AKG Acoustics WMS 4000 manual Signal propagation, Positioning the receiver

AKG WMS BASICS

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.

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