AKG Acoustics WMS 40 - Manual (Page 9)

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Signal propagation
As in a game of billiards, it is often impossible to reach a goal
via a direct path. In the diagram on the left the red ball repre-
sents the receiver, the while ball the signal, and the black ball
an obstacle between the two. However, in practice we are not
dealing with a single ball, but with very many simultaneously.
Thus there is a comparatively high probability that at least one
of the balls will reach its goal.
1. The signal hits the obstacle directly and fails to reach its
goal.
2. The signal reaches its goal after being reflected off the wall.
3. If two signals reach their goal simultaneously, they cancel
each other out in a situation referred to as dropout.
4. In spite of being reflected, the signal fails to reach its goal,
and continues to be propagated until its energy is expen-
ded.
fies it as necessary. An FM radio set – which
everyone is familiar with – works on exactly
the same principle.
Now that we have seen that a wireless micro-
phone system functions just like an FM radio
station on a narrow bandwidth, we can also
see that it is subject to exactly the same phy-
sical laws that govern the propagation of
radio waves. The challenge here is to find a
technical solution to the typical phenomena
of wave propagation, such as cancellation,
heterodyning, and absorption. Other factors
such as RF interference, electrical and elec-
tromagnetic fields, and intermodulation are
also severely disruptive, and have a negative
impact on the transmission quality of a wire-
less installation, significantly impairing the
quality of the radio link and the audio signal.
But more about that later!
It is particularly important to choose the right
antenna and set up the antenna and receiver
correctly if we wish to avoid loss of quality.
The rule of thumb is that the best radio
transmission path is always the line of sight.
Antennas and receivers should therefore
never be hidden behind walls or other objec-
ts – it is vital to have a free line of sight bet-
ween them! Problems with wireless systems
during club gigs are frequently caused by
incorrectly positioned antennas, although
everything functioned perfectly during the
sound check. In this case the audience forms
an imaginary “wall” between the transmitter
and the receiver, severely attenuating the
radio signal. The best thing you can do is
position the antenna high enough above the
audience to avoid such a situation.
Do not place the antenna behind gratings,
metal surfaces, or behind curtains if you can,
as the signal will be unable to penetrate such
obstacles. Even neon lights, dimmers,
lighting systems, electronic equipment with
strong RF radiation, like computers and stage
effects, power switching equipment, keybo-
ards, etc. are potential sources of interferen-
ce. They produce high-frequency harmonics,
and are therefore totally unsuitable for use
near the antennas of wireless systems. The
minimum distance between walls and anten-
nas should not be less than 3 feet, and the
recommended minimum distance between
transmitter and receiver is 10 feet.
You can get even better results by using
diversity systems, carefully selecting the car-
rier frequency band (UHF or VHF), and pla-
cing the antenna(s) carefully to prevent any
dead spots. If none of the above methods
alone improves the situation, try them in dif-
ferent combinations. If this still does not
help, you should secretly check that the
receiver is connected to power, and that fresh
batteries have been inserted in the transmit-
ter! But seriously, even professionals have
made this mistake in the commotion of set-
ting up the equipment for a gig!
Positioning the receiver
One of the most common mistakes in practice is failing to take the audience into account when positioning the receiver. The basic rule of thumb is: always maintain a line of sight between the
transmitter and the receiver. In order to prevent the signal from being absorbed by the audience, always place the receiver as high as possible and in the vicinity of the transmitter – ideally
on the stage itself.
AKG WMS BASICS