems, each with 4 outputs, connected to receivers, with 'A' and 'B' antenna inputs.
13-16 receivers
Diagram Description: Illustrates five active antenna distribution systems, each with 4 outputs, connected to receivers, with 'A' and 'B' antenna inputs.
Large system: 50 channels (dual receivers)
Diagram Description: Shows six active antenna distribution systems, each with 5 outputs, connected to dual receivers, with 'A' and 'B' antenna inputs.
Antenna combining: 2-4 systems
Diagram Description: Depicts a 4-to-1 antenna combiner setup, showing connections to transmitters and a combiner unit.
Antenna combining: 5-8 systems
Diagram Description: Illustrates an 8-to-1 active combiner setup, showing connections to transmitters and the combiner unit.
Antenna combining: 9-12 systems
Diagram Description: Shows a setup with an 8-to-1 active combiner and a 2-to-1 passive combiner, along with a 4-to-1 active combiner, connecting to transmitters.
Antenna combining: 13-16 systems
Diagram Description: Depicts a setup with two 8-to-1 active combiners, each with a 2-to-1 passive combiner, connecting to transmitters.
Remote antenna – 100 feet (~30 m)
Diagram Description: Shows a remote antenna setup with a 100 ft (30 m) RG213 cable. Includes a 'Net Gain Calculation' table showing 0 dB for Antenna, +10 dB for Amplifier, and -7 dB for the cable, resulting in a Net Gain of +3 dB.
Remote antenna – 75 feet (~20 m)
Diagram Description: Illustrates a remote antenna setup with a 25 ft (7 m) RG8X cable to Amp #1, then a 50 ft (15 m) RG8X cable to Amp #2. Includes a 'Net Gain Calculation' table showing 0 dB for Antenna, +3 dB for Amplifier #1, +10 dB for Amplifier #2, and -10 dB for the 75 ft RG8X cable, resulting in a Net Gain of +3 dB.
Remote antenna – 50 feet (~15 m)
Diagram Description: Shows a remote antenna setup with a 50 ft (15 m) RG8X cable. Includes a 'Net Gain Calculation' table showing 0 dB for Antenna, +10 dB for Amplifier, and -6 dB for the 50 ft RG8X cable, resulting in a Net Gain of +4 dB.
Remote antenna – 30 feet (~10 m)
Diagram Description: Depicts a remote antenna setup with a 25 ft (7 m) RG8X cable. Includes a 'Net Gain Calculation' table showing 0 dB for Antenna, +3 dB for Amplifier, and -3 dB for the 25 ft RG8X cable, resulting in a Net Gain of +0 dB.
Remote antenna – <30 feet, 10 m
Diagram Description: Shows a remote antenna setup with a 6 ft (2 m) RG8X cable. Includes a 'Net Gain Calculation' table showing 0 dB for Antenna and -1 dB for the 6 ft RG58 cable, resulting in a Net Gain of -1 dB.
More system diagrams are available on the Shure Knowledge Base at www.shure.com/support.
Quick Tips
The following tips offer workarounds for situations where specific accessories may not be readily available. While system failure is rarely due to a single factor, an accumulation of poor practices can lead to performance issues. Implementing one or two of these 'in a pinch' solutions is acceptable.
- For remote antenna applications lacking a 1/2-wave antenna, a 1/4-wave antenna can be used if connected to an amplifier. Performance should be comparable to a 1/2-wave antenna remote-mounted with an amplifier.
- A 1/4-wave antenna can be used in remote situations without an amplifier, provided a ground plane is present. This ground plane should be a metal surface at least 1/4-wavelength in diameter and grounded to the BNC connector.
- Using 75-ohm cable is acceptable for remote antenna applications and is often less expensive than 50-ohm cable, though cable loss must still be considered.
- Antennas are frequency-sensitive. Use the correct antenna for your wireless system's frequency. While efficiency may decrease outside the designated range, the 'wrong' antenna, if close in frequency, can still be used with minor RF signal attenuation. VHF antennas should not be used for UHF systems, and vice versa. Active antennas are strictly band-limited.
- Dipole antennas designed for transmission (e.g., in wireless personal monitor systems) can also serve as receiver antennas (and vice versa), provided they operate within the correct frequency range. This also applies to passive directional transmitting antennas.
Suggested Reading
For further information on antennas and wireless microphone applications, the following publications are recommended:
- The ARRL Antenna Book - 19th Edition, The National Association for Amateur Radio, Newington, CT, 2000. ISBN: 0-87259-804-7
- Selection and Operation of Wireless Microphone Systems, Tim Vear, Shure Incorporated, Niles, IL, 2003.
Additional Shure Publications Available
Printed or electronic versions of the following guides are available free of charge. To obtain copies, call the phone numbers listed or visit www.shure.com:
- Microphone Techniques for Studio Recording
- Microphone Techniques for Live Sound Reinforcement
- Selection and Operation of Audio Signal Processors
- Selection and Operation of Personal Monitor Systems
- Selection and Operation of Wireless Microphone Systems
- Audio Systems Guide for Video Production
- Audio Systems Guide for Houses of Worship
- Audio Systems Guide for Meeting Facilities
Our Dedication to Quality Products
Shure offers a comprehensive range of microphones and wireless microphone systems for users of all levels, from beginners to professionals in the music industry, catering to nearly every application. For over eight decades, the Shure name has been synonymous with quality audio. All Shure products are engineered for consistent, high-quality performance under demanding real-life operating conditions.
