On Sat, 30 Sep 2006 01:16:44 GMT, Cecil Moore
wrote:

jawod wrote:
For the record, I did not have deviating from G5RV specs as part of my
"design" ...

If you used 300 ohm ladder-line, like The Wireman's
#561 or #562, you did indeed deviate from G5RV specs
by making the ladder-line length equal to 31 feet.
For that particular transmission line, 1/2WL on 20m
is around 28 feet.

Wireman's data for #561 and #562 shows vf=0.91, and for #563, vf=0.66.

I have found wide variation in line construction and hence vf of
nominally 300 ohm line, and suggest that it is worth measuring.

This information is not new, G5RV wrote "If it is desired to use 300
ohm ribbon type feeder for this section, it is strongly recommended
that the type with "windows" (ladder line) be used because of its much
lower loss than that with solid insulation throughout its length, and
its relative freedom from the "detuning" effect caused by rain or
snow. If this type of feeder is used for the matching section,
allowance must be made for its velocity factor in calculating the
mechanical length required to resonate as a half-wave section
electrically at 14.150 MHz. Since the velocity factor of standard 300
ohm ribbon feeder is 0.82, the mechanical length should be 8.5m (28
ft). However, if 300 ohm ribbon with "windows" is used, its velocity
factor will be almost that of open-wire feeder, say 0.90, so its
mechanical length should be 9.3m (30.6 ft)."

Talking about the physical length of the so-called matching section
without knowing the velocity factor of the actual line is incomplete,
it is the electrical length that matters.

Owen
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