Thanks for the clarification, Don. As I mentioned in my private email,
to eliminate all ambiguity, you really need to look at the slopes of
both impedance magnitude and phase (or alternatively resistance and
reactance). This will cover you for both sides of either series or
parallel resonance.
This is what I do with my Plot software. You could certainly do it in
your head, but it's little much to remember. If you have an analyzer,
like the MFJ, which displays both R and X, it might make sense to print
a little "what if..." sequence on a laminated card and tape it to the
back of the unit to help with going through the scenarios when
calculating sign. Based on my algorithm, I could print up a sample for
anyone interested. Or you could use the two plots I referenced in this
discussion to make your own. Remember though, that my plots show correct
sign, and you have to reverse all the negative values to arrive at the
correct input data for an analyzer that doesn't show sign.
73,
Larry N8LP
Don Wilhelm wrote:
Folks,
There is an error in the reply I sent last night (below) that is very
important - it makes a big difference.
When increasing or decreasing the frequency to determine the sign of the
reactance:
One must observe the increasing or decreasing of the IMPEDANCE (Z) and not
just the reactance.
I had erroneously stated the reactance, and Larry helped get my mind
straightened out - make that impedance instead of reactance and it will be
correct. In other words, when using an MFJ259B - look at the "Z" value when
increasing or decreasing the frequency, and not the "X" value.
The notes in the MFJ259B manual recommending how to determine the sign of
the reactance are placed under the instructions for measuring Capacitance
and Inductance, and they do state that one can look at the reactance value.
While that works fine for a pure reactance, it does not work for complex
impedances, where one must look at the magnitude of the IMPEDANCE (Z).
In other words, switch the MFJ259B to IMPEDANCE MODE and do the 'frequency
increase/decrease thing' before attempting to affix a sign to the value of
the reactance parameter.
73,
Don W3FPR
-----Original Message-----
Larry,
I see no inconsistency with Stuart's statement in your plot. Without
looking at the specific phase angle - if the absolute magnitude of the
reactance increases with increasing frequency, then the load is inductive,
it it decreases, then the load is capacitive.
The poles of the black curve on your plot do line up with the
zero crossings
for the phase. So if I look at any point on your plot, I see that the
absolute value of the magnitude of the reactance will increase with
increasing frequency anytime the load has positive reactance (inductance),
and the absolute magnitude of the reactance will decrease with increasing
frequency any time the load is capacitive.
Yes, a simple (or complex) phase angle detector working without regard for
the sign will not provide the information required, but 'dithering' the
frequency and looking only at the relative magnitude of the reactance
between 'dither' points can provide the sign of the reactance.
73,
Don W3FPR
-----Original Message-----
That won't work, Stuart. I wrestled with this for awhile when developing
my LP-100 Plot program. You also need to know which side of resonance
you're on, and whether it's a series or parallel resonance... at
a minimum.
Take a look at the plot at www.telepostinc.com/Files/OCF_Z_PH2.jpg. This
is a section of an off center fed dipole as measured by my HP 87510A
VNA. If you take the absolute value of the phase data (ie, have it
bounce off zero as it would with a meter that can't determine phase
sign), then you will see the problem. If you look at the phase in this
plot, starting at the left edge, and remembering that all the negative
values have to be reversed to look like a meter without sign detection,
by your definition, the reactance makes the following changes from left
to right edge...
C, L, C, L, C, L, C, L
In reality, as the VNA shows, there are fewer changes. The
sequence is...
C, L, C, L, C
Without the visual aid, it can be confusing, to say the least.
In my Plot software, I take into account a number of factors, based on
the slopes of the magnitude and phase of impedance. If you don't do
this, you could easily be exactly wrong ;-)
73,
Larry N8LP
Stuart Rohre wrote:
It is not too simple to measure phase with a additional
circuitry. However,
it is simple to move the frequency of the antenna analyzer and see the
Reactance change, IF it is increasing with frequency it is inductive
reactance, (+); if decreasing with frequency increase, it is
capacitive
reactance, (-).
Once you get used to that, and make a little reminder note to
paste on the
analyzer case, you can do the check easily and remember which type of
reactance change you are seeing re frequency.
Stuart
K5KVH
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