[TowerTalk] Re: linear loading

L. B. Cebik cebik@utkux.utcc.utk.edu
Thu, 19 Nov 1998 07:32:48 -0500 (EST)


>Exactly what is a "critically coupled" element? Where does one find that
>definition, or what makes it "critically coupled"?
>
>> Force 12 uses critically coupled and reactive antenna elements, in a
>variation
>> of an open sleeve coupling,

Since "critical coupling" is a term properly applied to inductive
coupling, it is likely better simply to classify the Force 12 multi-band
antennas (but not all of them) as using open sleeve coupling.  The
placement of the fed element and the other driver elements is critical
physically to obtain the correct source impedance when the antenna is fed
on other than the band to which the fed driver is tuned.  But the
criticalness of placement does make it either critical coupling or even a
direct analog of critical coupling.  The best short treatment of open
sleeve coupling available to hams is the chapter section on the subject
written by Roger Cox, WB0DGF, in The ARRL Antenna Book.  Open sleeve
coupling is a very practical feed method, but in most cases best modeled
in MININEC for accuracy, since NEC has its close-wire problems that tend
to over-report gain and under-report source impedance.  Even with a good
model, expect to fiddle with the exact placement (lengths and spacing) of
the driver set in the first prototype.  However, once found, the results
are reliably reproduced.  Several experimental designs at my site use it,
including an 80-75 m array, a couple of 12-17m beams, and a Yagi
alternative to the 10 m wide-band LPDA in my current research.  THis
sample will allow modelers who are interested in the technique to get a
start, if they have not used it before.  (Be sure to examine the current
magnitude and phase on the drivers not directly fed, since the same
conditions can often be replicated with parallel wire connections between
drivers for "closed-sleeve" coupling--but notice the "" around the coined
term.)

Because of traditional usage encased in electronics dictionaries, not
everything that is critical and coupled is critically coupled.  It is easy
to get caught using seemingly sensible ordinary word combinations only to
discover that there are some rigid definitions floating around that limit
use and possibly mislead readers--I know because I have done it, and more
recently than most folks suspect.

Hope this is useful.

-73-

LB, W4RNL

L. B. Cebik, W4RNL         /\  /\     *   /  /    /    (Off)(423) 974-7215
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