[Fwd:LF-antenna with top load]
Andre' Kesteloot
akestelo@bellatlantic.net
Thu, 16 Sep 1999 08:20:11 -0400
> Dick Rollema wrote:
>
> To All
>
> 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
> E-mail: d.w.rollema@gironet.nl
>
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