I'll take Tom's word on this.
I haven't used this for antennas with feedlines built in --e.g. the
GAP verticals(?). I think plotting the value of X vs frequency as
suggested, will tip you off. If it's shape differs considerably
compared to a simple antenna (e.g. dipole), that's a tip-off that the
results should not be trusted.
73 de Brian/K3KO
Tom Rauch wrote:
> > You can back out the value of X using the SWR and value of Z.
> > The formula is in the AUTEK manual. The sign of it is easy to
> > determine.
> Not true!
> > First find the resonant freq. If your freq of interest is higher then
> > the sign of Z is + (inductive) and vice versa. To get practice, try
> > this first on a simple antenna with well known characteristics. Take
> > about 5 points either side of resonance and plot the R and X values.
> > It will be pretty obvious when X changes signs.
> Unfortunately this is what some analyzers assume. It is reliable
> with lumped components.
> It is unreliable when you are measuring a system with transmission
> lines. It is possible to have increasing Z or X with decreasing or
> increasing frequency and the sign to be either capacitive or
> inductive randomly.
> Since antennas have transmission lines integrated in them, even
> measuring a simple antenna with that method can prove misleading.
> The only reliable way is to measure phase angle, or add a
> reactance of a known sign and see how impedance changes.
> 73, Tom W8JI
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