18WuD2x

directional wattmeter • directivity index 197
to a transmission line. Such meters fall into two
categories. One type has a single scale, calibrated
in watts, and sometimes also in milliwatts or kilowatts
(switch selectable). The meter reads either
forward power or reflected power, depending on
the position of a switch or rotatable internal element.
Another type has two needles in a single
enclosure, with a different calibrated scale for
each needle. Both of these scales are graduated
in watts, and sometimes also in milliwatts or kilowatts.
One needle/scale indicates forward power
and the other needle/scale indicates reflected
power. There is a third scale, calibrated for the
point where the two needles cross. This scale indicates
the standing-wave ratio (SWR). See also
CROSSED-POINTER INDICATOR.
direction angle In radar operations, the angle between
the center of the antenna baseline and a
line going to the target.
direction finder A receiver specially adapted to
show the direction from which a signal is received,
thus revealing the direction of the receiver
with respect to the transmitting station, and vice
versa. In its simplest form, it is a receiver with a
loop antenna that is rotatable over a map or compass
card. For increased accuracy, checks are
made with signals from two transmitting stations;
the exact location of the receiver is pinpointed
by triangulation.
direction finding The taking of bearings by means
of a direction finder.
direction of lay In a multiconductor cable, the lateral
direction of winding of the topmost conductors
as they recede from the observer; called
Rcvr
x
Bearing
y
Sea
Land
Bearing
x
direction finding
Rcvr
y
Position of
vessel sending
distress signal
left-hand lay or right-hand lay. If the cable is
viewed from either end, left-hand lay is equivalent
to conductors that rotate clockwise as they recede
from the viewer; right-hand lay is equivalent
to conductors that rotate counterclockwise as
they recede from the viewer.
direction of polarization The direction of the electrostatic
field in a linearly polarized wave.
direction of propagation The direction in which
energy moves from a transmitter, or between
equivalent points in a sector of space under consideration.
direction rectifier In a control system, a rectifier
whose direct-current (dc) output voltage has a
magnitude and polarity dependent on the magnitude
and phase of an alternating-current (ac) selsyn
error voltage.
direction resolution 1. The smallest difference in
azimuth that a direction-finding device can detect.
2. The smallest angular separation between
two targets that allows a radar set to show two
separate echoes rather than a single echo.
directive
In a computer source program, a statement
directing the compiler in translating the
program into machine language without being
translated itself. Also called control statement.
directive antenna An antenna designed for best
propagation or reception in one (often steerable)
horizontal direction. Also called beam antenna
and directional antenna.
directive gain For a directional antenna, a rating
equal to 12.566(Pr/Pt), where Pr is the radiated
power per steradian in a given direction and Pt is
the total radiated power.
directive horn A microwave antenna having the
shape of a (usually rectangular) horn.
directivity 1. In an antenna, a directional response.
2. The degree to which the radiation or
sensitivity of a transducer is concentrated in certain
directions. 3. The angle between the halfpower
points of a directive antenna in the
azimuth plane. 4. In an antenna system, the ratio,
in decibels, between the power in the favored
direction and the power in the exact opposite direction;
also called front-to-back ratio. 5. The forward
power gain of an antenna, with respect to a
dipole in free space. 6. The forward power gain of
an antenna, with respect to an isotropic radiator
in free space.
directivity diagram A graph of the radiation/
response pattern of a beam antenna or other
directional device, usually in a horizontal or
vertical plane. Also see DIRECTIVITY PATTERN.
directivity factor 1. A measure of the directivity of
an antenna or transducer. 2. In acoustics, the ratio,
in decibels, between the gain in the maximum
direction and the gain in the minimum direction,
for a transducer, such as a speaker or microphone.
directivity index 1. For an acoustic-emitting
transducer, the ratio, in decibels, of E 1 to E 2 ,