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As there has been some discussion on the
merits of different kinds of top
loading I have performed some computer
simulation, using the Antenna Optimizer program by Brian Beezley, K6STI, which
is similar to EZNEC.
The program computes the impedance in the
feed point over a perfect ground. This means that in the real world the
radiation resistance will be somewhat different, but for comparison purposes
(what is better, what is worse) the computed values can be used very well.
The loss due to the resistance of the
wires is taken into account and efficiency is calculated as the radiation
resistance divided by the total resistance (radiation resistance + ohmic loss in the wire). In the real situation the
loss in the loading coil and the earth resistance have to be added.
The goal is of course to make the radiation
resistance as high as possible. A large capacitance helps as well because
the loading coil - and thus its loss (X/Q) - becomes smaller. But in
the majority of cases the earth resistance forms the largest
component of the total resistance so the effect of the
smaller loss in the coil is diluted by the unchanged earth
resistance.
As many amateurs seem to be restricted to
a height of about 10 metres I have used a vertical radiator of 10 m for all
cases. The antenna is fed at the bottom of the radiator.
I assumed the vertical part and top
loading wires to be made of 1.5 mm copper wire (16 SWG).
1. No top
load
Z = 0.073 - j 17792 (ohm)
Efficiency: 10.8%
Rs: 7.8 milli-ohm
C: 65.2 pF
2. One horizontal top load wire of 5
m
Z = 0.111 - j 12550
Efficiency: 12.5%
Rs: 13.9
milli-ohm
C: 92.5 pF
3. One top loading wire of 5 m, sloping down
under 45 degrees
Z = 0.108 - j 12993
Efficiency: 10.4%
Rs: 11.2
milli-ohm
C: 89.3 pF
4. One horizontal top loading wire of 10
m
Z = 0.148 - j 9659
Efficiency: 11.9%
Rs: 17.6
milli-ohm
C: 120 pF
5. One top loading wire of 10 m, sloping down
under 45 degrees
Z = 0.142 - j 9898
Efficiency: 7.0%
Rs: 9.94
milli-ohm
C: 117 pF
6. One horizontal top loading wire of 20
m
Z = 0.221 - j 6626
Efficiency: 10.0%
Rs: 22.1
milli-ohm
C: 175 pF
7. Two horizontal top loading wires of 5 m
(T-antenna)
Z = 0.133 - j 9914
Efficiency: 13.1%
Rs: 17.4
milli-ohm
C: 117 pF
8. Two top loading wires of 5 m, sloping down
under 45 degrees
Z = 0.127 - j 10466
Efficiency: 10.4%
Rs: 13.2
milli-ohm
C: 111 pF
9. Two horizontal top loading wires of 10 m
(T-antenna)
Z = 0.173 - 6793
Efficiency: 12.7%
Rs: 22.0
milli-ohm
C: 171 pF
10. Two top loading wires of 10 m, sloping down
under 45 degrees
Z = 0.163 - j 6987
Efficiency: 6.5%
Rs: 10.6
milli-ohm
C: 166 pF
11. Two horizontal top loading wires of 20 m
(T-antenna)
Z = 0.230 - j 4142
Efficiency: 11.3%
Rs: 26.0
milli-ohm
C: 280 pF
12. Four horizontal top loading wires of 5 m (90
degrees between wires in horizontal plane)
Z = 0.156 - j 7394
Efficiency: 13.7%
Rs: 21.4
milli-ohm
C: 157 pF
13. Four top loading wires of 5 m, sloping down
under 45 degrees
Z = 0.147 - j 7948
Efficiency: 10.1%
Rs: 14.9
milli-ohm
C: 146 pF
14. Four horizontal top loading wires of 10
m
Z = 0.193 - j 4529
Efficiency: 13.4%
Rs: 25.9
milli-ohm
C: 256 pF
15. Four top loading wires of 10 m, sloping down
under 45 degrees
Z = 0.180 - j 4689
Efficiency: 6.0%
Rs: 10.8
milli-ohm
C: 247 pF
16. Four horizontal toploading wires of 20 m
(T-antenna)
Z = 0.232 - j 2493
Efficiency: 12.5%
Rs: 29 milli-ohm
C: 466 pF
The detrimental effect of downward
sloping wires is evident, especially when the ends come near the ground (cases
5, 10 and 15).
73, Dick, PA0SE
D.W. Rollema
V.D. Marckstraat
5
2352 RA
Leiderdorp
The Netherlands
Tel. +31 71 589 27
34
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