Five Golden Rules for Wireless Audio Antenna Placement
Guest contributor Alex Milne of RF Venue, Inc talks about the 5 rules for wireless audio antenna placement.
The following post on wireless audio antenna placement is a guest contribution from Alex Milne of RF Venue, Inc — a US based manufacturer of innovative products that make wireless audio systems work and sound better. For more information, please see his bio at the end of this post.
1. Create and Maintain Line-of-Sight:
In radio terminology for entertainment production, line-of-sight refers to radio waves traveling straight and across an unobstructed path between receiver antenna and transmitter antenna. It is essential that antennas of all kinds are deployed to maintain line-of-sight between performers and rack equipment in a configuration that minimizes interruptions to the line.
It is not always possible to maintain line-of-sight indefinitely. Even with good placement, performers' movements inevitably bring bodies or scenery between antennas for one or more brief intervals. The receiving antenna momentarily loses contact with the signal carried by the line-of-sight radio wave, and the receiver grabs hold of the next strongest signal on its tuned frequency. Usually this signal is a multi-path wave from the same transmitter—the performer's radio microphone transmitter, or rack-mounted IEM transmitter—that has "reflected" across walls and other materials (and traveled multiple paths) at locations other than the line-of-sight destination.
Multi-path waves carry signal containing the same audio information, but, because they have been absorbed and re-radiated several times by RF reflective materials within a building or outdoor venue, are weaker in amplitude than a line-of-sight wave.
The probability of a multi-path dropout (when two waves of similar amplitude arrive at a receiver 180 degrees out of phase) is much greater when the receiving antennas of a diversity system are presented with multiple reflected waves of similar amplitudes, versus between one strong, primary line-of-sight wave and comparatively weak reflected waves. Modern receivers weather multi-path propagation storms well, due to superior sensitivity and intelligent diversity switching circuits, but even the best receivers obey laws of physics and information theory, and will suffer frequent dropouts if line-of-sight is absent, regardless of how much they cost.
2. Create and Maintain Line-of-Sight:
Seriously. It's that important, and applies to all antennas, not just external antennas like paddles and helicals. Maintaining line-of-sight is the most important procedure in designing antenna deployments for minimal dropouts and interference, and obstructing line-of-sight is one of the largest threats to reliable wireless audio.
Some of the most common line-of-sight blunders include:
- Whip/dipole antennas kept in a rear facing rack, closed cabinet, or equipment closet. No line-of-site there!
- Elevating antennas enough to achieve line of sight in an empty presentation space or auditorium, but lower than the height of a standing audience once the show begins. Human bodies soak up RF like a sponge. Where an audience is likely to rise, place antennas on stands at least two meters/seven feet tall, or mount antennas on the sides of the venue, on the ceiling or catwalks/trussing, or in the left or right wings.
- Antenna elements in contact with metals and other conductive materials nearby. If even a single pinprick of an antenna element (exposed elongations that resonate with RF energy) make contact with any metal around it, like trussing, the antenna will perform erratically, or not at all, as the energy pattern is contaminated by the metal structure in contact with the element.
3. Closer is Better:
Strengthening signal-to-noise ratio (SNR) is the end-goal of most antenna deployments. Since radio waves dissipate in field strength non-linearly according to the inverse square law, reducing the distance between receive and transmit antenna significantly improves SNR.
In a live sound environment, an FOH antenna position 15 meters into the house will gather four times less energy than the same FOH position which has, say, 7 dBd paddle antennas remoted on 7.5 meters of low-loss coaxial cable, 7.5 meters closer to the stage.
Highly directional antennas may have coverage narrow enough that one can place them too close. Antennas with gains in excess of 10 dBd may not provide sufficient coverage for the entire performance space if too close—the performers are effectively lensed out of the pickup whenever they walk to the left or right of the coverage area. Beam angle along horizontal and vertical azimuths, a specification published by most antenna manufacturers, is used to roughly estimate the correct distance from the stage based on coverage pattern, and can be compared to the "angle of coverage" of a telephoto photographic lens or telescope. The smaller the angle, the tighter and more selective the coverage pattern.
4. Understand ERP:
In the same way that audio gain must be managed across devices and connections in an audio system, the gain structure of an RF system must be carefully designed.
A useful concept in RF systems design is ERP, which stands for "Effective Radiated Power." ERP is simple. It is the output power or ambient power originating from the transmitter, plus the gain of the antenna, minus the attenuation and losses incurred by cable runs and connectors in-between the antenna and destination device. It's important to know how ERP works, because you must understand how the relationship between signal strength, antenna gain, and in-line loss of coaxial cables changes the amount of RF gain given or taken away from an RF signal by an antenna + cable assembly and distribution accessories.
RF gain is measured with the same unit as audio gain—the decibel. This is a simplified statement, but it is possible to make because of the "dimensionless" quality of the decibel as a unit of measure, which is another discussion for another time.
RF gain in decibels represents the amplitude, or strength, of an increase or decrease a device or connection gives or takes away from an RF signal, or the amplitude of an RF signal as measured in the environment, or at a given point in a device chain.
When using remote antennas and antenna distribution, the transmitter output (whether that transmitter is located in an onstage microphone, or in the rack as an IEM) is but the first step. Physical devices and interfaces—cables, connectors, antennas, and amplifiers—also influence the amount of "effective" electromagnetic power that is radiated out into space, or, for receiving systems, the amount of effective power detected by the front-end of a receiver after the signal has traveled through all elements in an antenna distribution assembly.
This is much easier to show than it is to tell. Here is the formula for ERP in an IEM setup, visualized:
A more common scenario is the receive configuration, for radio microphones. ERP also applies here, but the "R" in ERP might be thought of as Effective Received Power.
Note how a receive antenna with +9 dB of gain is beneficial on a run of 50' of low-loss coaxial cable, because the gain imparted by the antenna exceeds the in-line attenuation of the 50' cable run, which takes away only 3 of the 9 dB provided by the high-gain antenna, giving a 6 dB improvement over the strength of the signal at the antenna site.
After a certain length cable run, however, the losses incurred across the cable become greater than the gain advantage given by the antenna, eliminating most improvements using a high-gain antenna brings.
Here, the receiver ends up receiving less power than what was available at the remote antenna location because of the excessive loss in the 150' run.
This is a common problem, with several solutions.
- Decrease the distance between entire assembly (including rack) between performers and antenna, without changing the cable run's length. I.e. moving FOH forward several seats.
- Use a higher gain antenna.
- Use in-line amplification to compensate for in-line loss. (in-line amplification and "active" antennas should always be used with caution. They increase RF gain, but also raise the noise-floor, which reduces SNR, and can easily cause a front-end overload if too much gain is applied)
- Use lower loss coaxial cable, like LMR-400, or LMR-800.
- Use an RFoF system and fiber-optic cable for antenna distribution, which has negligible in-line attenuation.
5. Sell the Antenna Farm:
An antenna farm is when many stock whips/dipoles are in close proximity to one-another, usually behind the rack.
Antenna farming is bad practice for two reasons. First, when antennas farms live at the rear of the rack, they're shielded from the performer by the rack furniture and electronics inside. Even with front-panel mounted antennas, those are at knee height—not good, because line-of-sight is always important! Second, a large number of omnidirectional antennas in a small space has lots of potential electrical and RF consequences, especially if transmit antennas are involved. IMD, near-field interactions, proximity to PSUs and other sources of interference are all reasons to sell the antenna farm.
Antenna farms are easily replaced with antenna distribution, which allows multi-channel system signals to travel through only two or three antennas, and eliminates wall warts, power strips, and the myriad cables that construct the byzantine cable spaghetti discovered behind unscrupulous wireless racks. Plus, proper antenna distribution almost always improves the quality and reliability of wireless audio.
About Our Guest Author:
Alex Milne writes for Audio Gloss, a blog by RF venue, Inc., a US based manufacturer of innovative products that make wireless audio systems work and sound better, specializing in remote antennas, RF distribution equipment and RF signal management and monitoring systems for audio/video installations and live sound events. Shure UK carries the full line of RF Venue antennas.