AKG Acoustics WMS 40 - Manual (Page 49)

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S
ELECTING
,
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