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

"The other John Smith" wrote
Is there any condition where a passive, real-world, network
could show a negative resistance?
____________

"Negative towers' exist in some MW broadcast directional arrays. Here is a
link to a paper about it: http://www.qsl.net/km5kg/negative.htm. Don't
know if that addresses your situation or not...

RF

"Richard Fry" wrote in message
...
"The other John Smith" wrote
Is there any condition where a passive, real-world, network
could show a negative resistance?
____________

"Negative towers' exist in some MW broadcast directional arrays. Here is
a
link to a paper about it: http://www.qsl.net/km5kg/negative.htm. Don't
know if that addresses your situation or not...

RF


Thanks, Richard. I'll take a look.

John

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

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.



I was thinking the neon lamp could display negative resistance at certain
voltage/current ranges. The tunnel diode can also.
Not sure right off it they are active devices or not without looking at some
old information I have.

"Ralph Mowery" wrote in message
ink.net...
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.



I was thinking the neon lamp could display negative resistance at certain
voltage/current ranges. The tunnel diode can also.
Not sure right off it they are active devices or not without looking at
some
old information I have.


Hi, Ralph -

Yes, I am familiar with tunnel diodes from way back in the 60's. They were
GE devices as I recall. In fact, until about 10 years ago, I had an even
older schematic from an EDN magazine article that showed how to connect two
NPN transistors together to emulate a tunnel diode. Looked sort of like a
bistable connection, as I recall.

Anyway, that's why I limited my statements to passive devices although, as
Roy pointed out, I did not state that I had only one source.

Thanks.

John

To test a device, connect a resistor to its terminals. If the resistor
gets warm, you have a negative resistance.

The neon lamp and tunnel diode exhibit a negative *dynamic* resistance.
That it, the *slope* of the V/I curve is negative at some parts of the
range. They don't exhibit a true negative resistance -- that is, under
no conditions will you find the value of V/I to be negative.

Roy Lewallen, W7EL

Ralph Mowery wrote:

I was thinking the neon lamp could display negative resistance at certain
voltage/current ranges. The tunnel diode can also.
Not sure right off it they are active devices or not without looking at some
old information I have.

"Roy Lewallen" wrote in message
...
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.

Okay. I have only the one source, so I should never see a negative
resistance is what I read from your reply.

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.

Yes. That's why I made sure I mentioned passive. I meant no active devices.

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.

Error was my assumption as well. Thanks for your input, Roy. I'll take a
look at EZNEC as you suggested.

John




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

We had to stop using 8405s in the field many years ago due to the external
RF problem Roy describes. Many of the newer analyzers use synchronous
detection which reduces the problem, unless the RF signal level from the
antenna exceeds the dynamic range of the instrument. Put a wideband power
meter or true RMS RF voltmeter at the measurement terminals and see how much
voltage/power is there from your antenna. You may have to wait for a
blackout to make measurements if the RF is from sky wave signals. If it is
ground wave from MW broadcast stations, move.

--
Crazy George
Remove N O and S P A M imbedded in return address

John, negative resistance is indicated when the magnitude of the reflection
coefficient is greater than one.
The solution of the impedance equation for rho 1 yields negative values of
resistance. This lends a lot
of insight into possible causes:

The calibration for open or short is slightly lossy. If a measured load
has a little more reflection than the
calibration loads, it would indicate as negative resistance.

Numerical errors creep into the results. The Smith chart is extremely
sensitive around the periphery.
A few tenths of a dB. result in large movements at the edge of the chart.
Thus, small calibration or
computational errors can result in crossing over the rho=1 circle.

And, as mentioned, detection of external RF fields increasing the output
of the reflection detector
past the calibration value can result in a measured value for rho 1.

-- Tom, N5EG






"The other John Smith" wrote in message
ink.net...

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?



John KD5YI