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.
- 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 VSWR50 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.
Above is a calculation based on the data above, estimated radiation efficiency at 6.95Mhz is just less than 1% for this “optimised loop”. Performance at 7MHz (when tuned) should be a tiny bit better, though challenging to measure.
The loop uses a tuning capacitor with PVC dielectric which the author says “costs nothing, and works just as well as multi hundred dollar vacuum variable capacitor”. I doubt it, I think it one of several factors that contributes to the loop's poor performance.
Though the presentation does not give meaningful direct performance data, we can use the published data to estimate the radiation efficiency and antenna gain. It is a very compact antenna, but it comes at a large performance cost.