Hi David:
I would think to get the most accurate result you should use Kelvin connections where the V & I leads for each terminal
contacted the load under test.
So, rather than using a mating connector you would use something like pogo probes where two probes would contact the
ground and two would contact the center terminal. For a nominal 50 Ohm value Dc measurements will be OK, but I found
for measuring very low resistance values AC must be used.
I've measured the real and imaginary components of various things with the HP 4395A 0 to 500 MHz combo box which is
essentially a measurement near DC.
Here's a page showing the the Zo of a transmission line is not constant across all frequencies. Scroll down for some
plots showing what amounts to the DC resistance.
http://www.prc68.com/I/Zo.shtml
Have Fun,
Brooke Clarke
http://www.PRC68.com
http://www.end2partygovernment.com/2012Issues.html
http://www.prc68.com/I/DietNutrition.html
Dr. David Kirkby (Kirkby Microwave Ltd) wrote:
I am looking to measure as accurately as possible the DC resistance of a
number of types of RF loads - all around 50 Ohms. Types of interest include
SMA, 3.5 mm, N, APC7, 7-16 & BNC. I want to see if the difference between
the actual resistance at DC and 50 Ohms correlates with the performance at
RF as measured on a vector network analyzer. I want to do both sexes, with
the exception of the APC7.
I would expect RF performance to correlate well at low frequencies, but as
the frequency is increased I would expect it not to correlate.
I am looking for advice on the best way to do this. I have a HP 3457A (6.5
digit DVM with 4-wire resistance capability), but will consider purchasing
a meter designed for low resistance measurements which I believe uses AC to
avoid thermal EMF issues. But if possible I would rather use the 3457A.
I was thinking of a plastic box with the RF connectors & 4 banana plugs. I
suspect running the sense and drive wires back to the banana plugs would
work. No signicant current would flow into the unterminated connectors as
they would have just the dielectric.
Ideally I would like to measure just the load and contact resistance and
not the resistance of the plug or socket I connect it to. That might be
next to impossible.
Can anyone offer any recommendations of how to wire it up, type of
connector (e.g. banana plug or similar)
PS I once see someone selling a dummy load on eBay with an SO 239 socket
and 47 Ohm wire wound resistor! Apparently the inductance made it 50 Ohms.
Dave
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