I see from my records that you have EZNEC. Open EZNEC example file
4square.ez, and click Src Dat. You'll see that element 1 has a negative
resistance. In the main EZNEC window, open the File menu, then click
Edit Current Antenna Notes. This contains an explanation of the
phenomenon. It can happen only when there are multiple sources, since
the power to create the negative resistance at one source comes from
power supplied by others. If you use a feed system to distribute power
to various elements from a single source, the resistance at that source
can never be negative.

Active devices can also exhibit a negative resitance, but again, there
has to be some source of power, since a negative resistance sources
power to whatever is connected to it.

Your result could be due to measurement error, erroneous setup or
procedure, or, if you're measuring an antenna, RF on the antenna from
outside sources. The latter makes antenna impedance measurement
difficult without a sharply tuned detector, which the 8405A doesn't have.

Roy Lewallen, W7EL

The other John Smith wrote:

Hi, Gentlemen -

I have an HP 8405A Vector Voltmeter. I have the App Note "Measuring Complex
Impedance" by HP (thanks to Wes Stewart). I have made a few measurements
following (maybe) the HP paper. I have probably been sloppy with the setup
and procedure because I came up with a negative resistance a couple of
times.

The HP paper discusses negative resistance but does not mention where one
would encounter it. I assume it would normally be associated with active
device measurements. Is this true? If I measure a (passive) load and find
that the resistance is negative, shouldn't I doubt my setup and/or
procedure? Is there any condition where a passive, real-world, network could
show a negative resistance?

By the way, I have not gotten a negative resistance reading while using a
dual directional coupler. I have gotten the negative resistance reading only
when using the power splitter method. So far.

Thanks for any insight on this.

John KD5YI