Handbook for Sound Engineers

562 Chapter 16

extremely small. The advantages of diversity

compared to conventional RF transmission are shown

in Fig. 16-148. Only the receiving chain with the

better input signal delivers audio output. Not only

does this system provide redundancy of the receiving

end, but it also combines signal strength, polarity and

space diversity.

• Antenna Combination Diversity. The antenna com-
  bination diversity system is a compromise of the
  other methods. This system uses two or more
  antennas, each connected to a wideband RF amplifier

to boost the received signal. The signals from both

receiving antennas are then actively combined and

fed to one standard receiver per microphone. In this

way, the receiver always gets the benefit of the sig-

nals present at all antennas. There is no switching

noise, no change in background noise, and only

requires one receiver for each channel. A drawback is

the possibility of complete signal cancellation when

phase and amplitude relationships due to multipath

provide the proper unfavorable conditions.

16.10.2.1 Antenna Placement

It is often common to use a near antenna and a far

antenna. The near antenna, which is the one nearest the

transmitter, produces the majority of the signal most of

the time; in fact, it may even be amplified with an

in-line amplifier. The far-field antenna may be one or

more antennas usually offset in elevation and position;

therefore, the possibility of dropout is greatly reduced.

Because the antennas are common to all receivers,

many wireless microphones can be used at the same

time on the same antenna system. This means that there

are fewer antennas and a greater possibility of proper

antenna placement.

The following will generally prevent dead spots:

• Do not set up antennas in niches or doorways.


• Keep the antennas away from metal objects including
  armored concrete walls. Minimum distance: 3 ft
  (1 m).


• Position the antennas as close as possible to the point
  where the action takes place.


• Keep antenna cables short to keep RF losses at a
  minimum. It is better to use longer AF leads instead.

Figure 16-146. Diversity antenna system used to reduce
multipath radiofrequency phase cancellation.

Figure 16-147. Functional diagram of a true diversity
receiver.

Transmitter

Reflecting object

Phase-

canceled

signal

Diversity

receiver

B

No reflected signal: thus,

no phase canceling

A

Receiver 1

Comparator

Receiver 2

AGC Voltage

AF

AGC Voltage

AF

Figure 16-148. Effect of switch-over diversity operation.

Solid line RF level at antenna 1 and the dotted line RF level

at antenna 2. Courtesy Sennheiser Electronic Corporation.

Antenna 2

Antenna 1

25 dB

RF Input voltage–

MV

Time–s

0 1 2 3 4

1000

100

10

1