This article uses a report of an experimental small transmitting loop (STL) for 20m to demonstrate application of the calculator Calculate small transmitting loop gain from bandwidth measurement to predict efficiency and gain of the circular STL from the loop radius, conductor radius and measured half power bandwidth.
The report includes the following:
I found a small plumbing shop open late and bought 4 meters of 1/2in copper tubing and made a loop this evening. I had no high voltage capacitors so cut various lengths of RG-58A to use as coax caps and kluged a Faraday shielded loop coupling system to drive the main loop. None of it is permanent yet but after only working on it for 30 minutes total, I have been amazed how well it works with the low power version K2. … The bandwidth for 2:1 SWR on 20 is 80khz without retuning the center of which is 1:1.
The loop radius is 4/pi/2m, conductor radius is 0.064m, frequency is 14MHz, and VSWR=2 bandwidth is 80kHz.
Above is the calculator result, estimated efficiency is just 8.5%, quite low for a loop of this diameter on 20m so there is lots of room for improvement.