From: "Rod Brink" <rodbrink@montereybay.com>
To: <towertalk@contesting.com>
Subject: [TowerTalk] Linear loading
Date sent: Thu, 17 Jun 1999 08:57:15 -0700
>
> Recently heard a guy on 40M say that he was operating with a
> "linear-loaded loop". What is that? My antenna books are in storage, but
> as I remember, linear loading refers to electrically extending radiating
> elements by folding them back on themselves at the endpoints, right? How
> would you apply this to a loop??
Hi Rod,
You are basically correct on the definition, but the one point
throwing you is the folding does not have to be at an "end". It can
be anywhere along an element. If you consider that, then you will
see he could "linear load" a loop.
The advantages of linear loading are mostly imagined, rather than
practical. This is especially true if the loading is folded in towards
the feedpoint.
Linear loading is really just a simple inductor that has been
stretched out of optimum shape so mutual coupling is less. Of
course this effect increases resistive loss for a given conductor
size and material because it takes more wire length to produce the
same amount of reactance when mutual coupling is reduced.
Linear loading also has more distributed capacitance because the
a high voltage difference (strong electric field) appears between
close spaced conductors. This effect increases the effective
inductance, but at the expense of additional loss. Loss increases
because current bunches at the inside edges of the wires (reducing
surface area that carries current) and because circulating currents
increase. Capacitance across a loading inductor (or stub) is
ALWAYS a bad thing for efficiency, even if it reduces the number of
turns and narrows the measured bandwidth.
The last effect of linear loading is it radiates. The radiation changes
efficiency depending on the direction the loading wires are folded.
Many antennas fold the linear loading the wrong way, with the
result that replacing the linear load with a conventional load
improves performance quite a bit. Folding the linear loading IN
towards the feedpoint is the WRONG way to fold linear loading,
when compared to an inductor placed at the same point where the
element is "loaded".
73, Tom W8JI
w8ji@contesting.com
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