The condition known as nulling occurs when one or more reflected waves arrive at the receiver out-of-phase with the main wave with such amplitude that the main wave's amplitude is cancelled. As illustrated in Figure 9.4, when reflected waves arrive out-ofphase with the main wave at the receiver, the condition can cancel or “null” the entire set of RF waves, including the main wave.
When nulling occurs, retransmission of the data will not solve the problem. The transmitter, receiver, or reflective objects must be moved. Sometimes more than one of these must be relocated to compensate for the nulling effects on the RF wave.
Increased Signal Amplitude
Multipath conditions can also cause a signal’s amplitude to be increased from what it would have been without reflected waves present. Upfade is the term used to describe when multipath causes an RF signal to gain strength. Upfade, as illustrated in Figure 9.5, occurs due to reflected signals arriving at the receiver in-phase with the main signal. Similar to a decreased signal, all of these waves are additive to the main signal. Under no circumstance can multipath cause the signal that reaches the receiver to be stronger than the transmitted signal when the signal left the transmitting device. If multipath occurs in such a way as to be additive to the main signal, the total signal that reaches the receiver will be stronger than the signal would have otherwise been without multipath present.
It is important to understand that a received RF signal can never be as large as the signal that was transmitted due to the significance of free space path loss (usually called path loss). Path loss is the effect of a signal losing amplitude due to expansion as the signal travels through open space.
Think of path loss as someone blowing a bubble with bubble gum. As the gum expands, the gum at any point becomes thinner. If someone were to reach out and grab a 1-inch square piece of this bubble, the amount of gum they would actually get would be less and less as the bubble expanded. If a person grabbed a piece of the bubble while it was still small (close to the person's mouth, which is the transmitter) the person would get a significant amount of gum. If the person waited to get that same size piece until the bubble were large (further from the transmitter), the piece would be only a very small amount of gum. This illustration shows that path loss is affected by two factors: first, the distance between transmitter and receiver, and second, the size of the receiving aperture (the size of the piece of gum that was grabbed).
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