Picking the Right Antenna for Digital Wireless Audio
Antennas are critical components of a wireless system. In many ways, they perform like microphones with receivers, and loudspeakers with transmitters. Just like with microphones and loudspeakers, there is no single antenna suitable for all applications. A strong, predictable RF performance requires the use of the right antenna.
¼ Wave Whip
¼ wave whip antennas are included with most wireless systems, including the A10 Digital Wireless System, for both transmitters and receivers. Whips are omnidirectional antennas suitable for many applications, and are used on bodypack transmitters for their simplicity and portability. ¼ whip antennas must be mounted directly to the transmitter or receiver. They cannot be remotely mounted because they rely on the chassis of the transmitter or receiver for their ground plane.
When using bodypack transmitters, keep some distance around the antenna. Allowing the antenna to touch skin or damp clothing directly will dramatically reduce the transmitter’s RF output. Just a few mm of distance from the skin can result in 10 dB more output, which has a direct impact on the range of the system. It is also important to use the proper length ¼ wave antenna for the operating frequency. Folding or bending the antenna by inserting antenna-first into a pocket, etc, dramatically reduces the effectiveness of the antenna.
When to use: ¼ wave whip antennas are normally used in portable systems, such as one or two RXs being deployed in a bag, or when the RX is mounted on and linked to a camera.
Half Wave Dipole
Compared to ¼ wave whips, half wave dipole antennas have an improved system range. Like ¼ wave whips, dipoles are omnidirectional. An important benefit of the half wave dipole is its ability to be remotely mounted due to its built-in ground plane.
Dipole antennas have an operating bandwidth of approximately 50-60 MHz, so check that the antenna is designed for the intended frequency range. This is especially important when using receivers which tune across a broad range. Some dipoles are frequency adjustable, which allows the center frequency to be tuned.
When to use: For bag use, portable dipole antennas can be clipped to a bag or strap and connected via cable to an RF distribution amplifier or a slot mount rack, such as the Sound Devices SL-6. Dipole antennas can also be used on carts, especially in small studio environments.
LPDA
LPDA antennas, or log periodic dipole array, are a common directional antenna used for wireless microphone systems. On a sound cart, LPDA are used for both receiver and IFB transmitter applications. LPDA, sometimes referred to as “shark fin” antennas, are remotely mounted. These directional antennas have a theoretical gain advantage of up to 8 dB over a ¼ wave whip. Some LPDA antennas include active circuitry. Active antennas have built-in RF amplifiers which can be engaged to make up for signal loss with long cable runs.
When transmitters are in close proximity to an LPDA antenna, there is a potential to overload the receiver. Reduce TX power or deploy RF attenuation after the antenna, before the receiver.
When to use: LPDA antennas are best used outdoors or to increase the range of the system. LPDA are directional, so ensure that the transmitters are in the field of view of the antennas when in use. These antennas operate over a wider range of frequencies than the Yagi type (below).
Yagi
Yagi antennas are another type of directional antenna. The benefit of a Yagi is its high gain and directionality. The downside of using a Yagi is its narrow operating frequency range and the narrowing angle of pick-up as the number of elements on the antenna increases. For users requiring frequency agility, a Yagi may not be practical.
When to use: Like LPDA antennas, Yagi antennas are best used outdoors, or to increase the range of the system. Yagi antennas have higher gain than LPDA antennas.
Circularly polarised helical
Another high-gain directional antenna is the helical antenna. On-talent transmitters with fixed ¼ whip antennas can often end up in unpredictable orientations, or polarity, relative to receiving antennas. Helical, circularly polarised antennas operate with the same efficiency regardless of antenna polarization, resulting in fewer dropouts.
When to use: Helical antennas are often used for in-ear monitoring transmitter applications as well as receiver applications, and to circumvent issues with polarity on-set.
Choosing a Cable
Quality 50 Ω RF cables are an important component of a wireless system. As cable lengths increase, so too does signal loss. RG58 cable is commonly used for antenna-to-receiver connections because it is reasonable in size, flexibility, and cost. It has an RF loss of 4 dB every 10 m at 400 MHz (see chart below). When cable losses approach more than 6 dB, consider applying gain to make up for the loss. The goal is to achieve unity gain, no more.
WBC400 cable is much lower loss at the equivalent frequency, but it is also unwieldy and more suitable for fixed installations than location use.
Note: Coaxial cables used to interconnect video signals are typically 75 Ω with 75 Ω BNC connectors. While they may look identical to 50 Ω cables and connectors, there is an added signal loss when using 75 Ω cables.
Part 2: Picking the Right Antenna for Digital Wireless Audio
Part 3: Minimizing RF Problems When Using Digital Wireless Audio
Part 4: Maximizing Wireless Range When Using a Production Bag