[TowerTalk] Modeling antenna traps

[Jim Brown]

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
>