[TowerTalk] Modeling antenna traps

[Timothy Holmes]

Hi Jim

Ok, Ill dig out my analyzer manual to see what I can do.  It should be an
interesting project.  I know the original antenna was a 10,15,20 m
tribander, so that gives me a starting point

On Sun, Jun 5, 2016, 7:18 PM Jim Brown <jim at audiosystemsgroup.com> wrote:

> The answer to this question is not simple. It depends, for example, on
> what tools you have available, the physical construction of the traps,
> and the extent to which they can be taken apart.
>
> A simple antenna analyzer is NOT a good tool for this, simply because
> its accuracy is poor. That's in addition to the fact that the accuracy
> of ANY reflection-based measurement is good only for impedance values
> within 5X or 0.2X of the system impedance of the analyzer (usually 50
> ohms), and the greater the difference from 50 ohms, the greater the
> error. Nearly all ham antenna and impedance analyzers are reflection-based.
>
> There is a third source of error with analyzers -- the stray capacitance
> of their input circuit. The result of this error is that the resonant
> frequency is shifted down by the stray C, which adds to the error of L
> and C.  In the better vector analyzers, this can be subtracted out by a
> calibration routine, using known good short, open, and 50 ohm loads.
>
> Your MFJ with grid dip coils could probably find the resonance with
> reasonable accuracy. The key is to couple the analyzer very loosely, so
> that it does not shift the resonance up or down.
>
> 73, Jim K9YC
>
>
> On Sun,6/5/2016 3:40 PM, Timothy Holmes wrote:
> >
> > Hi Jim
> >
> > I guess that goes to the crux of my question, how do I go about
> > measuring those parameters, and, once I have done so, how do I then
> > model them. This is turning into quite an interesting discussion.  I
> > should mention that I have an MFJ antenna analyzer with the grid dip
> > coils if that will help, as well as a multimeter
> >
>
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