I'd been trying various simple methods of determining 'ball park' loss figures for broadband matching transformers for folks who don't have access to much test equipment.
The basic idea was to measure the input SWR of a transformer with the secondary Open or Short circuit, and I have been able to check this technique against a few different transformers in my collection.
It can be used as an indicator when the losses are relatively high, but it becomes much less accurate as the losses decreases, and the overall accuracy is likely to be made even worse by the use of a cheap inaccurate SWR meter as the measuring device.
The basic idea is not new, and a similar technique for measuring coax cable loss was described by Frank Witt, AI1H, in QEX back in 2005 (although that is based on earlier work too)
RF engineers tend to use 'Return Loss' as an alternative to SWR. The basic idea is that you measure the level of reflected signal in dB relative to the incident signal and this gives you a value of return loss, which tends to provide greater resolution than a basic SWR ratio.
The return loss is the two way loss though a device, as the signal travels out and then back again to the source. If you halve the return loss value it indicates the loss in one direction.
SWR can easily be converted to Return Loss and this in turn can be charted to show attenuation (1/2 RL value).
Below is a simple chart showing this relationship, which I hope will help with this measurement method.