AKG Acoustics WMS 40 SELECTING, Placing and Using Antennas, Signal loss caused by the audience

AKG WMS ANTENNA TECHNOLOGY

HOW TO USE ANTENNAS

SELECTING, PLACING AND USING ANTENNAS

Any radio system uses antennas to get a sig- nal from one place to another. To ensure the best possible signal quality, it is imperative to select the optimum antennas for the system and place the antennas correctly. Reflections, shadow loss, or deep fades may weaken or even cancel the radio signal (drop- out). If you obey a few simple rules for pla- cing your antennas, transmitters, and recei- vers, your system will operate smoothly.

Absorption by or reflections off metal grid structures, the audience, and the musicians on stage (see illustration below) will attenuate any radio signal.

For best results, place the receiver near the stage but at least 5 feet (1.5 m) away from any metal beams, spotlights, lighting con- trol consoles, computers, or other digital equipment. Make sure the transmitter will always be at least 21 feet (7 m) away from the receiver (see Antenna Position Check Applet on page 45). Since UHF signals pro- pagate in a similar way to light, always keep a direct line of sight between the transmit- ter and receiver.

Antennas, like microphones, have different polar patterns. Depending on the venue and type of system, you may need directional antennas, such as Yagi (cardioid to hyper- cardioid) or log periodic (shotgun) types, or omnidirectional antennas with no preferred

direction. Directional antennas are usually the best bet where the transmitters will only be used within a relatively small area, e.g., on a stage. Directional antennas can be used to overcome long distances or reject unwanted signals from off-axis directions. This is why they are very popular for open- air events. The greatest benefit of direc- tional antennas – provided their gain is high enough – is that you can place them far enough from the stage that all transmitters appear to be at the same distance from the antenna. This prevents transmitters nearer to an antenna from generating intermodula- tion products that may interfere with the weaker signals from more distant transmit- ters. Use active omnidirectional antennas in rooms that are too small for directional antennas. We recommend mounting the two antennas vertically polarized and as high as possible above the performers.

Select your antenna cables very carefully, too. The antenna cable must feed the out- put signal of a remote antenna to the recei- ver. Note that any antenna cable will attenuate the signal it carries (cable attenuation). Different types of cable have different amounts of attenuation so which type works best depends on the length of the cable run. So if you need very long antenna cables, go for a low-attenuation type even though it will be thicker and more expensive than high-attenuation cable.

To compensate for the attenuation of long antenna cables, use either active antennas or in-line boosters. In many situations, though, you may be able to save on active antenna components by using the next- higher (slightly more costly) grade of cable. Using the optimum type of antenna cable may be the key to a smoothly working wire- less system and helps reduce the levels of cost, stress, and aggravation.

Large open-air festivals are one example where antenna placement is of paramount importance because the transmitters are usually far away from the antennas, and more often than not there will be other radio links (radio or TV station O/B vans, etc.) to deal with as well. We recommend using directional antennas, and don't be a miser when it comes to buying antenna cable!

The only way to maintain good signal quali- ty over long cable runs is to use expensive, high quality antenna cable, e.g., a type with a foam dielectric. A booster such as the AKG AB 4000 can compensate for 17 dB of cable attenuation, allowing you to add ano- ther 200 feet (60 m) or so of RG 213 cable to your antenna line. For extremely long lines, you can even use two boosters in series. If you have to route the antenna cables through a cable duct that may be prone to RF interference, use double-shiel- ded cable.

Signal loss caused by the audience

The human body reflects and weakens radio signals. One problem that has plagued cellular telephone systems is the absorption of microwaves by human body tissues. Similar to the reverberation time, the RF level in a room decreases as the room becomes more crowded and absorption increases.

Even if there is a line of sight between the transmitter and receiver, the audience in between will weaken the RF sig- nal because part of the RF energy hits the people and is absorbed by their body tissues. We therefore recommend placing the antennas so that the line of sight will be at least 3 1/2 feet (1 m) above the audience's heads to re- duce this absorption effect.

Penetration depth in human body tissues for 4.3 dB attenuation:

Skin	4.3 mm
Fat	10.4 mm
Muscle	2.8 mm
Cartilage	21.0 mm

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