Welcome to Part 4 of our discussion.
In Part 3, we discussed the advantages of using ladder line
to greatly reduce feedline loss. You can reread it if you like, just
below this section.
Using ladder line does come with a few challenges, however.
Unlike coax, it cannot be coiled, run on the ground, or buried, without
paying a price in performance. And most importantly, it does not like
to be near other conductive objects. Ladder line is -- in theory, at
least -- a balanced feedline. That means that the currents in each
conductor at any given point are equal in magnitude (voltage) but 180
degrees out of phase. When that happens, the RF fields in each leg
cancel each other out, and the line does not radiate. That's generally
what we want. But...
When we bring the line close to a metal object, that
balance is disrupted. Outdoors, it's usually easy to keep the line away
from metallic objects. The problem surfaces when we try to bring the
line inside the shack. Window frames, water pipes, electrical wires,
radiators, and similar things are difficult, at best, to avoid. Many
hams opt for a "hybrid" approach, running ladder line from the antenna
to just outside the shack, the transitioning to coax for the short run
to the rig or tuner. We want to keep that coax as short as possible, of
course. That may mean rearranging the shack a bit, but I think you'll
find it worth the effort.
In choosing ladder line, please do yourself a huge favor --
spend a few more dollars to get stranded wire conductors. The line will
flex quite a bit in the wind, and the solid wire stuff will quickly
fatigue and break. Let me tell you, tracking down and fixing those
breaks is a huge pain in the butt! And you can bet that your line will
break just before a major contest or DXpedition, in the middle of a
torrential downpour or howling blizzard. Stranded line -- always!
When you run the line, put a gentle twist on it. Maybe one
full turn every 3 feet or so. This greatly eases wind stress on the
line and increases longevity. If your antenna has a support at the feed
point, run a light rope down to a convenient tie-off place near the
shack entry point. Weave the rope through the windows in the ladder
line. That will also help to keep it from flapping in the wind. Also, I
recommend using a strain relief like the ladder-lock (on sale this
month, above) where the line attaches to the antenna wire.
"But wait", you ask. "You say ladder line is balanced.
That must mean coax is unbalanced. Don't we need some sort of gizmo to
get from balanced to unbalanced?" And that, gentle reader, is exactly
what a balun does -- or should do. And amazingly enough, baluns will be
the next topic that this "unbalanced" author will tackle. Stay tuned.
Until then,
73 for now,
John Bee, N1GNV
Quicksilver Radio Products
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