Selecting the appropriate antenna for wireless operation is very important when it comes to planning any wireless system. Confirm the antennas in use support the operational frequency range being used by the transmitter and receiver. As we will discuss later, some antennas offer different characteristics such as gain or directivity and can be used to increase RF coverage under certain scenarios. Depending on location of the transmitter and receiver, desired RF coverage and available real estate onsite may require different options.
¼ Wave Whip
The ¼ Wave Whip antenna are very common in both transmitter and receiver wireless devices. Whips are omni-directional antennas and are mostly seen on bodypack transmitters and camera back receivers. The advantages of the ¼ wave whip antenna are the portability, but these antennas cannot be remotely mounted because it relies on the chassis of the transmitter or receiver for its ground plane. ¼ Wave Whip antennas are narrow bandwidth of around 60MHz and the center frequency of the antenna’s range is determined by the length of the antenna.
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. 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.
The Astral TX and Astral Mini ships with the following three antennas and set of antenna caps. Attach antennas with a length specific to the frequency in use. Using the wrong length of antenna reduces RF range and quality.
- 1 straight 470-548 MHz to SMA
- 1 straight 548-638 MHz to SMA
- 1 straight 17” uncut Astral VHF Whip Antenna to SMA)
To operate Astral Series wireless devices outside of the frequency ranges of the predefined antennas mentioned above, users must purchase the following antenna and cut it to length by following the instructions below. .
Cutting Astral VHF Whip Antenna to Length
Uncut antennas need to be cut to the length specific to the frequency for intended use.
- Position the antenna cap onto the cut-end of the antenna and push the cap firmly onto the antenna. Antenna Length Chart
- Measure the antenna for the cut point. Measurements should begin from the bottom of the SMA connector.
- Using a pair of cutters, cut the quarter-wave antenna to the specified cut point.
- Position the antenna cap onto the cut-end of the antenna and push the cap firmly onto the antenna. Antenna Length Chart
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.
Astral Monarch
The Astral Monarch is an omnidirectional, wide-bandwidth antenna that supports 470-1525 MHz. It provides uniform coverage and gain across the exceptionally wide SpectraBand 470-1525 MHz tuning range of the Sound Devices Astral series wireless products. This gives it its advantage over the ¼ wave whip and half wave dipole antennas. The Astral Monarch antenna is passive and does not require bias power so it can be used for both microphone receive and IEM/IFB transmit up to 5 watts. The antenna is paired with a multifunction clamp and articulating arm, as well as two accessory cables to be integrated into a bag mixer or in a stage configuration.
When to use: Indoors or outdoors in a bag mixer, cart or in a stage application. The antenna is omnidirectional and can provide even coverage around a performance or filming location. Advantages of wide frequency bandwidth to use with SpectraBand devices.
Log Periodic Dipole Array or 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. Make sure to activate bias power on the receiver when operating active antennas or booster amplifiers.
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 ¼ Wave Whip and Half Wave Dipole.
Circularly Polarized 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 polarized 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 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 average RF loss of 3.1 dB every 25 feet at 585 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.
If longer cable lengths are required, try using alternative larger diameter cables. The chart below displays the loss in dB per 25 feet. As cable diameter increases, loss in theory decreases. There are also benefits to better shielding in cables like Time Microwave LMR-240 UF and LMR-400 UF reducing unwanted induction from other RF/electronic devices from getting into the RF antenna circuit at the receiver. However, the tradeoff is the larger diameter cables are heavier, not as easy to coil and usually cost more.
The RG-8U and LMR-400 UF cable is much lower loss at the equivalent frequency than RG-58, but it is also unwieldy and more suitable for fixed installations and stage than location sound and portable 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.