Page 438 - Complete Wireless Design

Wireless Issues

Wireless Issues 437

Just as with any RF circuit, a good impedance match between the trans-
mitter, the feed line, and the antenna is important for the maximum transfer
of power. A proper match will also stop the energy generated at the transmit-
ter from being reflected back into its output, which can produce severe trans-
mitter damage and transmission line insulation breakdown. This match
between the transmitter, its feed line, and the antenna (Fig. 10.18) will occur
when the inductive reactances equal the capacitive reactances and cancel,
leaving only the resistances (which must equal each other), allowing maxi-
mum power to be radiated from the antenna. This prevents standing waves
from being created (see Sec. 1.4, “Transmission Lines”) on a mismatched trans-
mission line, with much of the reflected power then being given up as heat. So,
when the antenna is resonant and the impedances all match between the
transmitter, feed line, and antenna, maximum alternating current is sent into
the antenna and broadcast as an RF signal.
However, on the antenna itself, maximum output power will be possible
only if there are maximum standing waves. This will create maximum volt-
age at the ends of the antenna—and maximum current in the center—as
shown in the diagram of the dipole antenna of Fig. 10.19. Since the power
of the RF signal radiated from an antenna is also contingent on the RF cur-
rents in the antenna, the more current the higher will be the output power.
But for the energy to actually break free of the antenna and radiate far into
space, the frequency must reach a point where the field lines (created by the
RF currents) cannot fall back into the antenna’s elements before the RF cur-
rents change polarity. The minimum frequency at which this can occur is
approximately 30 kHz.

Figure 10.18 Maximum power output results when a perfect
impedance match exists between the transmitter, feed line, and
antenna.

Figure 10.19 Current and voltage along a half-wave dipole.

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