On 1/9/2011 3:31 PM, Paul Christensen wrote:
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 log formula has a tiny error at 600 ohms, but not enough to be
concerned about. The inv cosh formula and the log formula agree closely
at that impedance. A line the inv cosh formula says has 600 ohms
impedance with a spacing over diameter ratio of 74.2099, the log formula
says has an impedance of 599.33 ohms. A difference of .11%, I'm not sure
wire diameters or roundness are that precise. And I know its close
enough to suit me. The periodic insulators introduce a greater error. If
you want to use that line at VHF, it would be a good idea to vary the
spacing between insulators so as to not introduce a SWR change when the
spacings add up to quarter waves repeatedly.
That makes me wonder if the even spacing of the windows in the window
line may contribution to the 10m loss, wet or dry. Air spaced should
have lower loss that plastic insulated line.
As for taking a twin lead fed antenna down when high winds are forecast,
I'd have my antennas on this prairie down more than up if I did that. I
want to be inside operating when its windy, not droping an antenna.
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.
That reasonable impedance for the transmission line is why 400 to 600
ohm line is so nice feeding a center fed wire that might be run shorter
than a half wave, a half wave, and multiples of a full wave long. Even
when the feed line is an odd multiple of a quarter wave, the impedance
seen at the tuner is always reasonable and easy to tune. The problem
when feeding that same antenna with coax and a tuner is that if the
antenna is any integer multiple of a full wave long and the coax is any
odd multiple of a quarter wave long the 1000 ohms antenna Z transformers
through the coax to 2.5 ohms which take a whole heap of C to match. And
means high currents at the low Z points in the transmission line, even
if not an odd multiple of a quarter wave long. Some auto tuners do it,
some smoke trying. And nearly all show considerable loss tuning that low
impedance.
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
73, Jerry, K0CQ
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