[BC] Null Distortion

[Thomas G. Osenkowsky]

 > In some instances, can't there be overmodulation in or near the nulls due
to having
 > more power in the sidebands than carrier power?

This is the simplest explanation of the importance of pattern bandwidth.
Draw a horizontal line on a blank piece of paper. This will be the X axis
of a graph and will represent frequency. Draw a vertical line in the center
of the X axis 4 inches in height. This will be one part of the Y axis and
represent Fo, the carrier frequency power. Now draw a vertical line on
each side of Fo equally separated by one inch. The left Y axis line will
represent the Lower sideband, the right Y axis will represent the Upper
sideband.

Let the Fo power be 1000 watts, modulated by a sine wave 100%.
Each sideband will be symmetrical about Fo and contain 25% power
for a total power of 50%, or 500 watts. Ideally this is what we should
see at the final amplifier in the transmitter and at the antenna terminals
of the receiver. All components in between should ideally not distort
this relationship. If the carrier is attenuated more than the sidebands,
overmodulation and distortion will result. In most cases, the sideband
relationships become asymmetrical with change in frequency. This can
be a function of both impedance and pattern bandwidth deficiencies.

In a DA, many variables change with frequency. Even in a ND system
there are changes that occur with frequency. The goal of the design of an
antenna system is to preserve impedance and pattern bandwidth. Often
times, a compromise(s) is necessary.

Tom Osenkowsky, CPBE