Figure 2-6.—Anistropic radiator.
resonance. Changing the antenna dimensions physically is impractical, but changing them electrically is relatively simple. To change the electrical length of an antenna, you can insert either an inductor or a capacitor in series with the antenna. This is shown in figure 2-8, views A and B. Changing the electrical length by this method is known as LUMPED-IMPEDANCE TUNING or LOADING. If the antenna is too short for the wavelength being used, it will be resonant at a higher frequency. Therefore, it offers a capacitive reactance at the excitation frequency. This capacitive reactance can be compensated for by introducing a lumped inductive reactance, as shown in view A. Similarly, if the
Figure 2-7.—Maior and minor lobes.
antenna is too long for the transmitting frequency, it will be resonant at a lower frequency and offers an inductive reactance. Inductive reactance can be compensated for by introducing a lumped capacitive reactance, as shown in view B. An antenna with normal loading is represented in view C.
Figure 2-8.—Electrical antenna loading.
GROUND EFFECTS
As we discussed earlier, ground losses affect radiation patterns and cause high signal losses for some frequencies. Such losses can be greatly reduced if a good conducting ground is provided in the vicinity of the antenna. This is the purpose of the GROUND SCREEN (fig. 2-9, view A) and COUNTERPOISE (fig. 2-9, view B).
2-5
