There is a BIG difference between ladder line and open wire transmission
line.
Even low loss ladder line can have significant loss, especially when wet.
Open wire feeders can truly be low loss even when operated at high SWR.
So how do you construct proper open wire line? It is most easily done
when the 2 wires are under tension and spaced about 6 inches apart with
as few insulators between them as possible - the tension keeps the
spacing relatively constant - that is practical over a long horizontal
run of the transmission line. Then bring it up to the antenna feedpoint
with wires spaced apart with insulators, and likewise on the run to the
house entry or better yet to the shack - the more insulators, the
greater the loss can be. You can use ladder-line from there into the
shack, or you can put a good current mode balun at the house entry and
from there run a short length of coax to the tuner.
As I have mentioned before, whether a 1:1 balun or a 4:1 balun will work
better has to be determined - it depends on the feedline length, the
frequency, and the feedpoint impedance of the radiator for that
particular frequency. If the feedpoint impedance at the tuner end of
the feedline is already low, a 4:1 balun will make it 4 times lower and
difficult for your tuner to deal with.
A G5RV antenna on 160 meters will not be very efficient (the high
current point will be somewhere down the transmission line and not at
the center of the antenna). One thing is true, the RF voltage at the
ends of the antenna is the highest, and the highest current is 1/4
wavelength away from the end, even if that is at a point well down the
feedline. The equal and opposite currents on the feedline will cancle
making that highest current point not useful for radiating a signal.
Radiator lengths greater than 1/2 wavelength can be efficient, but
shorter than 1/2 wavelength will be less efficient because the balanced
currents on the feedline cancel each other out. That is just plain physics.
73,
Don W3FPR
On 1/7/2019 10:49 AM, Wes Stewart wrote:
Try running that 102' long "G5RV" up 50' on 160 meters. Feed it with
100' of Wireman 553 "low loss" ladderline. The 0.1 dB matched loss
turns into over 13 db at the input and that's without considering tuner
loss. (Source: EZNEC and TLDetails)
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