Practical_Antenna_Handbook_0071639586

108 P a r t I I : F u n d a m e n t a l s
when the antenna is made somewhat longer than the resonant length, inductive reactance
will be seen.
Note: If the transmitted frequency is changed, the electrical length of the antenna
also changes in accordance with Eq. (3.30). If the frequency is increased, the physical
length of a given antenna stays the same but the electrical length becomes a greater fraction
of a wavelength, with a corresponding increase in the amount of inductive reactance
seen at the feedpoint. Conversely, if the frequency is lowered, the electrical length
decreases, and the feedpoint impedance becomes more capacitive in nature.
No transmitter power is lost in a pure reactance. However, the mismatch caused by
the reactive part of the feedpoint impedance does have the potential to cause reduced RF
output from the transmitter, depending on the design of the transmitter. Further, for
lengths other than an odd multiple of l/2, most dipoles will exhibit sufficient feedpoint
reactance to make the use of a matching network in the circuit somewhere between the
transmitter and the feedpoint mandatory. Circulating currents in any wires or inductive
components in the network will also contribute to resistive losses, lowering the efficiency
of the antenna even more.