[volt-nuts] Building a box to measure DC resistance of multiple RF loads.

[Brooke Clarke]

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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