BNC 50 Ohm Termination: a rather surprising construction

Doing some precise level measurement on 50 Ohm system, I came across this termination, Model 24931 28P296-1.

termination 1

It’s resistance, not quite 50 Ohms, but 51.1 Ohm… not that the DC resistance is the most important characteristics of a 50 Ohm termination, but nevertheless, a reason to check it.

This termination is constructed using a BNC connector, so I expected some kind of thin film resistor inside, maybe damaged by overload or age, leading to the incorrect resistance.

Opening it up, this it what I found:

termination rn55d 51r1 resistor


The resistive element, a Vishay Dale R55D metal film resistor:
vishay rn55 resistor

And, in fact, it is a 51.1 Ohm, +-100 ppm/K tempco resistor. 0.1 Watts so it is running close to its limit already at 20 dBm! Don’t have the right tools here to measure ‘low frequency’ (< 2 GHz) SWR... but will check back home at the main workshop one time in the future, just for curiosity.

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