Getting The Most From Wireless: Antenna Issues
By Gary Stanfill
Colmar Systems

The usable range of a wireless system greatly depends upon the technical characteristics of the transmitter and receivers. It’s unrealistic to expect that a low-cost system will equal the range of a professional broadcast system costing $2,000 or more. However, attention to antennas is one very helpful way of improving range and reliability, regardless of system type and quality. (Part 1, with further chapters focusing on other facets).

Antenna height. For two reasons, the range of a wireless system can be improved by increasing antenna height. First, greater height usually means fewer obstacles in the way to absorb or reflect RF energy, so more signal arrives at the receiver. Second, as height increases, transmission losses are reduced. This is sometimes called “ground effect”, an interaction between radio waves and the ground and/or other reflective/absorptive surfaces. Loss can be reduced if either transmitter and receiver antennas (or both) are raised.

It’s usually more practical to elevate the receiver antenna (s), with a height of at least 8 feet (2.5 m) recommended. Still greater height will further reduce loss, but care must be taken that antennas are not positioned too close to metal in walls or ceiling.

Also note that cable loss will eventually offset gains made with increased height. As a rule of thumb, cable loss should be kept to 3 to 4 dB at the operating frequency. A loss of 6 dB may be tolerable, especially if the receiver must be located in an especially unfavorable area and the distance between the transmit and receive antennas is short. Keeping loss this low often requires the use of special large-diameter low-loss cable, such as Belden www.belden.com type 9913.

Receiver antenna orientation. With diversity receivers, one whip antenna should be at a 45- degree angle clockwise from vertical, the other at a 45-degree angle counterclockwise from vertical, and 90 degrees in relation to each other. Normally only a small improvement in range is realized, but every little bit helps.

If remote antennas are used, the same orientation should be used if feasible. Dipole, yagi, log periodic and similar antennas should be oriented at 45 degrees. However, some types of remote antennas, such as ground planes and 5/8-wave types don’t lend themselves very well to the 45-degree orientation and should be mounted vertically.

By design, ground plane antennas have poor reception beneath the ground plane. If a ground plane antenna is mounted high, as recommended for best range, it should be tipped forward in the general direction of the transmitter. If mounted near a ceiling, it can simply be turned upside down. When used with a false ceiling, a small hole can be made for the whip, with the ground plane simply laid on top of the ceiling. And since the cable can affect performance, be sure it is kept away from the antenna elements.

Gain antennas. At very long distances, antennas with gain may be necessary. The most common types are yagis and log periodics. A good gain antenna can increase the useful range of a system by two to three times. However, this improvement is only realized over a relatively small angle, and the more the gain, the smaller the angle. Therefore, these types of antenna are not suitable for situations where wide-angle coverage is required.

Bodypack antennas. When range is important, antennas for bodypack transmitters should be positioned as high on the body as feasible. The common practice of clipping the transmitter to the belt and letting the transmit antenna hang down can significantly reduce range, especially for VHF systems. The transmit antenna should also be kept straight and positioned away from the mic cable and transmitter case. And where possible, position the receive antennas to avoid having the wearer’s body between them and the transmit antenna.

If the wearer is likely to perspire heavily or become wet, the antenna should be kept separated from the skin by layers of clothing or other means. Under such conditions, the transmitter itself may require protection to prevent internal moisture that can greatly reduce power output or even cause damage.

Dealing with crowds. The human body absorbs RF energy, sometimes as much as 90 percent of the transmitter output. As a result, signals from transmitters in the middle of a large crowd will be greatly attenuated - a major problem at public events and theatrical productions. If range is long, the potential for trouble increases.

Solutions include raising the receiving antenna as high as feasible, so that transmission has a clearer path. It’s also helpful to position the transmit antenna as high as possible on the body. When practical, handheld transmitters may be more effective in tight situations because they’re normally held higher and away from the body.

With planning, it may be possible to position the receiving antennas behind the wireless wearer or at another location that minimizes the number of persons in the direct signal path. Using wireless systems with higher power output and more sensitive receivers may also be a solution. (Back to that system quality issue!)

A regular contributor to ProSoundWeb Install Sound, Gary Stanfill is principle consultant for Colmar Systems, based in Southern California. His company has provided engineering and marketing consulting services to wireless microphone manufacturers for several years. Stanfill was president and general manager of Vega, a leading manufacturer of wireless microphone systems, for a total of 18 years. He has over 30 years experience in audio, RF technology, communications and wireless systems. He can be reached at gjstanfill@earthlink.net

For more articles by Gary, including a look at the potential impact that DTV will have on wireless systems, click here.