One way of evaluating the efficiency of a wire as an antenna is to compare its radiation resistance with the
radiation resistance of a quarter-wave
antenna. The radiation resistance of an antenna is the resistance
which would consume the same amount of power as is radiated by the antenna. Thus the radiation resistance is
a direct measure of the energy radiated from the antenna. A monopole antenna one-quarter of a wavelength
long has a radiation resistance of 36.5 ohms (6-4). An antenna which transmits or receives ten percent or less
of the energy that would be transmitted or received by a
monopole can be considered relatively inefficient.
Thus an inefficient antenna would exhibit a radiation resistance of 3.65 ohms or less. Monopoles of length
meet this criterion (6-4). Greater convenience in calculations results if
is chosen instead of
is chosen to represent the length below which a conductor does not perform effectively as an antenna.
6.3 COMMON-MODE NOISE.
Common-mode noise is an unwanted noise voltage which appears identically on both sides of a signal line when
measured from the system ground or common point. It, like normal-mode noise, can be caused by resistive
coupling, capacitive coupling, or magnetic coupling from the unwanted source. In addition, many measuring
transducers intentionally have a dc or ac common-mode voltage present on both output lines, the presence of
which is necessary for proper operation of the transducer, Although not a noise source, these common-mode
voltages require careful design and use of data and instrumentation amplifiers to prevent interference with the
desired signal components.
The source of most common-mode noise is resistive coupling between separate ground points in a circuit or
system. A simple example of this is illustrated in Figure 6-13. An oscilloscope probe is used to couple a signal
from some point in a circuit to the oscilloscope terminals. The probe ground is connected to circuit ground
which is in turn referenced through the facility ground system. Since there are generally currents flowing in
the facility ground system (these are primarily at the 60 Hz power line frequency), it follows that the ground
reference potential for the circuit is different from that for the oscilloscope. This difference in potential is
produced by the flow of the stray ground currents through the impedance of the facility ground system. Thus,
both the ground reference for the circuit and the signal point in the circuit have identical noise voltages
impressed on them with respect to the ground reference for the oscilloscope. This noise is called
common-mode noise by virtue of the fact that is common to all points in the circuit, including the circuit
ground. Not only do these noise sources introduce measurement errors but they also produce interference
between interconnected equipments.
Resistively coupled common-mode noise can also occur in a single equipment rather than between equipments.
The coupling arises from multiple signal currents and power frequency currents flowing in a common ground
lead, chassis, or ground plane.