MFJ-261 – review of review

A recent review of the MFJ-261 (Bogard 2021) was interesting.

 

From MFJ’s web site listing:

Connects directly to the transmitter with PL-259 connector. No patch cable used, reduces SWR. Finned aluminum, air-cooled heatsink. Handles 100 Watts peak, 15 Watts average. 50 Ohms. Covers DC to 500 MHz with less than 1.15:1 SWR. 1 ⅝” round by 3″ long.

That is pretty stunning for a device with a UHF connector, more on that later.

Bogard wrote a review of the device, making some VSWR measurements using a spectrum analyser with tracking generator and a VSWR accessory (a directional coupler).

The VB1032 VSWR bridge specification directivity is modest at 30dB. That challenges making really low VSWR measurements (as the article does) with low uncertainty.

Above is a calculation of the uncertainty in measurement for indicated VSWR=1.15 using a 30dB directivity coupler, the actual VSWR uncertainty range is 1.08 to 1.23.

Note that the test equipment does not use UHF connectors, so there was some kind of N(M) to UHF(F) adaption used, but no detail.

Above is the reported VSWR response.

In summary, it is pretty good at low frequencies and rises fairly uniformly to just over 1.5 at 500MHz… so on the surface of it, it fails to meet spec in a big way.

UHF connectors do not have a controlled characteristic impedance, and experience is that most UHF connectors are well represented by a transmission line section with Zo in the range 35-40Ω.

Realise that there are at least two cascaded UHF connectors in this test setup, and that the effect of a very short series line section of low Zo is to cause InsertionVSWR.

Let’s look at a model in Simsmith to illustrate the effect.

Above, the Simsmith model that models the UHF connectors as a 37mm length of line with Zo=35Ω and VF=0.66. These values were arrived at by calibrating the model to the published measurements.

The model assumes that the load has little reactance at low frequencies, but the resistance is wrong and causes the low frequency VSWR=1.045. This gives two choices.

Calibration included adjusting the load resistance for low frequency VSWR and the transmission line parameters to achieve the shape, slope and 500MHz VSWR.

It is not a perfect fit, but it gives a very plausible possible explanation the measurements.

So, the question is how much of the non-ideal behavior is due to the test fixture and how much is in the DUT? It is possible that although the DUT has a UHF connector, that the effect of the connector has been compensated inside the DUT and that at some reference plane, it meets the stated specifications. The very slight wavy nature to the VSWR response might hint some level of internal compensation.

So, (Bogard 2021) is a bit unsatisfying. On the surface it shows non-compliant VSWR, but does not address it leaving one wondering about whether a significant part is due to the test setup (coupler directivity and UHF connectors / adapters).

References

    • Bogard, J. Sep 2021. MFJ-261 Dry dummy load. In QST Sep 2021.