RE: NEC-LIST: Negative resistance?

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From: Steven Best (sbest@cushcraft.com)
Date: Wed Apr 25 2001 - 07:04:50 PDT


Jian:

Measuring a negative resistance is actually quite common, especially with
vector network analyzers.

Several things can cause this. First, with a calibrated network analyzer
you must realize that the "measured resistance" is a calculated number. The
network analyzer software essentially creates a calibration algorithm and
processes the measured voltage and current to arrive at an impedance
refereed to the calibration point.

Sometimes, it calculates a negative resistance. This can typically occur
when the resistance is low and the reactance is high. This is usually at
frequencies well below the first resonance of a monopole. It may also occur
at frequencies where the monopole's impedance is going through a parallel
resonance. This is characterized by a rapid change in the sign of the
reactance, where the reactance rapidly transitions from a +X to a -X value.
Over this same range of frequencies, the resistance may also be rapidly
changing from a low value to a very high value and then back to a very low
value. This exact problem happended with a monopole I was measuring the
other day. I also measured a negative feedpoint resistance.

In your case, where the monopole is mounted on the interior side plate of a
rectangular metal box, you will find that the presence of the metal box will
dramatically change the feedpoint impedance characteristics of the monopole
as a function of frequency. I suspect that at the frequencies where you
measure the negative resistance, the monopole's impedance is rapidly
changing and perhaps going through a parallel resonance. You may find that
at these frequencies, the box dimensions or distance from the monopole to
the box sides is a fractional resonant length. If the frequencies where you
measure the negative resistance are of little interest operationally, ignore
the measurements. If they are operationally critical, you may have other
design issues (like the size of the box, or location of the monopole) that
need to be resolved.

Steve Best

-----Original Message-----
From: davem@ece.ubc.ca [mailto:davem@ece.ubc.ca]On Behalf Of
rjianwei@cc.curtin.edu.au
Sent: Friday, April 20, 2001 6:01 AM
To: nec-list@ee.ubc.ca
Subject: NEC-LIST: Negative resistance?

Dear all,

I am measureing an input impedance of a monopole , which is mounted on
the interior side plate of a rectangular metal box. The box has two
slots on the top plate. However, we get megative resistance at some
frequencies. Could you please give me some suggestion which kind of
things can cause negative resistance in the measurement? Thank you
very much.

Best regards,

Jian

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