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3-20. Compared to omnidirectional antennas,
directional antennas provide which of the
following advantages?
1. Power gain and selectivity
2. Power gain and directivity
3. Sensitivity and selectivity
4. Sensitivity and directivity
3-21. If the vertical beam width of a radar
antenna is decreased, what will be the
effect on (a) power gain and (b) vertical
directivity?
1. (a) Decrease
(b) decrease
2. (a) Decrease
(b) increase
3. (a) Increase
(b) increase
4. (a) Increase
(b) decrease
3-22. An array of twelve dipoles are set in the
same position as a reference dipole and
are fed with the same line. The power
gain will be
1. less than unity
2. one-twelfth the reference
3. twelve times the reference
4. dependent on the directivity of the
array
3-23. If the slant range and altitude of a target
are known, which of the following
coordinates can be computed using
trigonometric functions?
1. Elevation angle
2. True-bearing angle
3. Relative-bearing angle
4. All of the above
3-24. To convert diverging waves into parallel
waves, where must the radiating element
be placed in relation to a parabolic
reflector?
1. At the focal point of the reflector
2. 1/4 wavelength from the reflector
3. 1/2 wavelength from the reflector
4. At the focal point of the
hemispherical shield
3-25. What is the function of the hemispherical
shield of the parabolic reflector?
1. To polarize all reflected waves in the
vertical plane
2. To polarize all reflected waves in the
horizontal plane
3. To convert the spherical waves
radiated by the dipole into vertical
lines of rf energy
4. To reflect rf energy radiated forward
of the dipole back to the parabolic
reflector
3-26. Which of the following types of
parabolic reflectors has a focal line rather
than a single focal point?
1. Truncated
2. Rotational
3. Orange-peel
4. Cylindrical
3-27. A broadside array causes maximum
energy to be radiated perpendicular to the
plane of the dipole for which of the
following reasons?
1. Because dipoles are excited in phase
with each other
2. Because dipoles are parallel to each
other
3. Because dipoles are 1/2 wavelength
apart
4. Because dipoles are 1/8 wavelength
away from the reflector
3-28. The directivity of a horn radiator is
dependent on which of the following
physical dimensions of the horn?
1. The shape
2. The length
3. The mouth size
4. All of the above
