TELE-TECH & - AmericanRadioHistory.Com

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220° 210° 200 190° 170° 160° 150°
140° 150° 160° 170° 180° 190° 200° 210° 240°
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230°
300 MV/61 130°
130°
,4
230°
240°
120°
2.0
250° ,*tat*.
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120°
`' ,I'..!
240°
I
110°
110°
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250°
+i1.:0
260°
100 °'
100°
....
260°
270°
90°
90°
9/I1
280°
1=s1 y1
270°
80°
80°
280°
290°
. .
70°
70°
290°
,
300°
60°
33000°
10
4.
77F
310°
/
50°
=
7y°
9o. 90'
.L0,.....í.A
320°
40° 330° 340° 350° 0° 10° 20° 30° 320°
30° 20° 10° 350° 340° 330°
,. iu.
Fig. 1: Field strength of two 60' radiators spaced at 90°
Calculating The
Efficiency of
By making use of a broadcast antenna array
impedance and power analysis, it is possible
to compare the pattern RMS current of the
array with the base current and effective
field of an omni -directional radiator of
equivalent height and power input.
Essentially for radiators of equal height, the
method described may be adapted to
arrays with unequal tower heights
THIS article illustrates a relatively
straightforward method
for determining the efficiency of AM
broadcast antenna arrays. The
method described is essentially for
arrays with radiators which are of
equal height, but may be adapted to
arrays with unequal tower heights.
The point of particular merit is that
integration is not required, but
rather the results are obtained by
merely making full use of an array
impedance and power analysis.
When a broadcast antenna array
is comprised of equal height radiators,
the parameters are proportional
to either field strength or base
current. It then follows that if the
pattern RMS current is evaluated in
terms of reference units, these may
be converted to absolute current.
This is so because through an impedance
analysis it is possible to express
the reference parameters in
terms of current. The conversion to
field strength is accomplished by
comparing the pattern RMS current
with the base current and effective
field of an omni -directional radiator
of equivalent height and power input.
Direct proportion applies since
field strength is proportional to input
current.
It is interesting to note that the
Root Sum Square of the fields from
individual elements comprising a
directional array is equivalent to the
RMS field of a single identical radiator.
Assume for example a 3 element
array with the nomenclature as outlined
below:
102
where E.5 = field of single element
with input power
of P
P = power input to the
array
Pa = base power at radiator
n
In = base current at radiator
n
Rn = operating resistance
of radiator n
En = calculated field of radiator
n with input power
of Pn
Kn = relative current at
radiator n
Fig. 2: Field strength
of two 60' radiators
spaced at 210'
130
250°
110'
100'
270°
280°
80°
290°
70°
300°
60°
310°
50°
by definition
Er.. = (E; + E6" + E°-) (1
C
E
rm.
)=+(
E .m.
)
P./ \ P6/
Erm \214
P
P°
1 1 1 1
=E. i + P+ P 1
220° 210° 200° 190° 170° 160° 150° 140°
140° 150° 160° 170° 180° 190° 200° 210°
320° 330' 340° 350°
40° 30° 20° 10°
Li
° 10° 20° 30°
350° 340° 330°
40°
320°
130°
230°
120°
240°
110°
250°
100°
260°
90°
270°
80°
280°
70°
290°
60°
300°
50°
310°
Tele -Tech & ELECTRONIC INDUSTRIES June 1956