When was the last time you paid attention to the antennas in your wireless system? Maybe it was at that gig when you realized they were missing and needed last-minute replacements. Then asked if anyone in-house had antennas that you could borrow, which is risky business because substituting antennas from another wireless system is a bad idea.
Antennas from another manufacturer (or even from the same manufacturer) may look similar to the ones in your system, but that doesn’t mean they’ll work properly. The “whip” antennas furnished with wireless systems are usually tuned and band-limited — meaning that they’re designed to work within the same frequency range as the system’s wireless transmitter and receiver. This reduces interference from other nearby RF sources, but you can probably guess what happens when you substitute antennas that are band-limited to a different frequency range. While we’re on the subject, note that changing the length of an antenna changes its tuning, so never cut an antenna, and make sure telescoping antennas are extended to their maximum length.
How Well Does Your Wireless System See?
Manufacturer supplied transmission range specs generally refer to line of sight, which means the transmitter and receiver have a direct path with no obstructions between them. Unfortunately, just about everything on a stage is an obstruction: drums, cymbals, mic stands, equipment racks, speaker cabinets, walls, stage props and even people are all obstacles to wireless transmission. That’s why antenna placement is so important. Get the antenna out of the back of the rack. Kits available from an assortment of manufacturers enable you to move the antenna to the front of the rack, and on smaller stages or houses-of-worship, that might be enough to achieve line-of-sight from the transmitter to the receiver. The antennas in Audio-Technica’s System 10 PRO (Fig. 1) are designed to be easily relocated for best reception.
Diversity wireless systems have two antennas, which should be spaced three to six inches (¼-wavelength) apart. Spacing them one wavelength apart would be even better: 12 to 24 inches for UHF systems, or 16 to 36 inches apart for VHF systems. Increasing the spacing beyond that may not improve the diversity performance, but might increase the range or cure that pesky line-of-sight issue. To improve reception, angle each antenna 45° from vertical to form a “V.”
The transmission pattern of a whip-type antenna is “vertically polarized,” meaning that it’s omnidirectional in the plane perpendicular to the whip (picture the transmission pattern as a donut placed around the whip). Positioning the receiver’s antennas the same way will improve reception, but that may be difficult when dealing with a handheld vocal mic or a guitar player who likes to stalk around the stage.
Quarter-wave antennas should be electrically grounded to the receiver, and they also require a “ground plane” for proper operation. (A ground plane is a reflective metal surface approximately the same size as the antenna in at least one dimension.) The chassis of the rack unit normally serves as the ground plane, making it a bad idea to remote-mount a ¼-wave antenna. Half-wave antennas such as the Shure UA8 in Fig. 2 do not require a ground plane and are more appropriate for remote use.
More effective options for remote use are LPDAs and helical antennas. LPDAs — Log-Periodic Dipole Arrays — look like shark fins and have a cardioid pattern with a coverage angle of roughly 120 degrees. The cardioid pattern provides a higher degree of interference rejection and higher gain than you’d get using ¼- or ½-wave antennas. The Lectrosonics ALP500 in Fig. 3 is a broad-bandwidth, LPDA that receives DC power via the coax cable.
Helical antennas (such as the PWS model in Fig. 4) typically have higher gain and are more directional than LPDAs, with a beamwidth of around 60°. They transmit RF in a circularly polarized field, which is useful in IEM applications because RF can be received from a body pack regardless of whether the pack’s antenna is oriented vertically or horizontally.
Directional antennas can be broad-band or band-specific, so make sure the one you plan to use operates in the same frequency range as the transmitters and receivers.
Coax cables used to connect remote antennas should have an impedance of 50 ohms, which results in less transmission loss over the length of the cable; off-the-shelf coax that you’ll find in a big-box store has a 75-ohm impedance that will yield a higher signal loss.
Splitsville
These days, it’s fairly common for even small acts to use 10 or 20 RF systems on stage, and that means a rack full of wireless gear and antennas that can interfere with each other (not to mention the visual mess). Splitters (such as the Shure UA844+ in Fig. 5) and combiners help manage the chaos.
There are two basic types of splitters: passive and active. A passive RF splitter allows one antenna to feed two receivers (double that equation for diversity systems), but there’s a signal loss of about 3 dB. If you try to feed more than two receivers from a passive splitter, RF performance will likely drop to an unacceptable level. Some receivers have antenna outputs that permit daisy-chaining to a second receiver, but check with the manufacturer to see if you can chain a third receiver while maintaining an acceptable level of signal quality.
As the name implies, active splitters are powered and use amplification to facilitate distribution of the signal. An active splitter usually can feed four to eight receivers and often has the capability of powering the attached receivers through the coax cables used to distribute the RF signals, thus simplifying the power requirements of the system.
An RF combiner (such as the Sennheiser AC3200-II in Fig. 6) is used to merge the outputs from multiple transmitters to a single antenna, which comes in very handy when dealing with IEMs.
Used in conjunction with a directional antenna, a combiner allows RF transmission to be targeted at the stage so performers’ IEM receivers receive a strong signal, while reducing the mess associated with using separate antennas for multiple IEM systems.
Steve “Woody” La Cerra is the tour manager and front of house engineer for Blue Öyster Cult.
