Alan, VK2ADB wrote:
"What sort of feed warrangement did you use?"

600-ohm open-wire line. Many of the rhombics I erected were made from
U.S. Army WW-2 Signal Corps kits. These used three cables in the diamond
shaped curtain which came together at the end supports but were spread
apart by several feet at the side supports. This construction tended to
further reduce impedance variations as frequency changed. We didn`t
bother with this refinement with receiving antennas.

Actual antwenna driving point impedance tended to exceed 600 ohms so
width of the attachment point to the anteena was wide for the cable size
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.

The cable used in the kits was made from (3) AWG 12 Copperweld wires
twisted together. There was also stainless steel wire for a dissipation
line at the far-end of the rhombic. We had to replace this with larger
wire as we were broadcasting with 100 KW transmitters, far more power
than anticipated by the Signal Corps. Under some conditions and at some
frequencies, it is possible to dissipate up to 50% of the power fed into
the rhombic in its dissipation line.

It is possible to operate without a dissipation line or resistance. The
rhombic becomes bidirectional without the termination. I know from
experience during a period when the Signal Corps dissipation lines
melted from the broadcast power and our fan mail came from South America
as well as Central Europe.

Best regards, Richard Harrison, KB5WZI

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.

Best regards, Richard Harrison, KB5WZI


Alan

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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