The findings at InsertionVSWR of Revex W560 on HF and the suggestion that the low frequency problem is characteristic of poorly designed Sontheimer couplers (Sontheimer, C & Frederick 1966).
These couplers were popularised by (Grebenkemper 1987) in his Tandem Match – An Accurate Directional Wattmeter and have appeared in ARRL handbooks over the decades, and may have inspired the many commercial implementations of the coupler.
Grebenkemper claims his meter is ‘good’ down to 1.8MHz, but does not clearly claim any particular InsertionVSWR. There is limited value in an instrument that can measure down to 1.05 when it causes significantly higher VSWR itself.
Lets drill down on Gebenkember’s article, specifically the coupler design.
Continue reading InsertionVSWR of Grebenkemper’s Tandem Match