On Sun, 01 Oct 2006 17:27:31 +1000, Alan Peake
wrote:



to appear as about 800 ohms and then the spacing tapered steadiy down
during the descent to the horizontal transmission line which was spaced
for 600 ohms.
I like the tapered arrangement. What's the minimum distance needed to
taper from 800 to 600 at freqs from say 3.5MHz to 14MHz? I did one years
ago from 130 to 200 and it was about a foot long at 1.8GHz.

Alan,

Are you tapering for impedance transformation, or just to reduce
transmission line losses?

I make the matched line loss on a 600 ohms open wire line with 2mm dia
copper spaced 150mm something approaching 0.1dB/100m, with 2:1 VSWR
you are talking ~0.14dB/100m. You should find the rhombic no worse
than 2:1 unless it is too short.

My loss calculator has an ideal air spaced 600 ohm copper line (copper
loss only) 2mm/150mm, the label is "Open / air dielectric (150/2.00)",
you can try the various loads from the NEC models for more exact loss
figures.

Owen
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