RE: NEC-LIST: Enhancing low-angle radiation

Keith Lysiak wrote:
>
> Chuck,
> I am not familiar with how to respond through NEC-LIST, so
> feel free to send a copy of this in if you would like.
>
> I ran an optimization on your problem with some assumptions
> and this is what I found. The assumption was that you wanted to
> restrict the height of the highest dipole to 0.75 wl or 16.0 meters.
> I first optimized the dipoles for minimum VSWR (1.43) at 14 MHz. I
> then allowed the height of the top dipole excited with 1.0 volts to
> vary from 0.5 to 0.75 wl. The bottom dipole could vary from 0.1 to
> 0.5 wl and its excitation from 0.0 to 1.0.
>
> This is a summary of what I found:
>
> Length of dipoles -- 10.32 meters
> Height of top dipole -- 0.75 wl (16.0 meters)
> Height of bottom dipole -- 0.18-0.19 wl (4-4.11 meters)
> Excitation of bottom dipole -- 0.44-0.47 volts
> VSWR of top dipole -- 1.75 (up from 1.43)
> VSWR of bottom dipole -- 2.07
>
> GAIN -- 9.23 dBi up 1.8 dB from 7.43 dBi
>
> This was a fun exercise! I don't suspect these exact numbers will
> work out in the real world, but they should be close. I'm not sure
> if the gain vs VSWR trade-off is worth it -- I need to look at this.
>
> Later,
> Keith
>
> Keith A. Lysiak, Ph.D
> Senior Research Engineer
> Signal Exploitation and Geolocation Division
> Southwest Research Institute
> San Antonio, Texas
> (210) 522-5776

1. Nice work. What s/w did you use for optimization?

2. To post to NEC-LIST, just address to <nec-list_at_ee.ubc.ca> and
include "NEC-LIST" in the Subject header f/b/o those who filter their
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3. People will want to know whether you had copper wire loss turned on
and what diameter wire you used (I used 2E-4 wl), and more importantly
whether you used the Somerfeld-Norton ground calculation (as I did).
I suspect that you did because your results are so consistent with
mine (see below).

4. Your results appear quite consistent with and similar to mine. Direct
comparison is not possible because, e.g., you used voltage sources instead
of current sources, but if we overlook such differences...

         Yours Mine
Hgt top 0.75 wl 0.75 wl
Hgt bot 0.18 wl 0.25 wl
Excit. 0.45 v 0.50 a
Gain 9.22 dBi 9.06 dBi with lower dipole
Up from 7.43 dBi 7.27 dBi without " "
Improvmt 1.79 dB 1.79 dB from addition of lower dipole

5. I don't know what you mean by "gain vs VSWR trade-off". There's no
significant VSWR problem here. I.e., feedline loss is not
significantly changed. However, I agree that 1.79 dB isn't much
improvement in view of all the work involved in hanging another dipole
below the first! We need a breakthrough.

73 de Chuck W1HIS
Received on Fri Oct 10 1997 - 08:59:37 EDT