TELE-TECH & - AmericanRadioHistory.Com

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Antenna Arrays
By G. MATHER
P. E., George Mather and Associates,
Port Credit, Ontario, Can.
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220' 210' 200'
140 150° 160' 170' 180° 190'
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140°
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250°
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100°
260°
90°
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70° 290°
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5Ó° 310°
6 90'
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320° I
40°
40° 330° 340° 350° 0° 10° 20° 30°
270°
90° fttt
3° 1=.. 3
30° 20° 10° 350° 340° 330°
Fig. 4: Three 90 radiators are converted to a single 90 radiator
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By applying the above result to an
array of unequal tower heights, an
Er.,, value may be evaluated which
could be presumed to be representative
of hypothetical equal height ra-
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220° 210° 200° 190 170° 160' 150° 140
140 150' 160° 170° I 0° 190° 200' 210 220°
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40' 30° 20' 10' 35011 340' 330° 320°
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Tele -Tech & ELECTRONIC INDUSTRIES June 1956
J
diators in the array. Hence the array
aMS may be approximated.
Method
Assuming first that the array is
comprised of radiators of equivalent
height, the pattern shape is derived
from the parameters as specified in
terms of relative current (or field)
ratios. Assuming further that the
reference tower is assigned a relative
current (or field) of unity, the
pattern RDAs is then evaluated.
The second and parallel step is
the impedance and current analysis.
As a result of this computation it is
130°
230°
120'
240°
no°
250°
100'
260'
90°
270'
eó°
70°
290'
3o
Fig. 3: Radiators at
90' and 60 are
equivalent to one radiator
at 75
then possible to assign current magnitudes
to what were previously relative
values. In particular the reference
radiator with an arbitrary
value of unity may now be described
by current magnitude.
By now combining the previous
two steps it is possible to compute
the RMS in terms of field strength. It
is known that a reference vector of
unity may be specified in current
magnitude, hence the pattern RMS
may also be converted to a current
magnitude. By comparison of this
latter value of effective Rids current
with what would be normal in terms
of current and efficiency for an
omni -directional single radiator (of
equal height) system, it then follows
that the array efficiency or RMS
value may be computed by direct
proportion, and expressed in field
strength.
The second case for consideration
concerns an array which may be
comprised of radiators of unequal
height. While the solution here is
not as straight forward as for the
previous case, nevertheless it is possible
to evaluate an RMS field
strength for an array which may be
quite realistic. It has been shown
that where radiators are of equal
height an ass (Root Square Sum)
combination of the fields (based on
input power) for the individual
towers is equivalent to the RMS field
of any element operating independently
with full power applied. By
applying this principle to the situ -
(Contitwed on rage 205)
103