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Figure 7-6. Center-fed half-wave doublet antenna,

(1) Compute the length of a half-wave antenna by using the formula in paragraph 7-5. Cut the wires as close as possible to the correct length; this is very important.

(2) Uses transmission line for conducting electrical energy from one point to another and for transferring the output of a transmitter to an antenna. Although it is possible to connect an antenna directly to a transmitter, the antenna is usually located some distance away.

(3) Support center-fed half-wave FM antennas entirely with pieces of wood. (A horizontal antenna of this type is shown in A, Figure 7-7, page 7-8, and a vertical antenna in B, Figure 7-7.) Rotate these antennas to any position to obtain the best performance.

(a) If the antenna is erected vertically, bring out the transmission line horizontally from the antenna for a distance equal to at least one-half of the antenna's length before it is dropped down to the radio set.

(b) The half-wave antenna is used with FM radios (Figure 7-8, page 7-8). It is effective in heavily wooded areas to increase the range of portable radios. Connect the top guidelines to a limb or pass it over the limb and connect it to the tree trunk or a stake.

7-4. FIELD-EXPEDIENT DIRECTIONAL ANTENNAS The vertical half-rhombic antenna (Figure 7-9) and the long-wire antenna (Figure 7-10) are two field-expedient directional antennas. These antennas consist of a single wire, preferably two or more wavelengths long, supported on poles at a height of 3 to 7 meters (10 to 20 feet) above the ground. The antennas will, however, operate satisfactorily as low as 1 meter (about 3 feet) above the ground—the radiation pattern is directional. The antennas are used mainly for either transmitting or receiving high-frequency signals.

COUNTERPOISE WD-1 TT, 60 FEET

Figure 7-9. Vertlcle half-rhombic antenna.

COUNTERPOISE WD-1 TT, 60 FEET

Figure 7-9. Vertlcle half-rhombic antenna.

FIELD WIRE,

FIELD WIRE,

Figure 7-10. Long-wire antenna.

a. The V antenna (Figure 7-11) is another field-expedient directional antenna. It consists of two wires forming a V with the open area of the V pointing toward the desired direction of transmission or reception. To make construction easier, the legs should slope downward from the apex of the V; this is called a sloping-V antenna (Figure 7-12). The angle between the legs varies with the length of the legs to achieve maximum performance. (to determine the angle and the length of the legs, use the table in Table 7-l.)

b. When the antenna is used with more than one frequency or wavelength, use an apex angle that is midway between the extreme angles determined by the chart. To make the antenna radiate in only one direction, add noninductive terminating resistors from the end of each leg (not at the apex) to ground. (See TM 11-666.)

Figure 7-11. V antenna.
Figure 7-12. Sloplng-V antenna,

ANTENNA LENGTH (wavelength)

OPTIMUM APEX ANGLE (degrees)

1

90

2

70

3

58

4

50

6

40

8

35

10

33

Table 7-1. Leg angle for V antennas,

Table 7-1. Leg angle for V antennas,

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