_______________________________________________________________________ Antennas
4-68. The positions marked off on the two polar-coordinate graphs in figures
4-11 and 4-13 were selected and numbered arbitrarily. However, a standard
method allows the positions around a source to be marked off so that one
radiation pattern can easily be compared with another. This method is based
on the fact that a circle has a radius of 360 degrees. The radius extending
vertically from the center (position 0 in figure 4-11) is designated 0 degrees.
At position 4 the radius is at a right angle to the 0-degree radius.
Accordingly, the radius at position 4 is marked 90 degrees, position 8 is
180 degrees, position 12 is 270 degrees, and position 16 is 360 degrees. The
various radii drawn on the graph are all marked according to the angle each
radius makes with the reference radius at 0 degrees.
4-69. The radiation pattern in figure 4-13 is obtained by using the same
procedure that was used for figure 4 -10, view B. The radiation measured at
positions 1, 2, 3, and 4 is 0. Position 5 measures approximately 1 unit. This is
marked on the graph and the rotating radius moves to position 6. At this
position a reading of 5.5 units is taken. As before, this point is marked on the
graph. The procedure is repeated around the circle and a reading is obtained
from positions 6 through 11. At position 12 no radiation is indicated, and this
continues on to position 16.
4-70. The polar-coordinate graph now shows a definite area enclosed by the
radiation pattern. This pattern indicates the general direction of radiation
from the source. The enclosed area is called a lobe. Outside of this area,
minimum radiation is emitted in any direction. For example, at position 2 the
radiation is 0. Such a point is called a null. In real situations, some radiation
is usually transmitted in all directions. Therefore, a null is used to indicate
directions of minimum radiation. The pattern of figure 4-13 shows one lobe
and one continuous null.
4-71. You will sometimes want to use one antenna system for transmitting
and receiving on several different frequencies. Because the antenna must
always be in resonance with the applied frequency, you may need to lengthen
or shorten the antenna physically or electrically.
4-72. Except for trailing-wire antennas used in aircraft installations (which
may be lengthened or shortened), physically lengthening the antenna is not
very practical. But you can achieve the same result by changing the electrical
length of the antenna. To change the electrical length, you can insert either
an inductor or a capacitor in series with the antenna. This is shown in figure
4-14, views A and B. Changing the electrical length by this method is known
as lumped-impedance tuning, or loading. The electrical length of any antenna
wire can be increased or decreased by loading. If the antenna is too short for
the wavelength being used, it is resonant at a higher frequency than that at
which it is being excited. 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 figure 4-14, view A.
Similarly, if the antenna is too long for the transmitting frequency, it offers
an inductive reactance. Inductive reactance can be compensated for by
introducing a lumped-capacitive reactance, as shown in view B. An antenna
without loading is represented in view C.
4-21
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