PD7MAA’s BN43-202 matching transformer for an EFHW – full measurement set

I have written some recent articles about or relevant to PD7MAA’s BN43-202 EFHW matching transformer. At about the same time a discussion started on and through that discussion, one ‘online extra expert’ stated that my analysis was bogus (dictionary meaning: not genuine, faked, a misrepresentation).

This article presents detail that was not included in the earlier articles as it distracts from the issue for most readers.

PD7MAA’s measurements

PD7MAA gave measurements of his 7-29MHz transformer with a 3300Ω load. He does not explain the choice of load resistance, but it does give a lowish VSWR mid band.

He has not given us R,X values, but he has given VSWR and |Z| at 7.01MHz (1.48,60.3). From those, we can calculate R,|X|.

So, R is 56.9 and |X| is 19.9. It is most likely that X is positive, it is the nature of these things at lower frequencies where the magnetising impedance dominates.

My measurements

A prototype transformer was constructed to PD7MAA’s design and loaded with a 3300Ω resistor and measured. Note that the tolerance of ferrite components is quite wide, and the results will not replicate PD7MAAs exactly, but it should be close.

Above is the prototype transformer in a calibrated test jig. The total transformer load is 3300+50Ω.

The measured R,X at 7MHz are similar to PD7MAA’s (allowing for component tolerances), and there is good reason to think that this is in the region of what any other constructor would obtain with the design..

It seems that people with a superficial knowledge of transformers insist that it is bogus because Zin=Rl/n^2=3350/5.5^2=111+j0Ω, not what was measured by PD7MAA and myself, but the problem is not the measurements, it is their superficial knowledge.