From: rkarlqu@scd.hp.com (Richard Karlquist)
Newsgroups: rec.radio.amateur.misc
Subject: Re: Linear Loading of antennas
Date: 19 Nov 92 01:56:33 GMT
Organization: Hewlett Packard Santa Clara Division
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Fred McKenzie (fred-mckenzie@ksc.nasa.gov) wrote:
: In article <1992Nov18.163220.24344@cbnewsm.cb.att.com>,
: nk30@cbnewsm.cb.att.com (JEFF ZELL NK3O) wrote:
: 
: > Why is linear loading not a popular method of making antennas? From everything
: > I have read a linear loaded antenna, has less radiated power loss than an 
: > inductively loaded or trapped antenna. Is it a problem of mechanics 
: 
: 
: Jeff-
: 
: I doubt that the linearly loaded antenna, which is in fact an inductively
: loaded antenna, would have less radiated power loss in general.
: resistance of the wire.
: 
: A continuously loaded antenna will probably have a wider bandwidth than an
: end-loaded antenna, and not have as much power loss as an antenna with
: loading concentrated at a high current point.
: 
: As far as popularity, I recall seeing advertisements for linearly loaded
: fiberglass CB mobile antennas, as well as a "Slinky" halfwave dipole for
: hams.  However, that's about it.  They probably cost more to make, or
: something!
: 
: 73, Fred, K4DII
: 
: fred-mckenzie@ksc.nasa.gov

I think we have some semantic problems here.  A "Slinky" antenna is a
*continuously* loaded antenna.  A 40 meter commercial beam with 46 foot
(instead of half-wave 66 foot) elements is a *linearly* loaded antenna.
Linear loading in the beam is done by having have the signal travel
half way out and then double back on a separate conductor almost to the
boom.  Then it goes out again on a third conductor to the element tip.
This forms essential a long skinny one-turn inductor.  You could also
analyze the loop as a shorted parallel wire line.  Notice that the 
third conductor is centered between the first and second so there is
no coupling.

There are two very simple reasons why a continously loaded antenna is
suboptimal.  First, it is a combination of bottom, center and top loading.
Since top loading is better than the other two types, continous loading
cannot be as good as top loading.  Second, the Q of an inductor is mainly
determined by its diameter.  Usually, continous loaded antennas have a
diameter of only an inch or two.  A 1 inch diameter coil will have a Q
of at best 200 on 80 meters, whether it is 1 inch long or 100 feet long.
A 6 inch diameter coil 6 inches long will have much higher Q and hence
much lower loss. 

Linear loading works OK for the canonical case of shortening the element
from a half-wave to a third-wave (most commercial compact 40 meter beams).
However, you can't go much beyond that (at least with a single turn) because
even if the linear loading section takes up the entire element, you can't
get enough inductance.  I suppose you could make a two turn "coil" which
means you now need five conductors in parallel.  To get high inductance,
you need small diameter wires widely spaced.  The small diameter increases
ohmic losses and the wide spacing increases radiation losses, resulting in
mediocre Q.  This mediocre Q is adequate for a 1/3 wavelength element but
not a 1/4 wave or less element.  Note that radiation losses from the 
linear loading are "bad" radiation that screws up the beam pattern.
For a simple dipole, you could make a case for saying they don't count as
losses because any radiation in any direction is probably useful.

Rick N6RK
rkarlqu@scd.hp.com