Efficiency and gain of Small Transmitting Loops (STL) referred to an online calculator (Calculate small transmitting loop gain from bandwidth measurement) for estimating the efficiency and gain of an STL.
This article documents an NEC-4 article of a 1m diameter tuned loop of 20mm diameter copper at 7MHz with a driving loop of approximately 0.2m diameter.
The model calculated efficiency was 11.89% and gain at -7.4dBi.
Above is a VSWR plot from the model, points are spaced 100Hz.
From the graph, we can estimate the half power bandwidth as the VSW=2.62 bandwidth. My estimate from the graph is 7.02642 to 7.02957MHz or 3.15kHz.
Above, the results of calculation using the model data.
The calculator indicates slightly lower efficiency and gain, possibly the result of the driving loop interaction (some small -ve mutual inductance inserted in the main loop), but nevertheless quite within the bounds of my expectation of accuracy.