For example, when used in the multiband doublet application 200ft of
Wireman 553 has about 2.6dB of loss at 28MHz. When wet, that figure
rises to 9.3dB, based on the work of Wes Stewart N7WS.
And the reason I've been using homebrewed open 600-ohm line -- using the 276
log formula :-)
The line uses E.F. Johnson glazed ceramic spreaders spaced every 18-inches.
Total of roughly 50 ft. of line between a 130 ft flat-top dipole to a
ground-mounted motorized symmetrical balanced tuner. When QRT, the
balanced line is automatically disconnected from the tuner with a pair of
Jennings vacuum switches to form a quasi "knife switch." LMR-400 between
the ATU and shack. The open feeder length represents a reasonable end-Z
compromise on all bands from 80m-10m and remains under 1,000 ohms and over
50 ohms at the tuner terminals at all times. 50-ohm VSWR never changes,
wet or dry. Next time the line comes down, a re-application of Rain-X is
needed on the glazing. ATU details on my QRZ.com page.
Been tempted to replace the Johnson spreaders with Delrin but I doubt
there's much difference in total dielectric loss with such a short run of
line.
http://www.73cnc.com/73cnc/laddersnap.html
Paul, W9AC
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