[TowerTalk] Re: linear loading

w8ji.tom w8ji.tom@MCIONE.com
Mon, 16 Nov 1998 10:20:50 -0500

Hi Pete,

> I know that antenna has a 24-foot boom, and the elements are also longer
> than on the EF-240S, but I was surprised at the difference.  W9LT says
> there has been some empirical work done that says lumped-constant loading
> like Cushcraft's is inherently superior to linear loading.  I'd like to
> know more, and also wonder what the gurus of Towertalk-land think of

That's a tough question Pete. It is pretty easy to get an unfair

**Of course this might not apply to systems that are not properly
optimized, anything can happen when something is "wrong" with a certain

First, NEC based programs don't not allow for  increased resistance that
occurs when two RF carrying conductors are placed next to each other. This
is a minor effect in most cases, but it is always there. That makes NEC
models skew results unfairly towards linear loading, because the model does
not account for all the losses with linear loading but it DOES include all
losses for an inductor (since you "write them in" for the inductor). 

Second, linear loading can move the effective loading location to a "better
location" when the linear loading wire is parallel to the element. If you
don't consider this problem, you might be comparing two antennas that are
really loaded in different locations despite the fact the element loading
system insulators are at the same locations.

Third, the last thing you want to do is put capacitance across a loading
inductance (and the stub in a linear loading system is simply an inductor
with very poor form-factor). Capacitance shunting an inductor reduces
efficiency by increasing unwanted circulating currents in the inductor (and
stub). Linear loading always has a lot of shunt capacitance, just like some
poorly designed coils (and all traps) do. 

It has been my experience, based on direct FS measurements, that properly
designed lumped loading is better than linear loading when the loading is
at the same effective location on the elements and similar conductor sizes
are used in the loading systems, but this difference is so minor you'd
never notice it.

IMO, seeing a large difference means something was wrong with the design of
the poorer system rather than the fact the loading "method" was changed.
73 Tom

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