Hi Frank,
I gave this several passes with various segmentations.
The best, flattest response appeared at 400 (total) segments over a
10M length of line of #24 separated by 1". This resulted in a
characteristic 575 Ohm line with a 1.045:1 SWR ripple over the 2 MHz
to 30 MHz band.
When I doubled and then tripled the segments, the low end got a little
whacked out (getting worse as the segments went up).
I was working from many segments down, so I did not lower the count to
your longer segments - maybe tomorrow, I was paying attention to your
(and EZNEC's) advice about segment length equaling line separation.
73's
Richard Clark, KB7QHC
Hi Richard:
Found some three conductor zipcord and measured its dimensions: wire,
stranded, #14 AWG, center to center spacing 3.5mm (0.138"). Experimented
with a 100ft NEC transmission line model, with various segmentations from 4"
to 16". Characteristic impedance agreed with :- 276*log(2*S/d) at 171
Ohms, where S is the center to center wire spacing, and d is the wire
diameter. From your comments it seems EZNEC is in close agreement with my
NEC program.
Applied the above dimension to a 140 ft "Cobra" type antenna at 35 ft above
an average ground of: sigma 5 mS/m, and Er = 13. The antenna was segmented
at 6". Hope the formatting of the following results is not totally
destroyed by Microsoft's Outlook Express.
Cobra Antenna:
Freq Re Im S Eff.
(MHz) (450) (%)
2 29.8 93.8 15.7 66.3
3.8 109 637 12.6 83.4
5 251 58 1.8 95.4
7.2 999 -1381 6.8 99.0
10 516 1850 16.7 68
14.2 1287 -989 4.1 98.9
18 315 861 7.2 81.3
21.2 1096 -915 3.3 98.6
25 301 672 5.3 82.7
28.6 669 -712 3.6 98.2
140 ft Dipole compared
Freq Re Im S Eff.
(MHz) (450) (%)
2 8.8 -856 235 66.3
3.8 79 238 7.3 83.3
5 361 1127 9.8 95.4
7.2 1959 -2379 13 99
10 111 -195 4.9 68
14.2 2581 -2020 9.3 98.9
18 181 251 3.4 81.3
21.2 2225 -1871 8.5 98.6
25 188 233 3.1 82.7
28.6 1252 -1577 7.4 98.2
If the NEC models are correct there does not seem to be a lot of difference
between the "Cobra" and a 140 ft dipole. Interesting to note that the
Imaginary part of Zin, at 2 MHz is still 0. With 100 ft of 450 Ohm line on
2 MHz the loss is only 0.12 dB compared with 1.5 dB on the regular dipole.
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