AKG MP 40 User Manual download pdf (Page 49)

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SELECTING, PLACING AND USING ANTENNAS

HOW TO USE ANTENNAS

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

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.

AKG WMS ANTENNA TECHNOLOGY

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