Re: NEC-LIST: Negative resistance?

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From: Wan Juang Foo (fwj@np.edu.sg)
Date: Wed Apr 25 2001 - 00:53:37 PDT


Hi Jian,
I suppose you are using a network analyser. BTY, are the dimensions of the
features larger than say 0.1 LAMBDA?
Since you are dealing with measurement of a real physical system. It is
possible that there are resonance in your metal box.

Anyway this is just a shot in the dark, not knowing how you did the
measurement setup. The obvious thing to note is the inherent resonances
within the box which acts like a microwave oven. You could be measuring
any one of the resonance mode of the rectangular metal box acting as a
resonance cavity.

Here are some questions, see if you can help us by answerin them. At the
offending frequency (or frequiencies), what are the electrical length of
your monopole, size of the box and location and size of the slots. You may
want to attempt a NEC simulation of the system, if you not already doing
so. What is not certain is the electrical sizes of the box and slots you
are working on.

Tim Foo

<snip>
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.
<snip

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