Use of port extension or e-delay to measure a dummy load

I have an MFJ-264N which does not perform very well.

A measurement of the impedance at the inboard side of the N connector jack would be informative.

So, a NanoVNA-H4  (SOL calibrated at the Port 1 connector) with short SMA(M) to SMA(M) and SMA(F) to N(M) adapter are available, but how to set the reference plane to the inboard end of the N(F) connector?

Port extension, or e-delay is a means of correcting a lossless (ie short) ‘adapter’ where propagation time is independent of frequency, ie phase delay is proportional to frequency.

Above is a view of the inboard side of N(F) connector. If we can apply an effective short circuit centre pin to ground at that point, we can adjust e-delay to calibrate the ‘fixture’.

Above shows a small piece of kitchen foil scrunched up into a ball and pressed into the space to short the inner conductor to the connector body. The piece is held in place with moderate finger pressure whilst calibrating the e-delay value.

Above is the display after tweaking e-delay for approximately flat s11 phase over the frequency range (save the wrap from 180° to -180°), a necessary condition for validity of the technique.

The VNA is configured with VF=1, so the displayed distance is the two way free space electrical length which reconciles with the approximately 200mm cable length.

Having calibrated e-delay, we can now measure Z looking into the dummy load from the inboard end of the N(F) connector. There are two significant problems:

• R at low frequencies is close to 60Ω when ideally it would be 50Ω; and
• X rises linearly to 4.7Ω at 100MHz, suggesting there is about 7nH of series inductance when ideally X should be zero.

The main problem is the resistance of the carbon resistor. They tend to age high in value, and operation at high temperature accelerates aging. Given that these are rated at 1500W for intermittent operation and the derating curve suggests they are probably not good for more than about 50W continuously, users should keep in mind that every cycle to extreme temperature accelerates aging of the resistor and upwards creep in resistance.

This one has not been abused, abuse will exacerbate the problem.

Conclusions

Port extension or e-delay can provide a convenient means of shifting the reference plane given suitable test fixtures.