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100 Handbook of Intercom Systems Engineering
Figure 7.8
An example of combining waves that are not 180° out of phase.
Interference
As mentioned earlier, multipath can be described as a form of self interference caused
when a reflected RF carrier arrives at the receive antenna along with an RF carrier that has
taken a direct path. See Figure 7.9. The reason multipath is so detrimental to the successful
operation of an RF system has to do with the nature of the relationship of the reflected
signal to the direct path signal.
Figure 7.9
An example of multipath in its most basic form.
The direct path carrier takes the most direct, and consequently, the shortest path from
transmitter to receiver. The reflected carrier, on the other hand, takes a longer path, from
the transmitter to the reflective surface, and from the reflective surface to the receiver. The
waves leaving the transmitter antenna are all in phase, but because the direct carrier and
the reflected carrier travel different distances, thus taking slightly different lengths of time,
the two carriers are out of phase, and of different amplitudes (remember the inverse square
law), when they reach the receive antenna. The two carriers are combined at the receive
antenna and, being out of phase, they cancel each other out so that little or nothing can be
detected by the receiver. This causes a momentary interruption in the RF wave, which is
called a dropout. Dropouts are manifested in audio RF systems by a loud click or pop
surrounded by noise. Proper system design and careful antenna placement can go a long
way to reducing the effects of multipath on a wireless communications system. We discuss
how to avoid multipath later in this chapter.
The next concept that you must be familiar with to move forward in the design of your
wireless intercom system is receiver desensitization or desensing. As mentioned earlier,
desensing happens when a transmitter is in close proximity to a receiver, even if that
transmitter is not on or near the receiver’s operating frequency. Receiver desensitization
happens because receivers must maintain critical voltage and current levels throughout the
front end stages, and a strong (i.e. close by) transmitter can cause these levels to vary
REFLECTED SIGNAL
TX
Antenna
DIRECT SIGNAL
RX
Antenna