John, I built that bridge from that Ham Radio article and found it to
work reasonably well. I was appalled, however, to find just how very
critical every aspect of that device turned out to be, from component
values to tiny details of circuit layout. As you point out, it has been
surprising that such excellent and definitive work apparently fell into a
black hole.
Based on that experience, and the use of less competent noise bridges
before that, I have come to deeply mistrust any noise bridge on first
glance. Only if a particular unit has been properly "vetted" by
comparison with a known good impedance-measuring bridge should it be
trusted. There are just too many variables that significantly affect
accuracy, especially at the higher end of the HF spectrum. Even the
"manufactured" noise bridges are subject to tolerances of component
values and layout, etc. and cannot blindly be accepted as "accurate."
Kit-built units represent yet another layer of complication.
Plus, a lot of the end accuracy depends upon the operator, the companion
receiver, etc. These are elements of the system that the manufacturer
cannot control.
My bottom line is that I would tend to trust the reading of even a crude
SWR meter before I would put much faith in a noise bridge until that
bridge was fully qualified by comparison testing with a known impedance
bridge.
Sadly, the current crop of microprocessor-controlled "antenna analyzers"
has attained a cloak of "accuracy" largely because of the perceived
*precision* of their digital displays. From my work, only the AEA/Tempo
Complex Impedance Analyzer (CIA-HF) has demonstrated that it is at least
a semi-professional, lab-grade piece of gear.
72/73, George
Fairview, TX 30 mi NE Dallas in Collin county
Amateur Radio W5YR, in the 54th year and it just keeps getting better!
R/C since 1964 - AMA 98452 RVing since 1972 Kachina #91900556
John - G3JAG wrote:
>
> Probably much less than an extra inch .... the best article on this
> topic was perhaps the one in Ham Radio quite a few years back. The
> authors found that toroid lead length was remarkably critical and
> showed what they called "inductive rotation" because the error was
> frequency dependant.
>
> Their final design provided very simple and adjustable compensation; it
> gave a bridge with excellent accuracy over the HF bands. I have never
> seen any later reference to this article, which is pretty amazing
> considering the work that went into writing it. Maybe its the "not
> invented here" syndrome.
>
> I have the mag somewhere in my junk room, but so far I cannot locate
> it, or I would be able to give the date/page info.
>
> John G3JAG
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