Whilst researching another article, I came across a Youtube presentation on the subject of “Optimum Magnetic Loop Antenna.” It described the authors design of the “10-40m “epicenter” 42″ square 1.5″ dia loop” as an example optimised loop.

This article attempts to analyse the presented data to estimate the performance of the loop, specifically radiation efficiency at the lowest operating frequency.

Loop parameters:

- shape: square
- conductor diameter : 38mm
- perimeter: 4.47m

The presentation gives a measured VSWR curve at maximum capacitance. Minimum VSWR is just below the 40m band, so we might expect that performance on 40m is very similar.

I did not see description of the measurement environment (height above ground, soil type, proximity to structures etc) in the presentation, they are all relevant if the experiment was to be replicated.

Above, the VSWR curve at maximum capacitance. Unfortunately it is not matched for low VSWR_{50} so we must make assumptions. Lets assume the impedance at resonance is lower than 50Ω, we can estimate the half power bandwidth from the VSWR=10 bandwidth of 480kHz, see below. Continue reading What passes as an optimised small transmitting loop?