TELE-TECH & - AmericanRadioHistory.Com

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DISTANCES IN MILES
60 90 BO 90 00
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3 . 30 60 90 120 150 IBO 210 240 290 300 330 361
DEGREES
Fig. 5: Signal strength at 20 ft. and 26.6 ft. antennas
Fig. 6: System instrument error
Direction Finders
(continued)
the instantaneous direction of the
two fields are the same). Adjustable
magnetic damping is applied
to the phase meter shaft to reduce
needle swing due to noise and rapid
changes in bearing.
Fig. 7: Simplified
block diagram of
remote station
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AZIMUTH
INDICATOR
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Diversity Techniques
An analysis of propagation phenomena
in the 225 to 400 MC
frequency range reveals a continual
variation between a minimum and
a maximum in the r -f field at the
D/F antenna as the transmitting
aircraft's vertical angular position
varies with respect to the D/F
antenna. This condition results
from the fact that energy radiated
from the airborne transmitter
reaches the ground antenna by two
different paths, a direct path and
an indirect path that involves reflection
from the ground as shown
in Fig. 2.
COMPARATOR
SIMPLIFIED BLOC% DIAGRAM
Depending upon path difference
and corresponding phase relationships
of the direct and reflected
rays, the resultant effect may be
either additive or subtractive. In
the latter case, the field strength
at the receiving antenna is considerably
reduced and may drop
essentially to zero if the two interfering
rays are nearly equal in
strength and exactly out of phase.
When a null occurs, it may be impossible
to obtain a bearing due to
insufficient signal level. Moreover,
under conditions where a reflected
signal from nearby buildings or
TUATA
9 T. MOOT
Fig. 8: Comparator
automatically connects
antenna receiving
stronger
signal to indicator
ckts
other masses overrides the resultant
of the direct and ground
reflected wave, an erroneous bearing
may be obtained. However,
space and height diversity techniques
can be successfully used to
minimize these effects.
The succession of maxima and
minima produces a definite interference
pattern for an antenna at
a particular height. A second
antenna mounted at a different
height will not have the same interference
pattern because the distance
traveled by the ground
reflected wave is different. This
means that nulls do not occur at the
same instant for the two antennas.
It then becomes possible to choose
an optimum height difference so
that the minima of one antenna
will almost always be filled by the
maxima of the other. Fig. 4 illustrates
signal reception with two
D/F antennas at different heights.
Although interference minima
could be eliminated by intercepting
the rays reflected from the ground
before they reach the D/F antenna,
for example, by surrounding the
antenna with suit -screens, consideration
of the geometry involved indi-
(Continued on page 193)
92 Tele -Tech & ELECTRONIC INDUSTRIES June 1956