An online expert talking about compensation capacitors and EFHW ferrite cored transformers opined:
If the evaluation is done solely by the effect on measured SWR, whether it is measured with a standard reflectometer or a VNA, then it is just as likely the capacitor is changing the losses in the transformer rather than actually adjusting the match.
“Just as likely” + gobbledygook, is this just hand waving on social media?
Let’s explore it using the calibrated model used in a series of articles starting with 1:49 EFHW transformer using a Jaycar LO1238 core – design workup.
Above is the SimNEC model as calibrated to bench measurement of a prototype transformer. The compensation capacitor Ccomp is specified as 100pF with Q=1000 (reasonable for a silver mica capacitor that is well suited to the application).
Above is a zoomed in view of the chart, the dashed magenta line is the power passing right to left before the compensation cap, and blue is the power passing right to left after the compensation cap. The blue dashed line is mostly under the magenta dashed line, ie there is negligible reduction in power caused by the compensation cap.
With a suitable capacitor, capacitor loss @ 25MHz (the design limit for the transformer) is 0.11W, much less than core loss which is 3.2W.
Let’s reduce the Q of Ccomp to 100 representative of a low grade ceramic cap.
Above is a zoomed in view of the chart, the dashed magenta line is the power passing right to left before the compensation cap, and dashed blue is the power passing right to left after the compensation cap.
Now we can see the dashed blue line separate above about 15MHz, though the difference in power is very small even to 25Hz (the design limit for the transformer) where for 100W from the transmitter, less than 1.1W is dissipated in Ccomp (though that probably exceeds its power handling capacity).
Whilst I do recommend a good capacitor such as silver mica be used for this application, I note lots of published pictures use ceramic capacitors and their Q may range widely. It is misguided to select a 6kV ceramic capacitor, the voltage withstand is not needed and they often have Q below 200.
To the opening quote and my question: “Just as likely”, is this just hand waving on social media? Sound design should prevent that occurring, this does look like social chat without evidence to support assertions.