ARLA/CLUSTER: Episódio 123 da série " Foundations of Amateur Radio "

[João Costa > CT1FBF]

Foundations of Amateur Radio #123

*Ladder Line is not Evil*

The way we connect our antenna to our radio depends on a number of
different factors. If you've come through the ranks recently, like I have,
it's probable that you've only ever considered using COAXIAL cable. It's a
single conductor, surrounded by some type of insulator, which in turn is
surrounded by a conductive shield, which is protected by another layer.

There are variations where the shield has multiple layers, including layers
of foil and braid, so-called quad-shield COAX, and there are variants that
have several cores, sometimes two sets of COAX connected side-by-side and
so on.

In many ways, COAX is an invention of convenience, which has several
compromises as a result, loss over distance, termination issues, twisting
and deformation and others. It's compact, less susceptible to external
interference, it's relatively easy to route to its destination and if you
treat it well, it's easy to carry around, but it's not the only way you can
feed an antenna.

You may have come across the term Ladder Line or Twin-lead, or Twin-feed
line. You may also have heard horror stories associated with this "ancient"
- well at least in Amateur Radio Terms - technology. Essentially, Ladder
Line is two conductors, side-by-side, evenly separated by spacers. It's in
use all over the place. If you look up at your power-lines in the street,
or the high-voltage lines on top of towers, you'll notice that those are
essentially Ladder Line.

You've no doubt been told that you need to keep Ladder Line away from
everything, in order for it to work, but that's not actually what's needed.
What's required is that both conductors are exposed to the same fields.
This means that if you're running the Ladder Line through a metal window,
you need to ensure that both lines get the same amount of exposure to any
nearby metal, you might put a slight twist in the Ladder Line, or you might
put it in the middle between two bits of conductor, like a metal window
frame.

You might also have been told that Ladder Line radiates. It does, but only
if the antenna you're feeding isn't balanced, because what actually happens
is that the two sides of the antenna don't cancel each other out and the
difference is radiated by the Ladder Line. It's worse if the Ladder Line is
some resonant length on the frequencies on which you're using the antenna,
because it will receive the signal from your antenna and re-radiate that
too.

It really means that you need to pay attention, but the cost of that
attention pales into insignificance, if you think of the benefits.

You might recall that your radio is most happy when it's transmitting into
a 50 Ohm load. One of the measurements associated with that is an SWR
reading of 1:1. This has come to be interpreted as: "You need a 50 Ohm
antenna in order for it to radiate." and that's not actually true.

All antennas will radiate, and as long as they are at least half a wave
length long, their efficiency will be about 90%. The problem isn't the
antenna, it's how you feed the antenna. As I said earlier, if you're a
relatively new amateur like me, you might have put one and one together and
decided that you need to feed it with 50 Ohm COAX and that your antenna
needs to be 50 Ohm. If you do that, it will work, but it's not the only way.

The reason you need to have a 50 Ohm feed-point to plug your COAX into, is
because the COAX has a lot of loss if there is a feed-point mis-match. The
higher the mis-match, the higher the loss. For example, using an 80m Dipole
on 40m might mean an SWR of 65:1. This has about 80% loss on 100 feet of
RG-8 COAX at 7 MHz. All you're doing is heating up COAX.

However, if you were to feed it with 600 Ohm Ladder Line, the loss might
only be around 3%. Before you start getting out the calculator to prove my
maths wrong, this isn't about maths, it's about the difference between
Ladder Line and COAX.

COAX is wonderful as a tool, but Ladder Line should not be consigned to the
annals of history, because in many, many situations it out-shines COAX. The
combination of the thickness of the conductor and the separation width,
determines its native impedance. There is lots of documentation online,
including calculators on spacing and thickness, so you can build your own.

I've seen lots of different types of spacers, from watering tubes cut to
length holding the wire with cable-ties, to bits of Perspex, to cutting
board plastic and others. The simplest and cheapest one I've seen to date
was last weekend, made from two earth wires separated by strips of garden
edging that comes in rolls. You cut off little strips, drill two holes,
feed the wire through and if you're feeling that it needs permanency, glue
them in place.

Of course you can buy the stuff, but it's getting harder and harder to find
at reasonable cost, so experiment a little.

Ladder Line, it's not evil, it might surprise you and you will have another
feather in your cap when you go out portable to set-up a field station.

I'm *Onno VK6FLAB*

Listen online at http://podcasts.itmaze.com.au/foundations/ or search for
my callsign VK6FLAB on iTunes.
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