Here is my first small loop antenna. Small means that the loop circumference is around 10% of the operating wavelength.
I intended to use the antenna on the 40m band, so I cut my old coaxial cable for TV antenna to 4.5m and used only the blade. The shape of the loop could be almost anything, and I made it a triangle so that the T shaped wooden structure can support the loop.
The measured inductance of the 4.5m length loop turned out to be 5.1uH, so the capacitance should be around 100pF to get the resonant frequency of 7MHz.
I used a tiny tuning capacitor for AM/FM radios with four sections, 160pF x 2 and 40pF x 2, starting my experiment with just two 160pF sections in parallel.
The coupler is a one-turn to eight-turn transformer with FT-140-43.
This what I get. Note that the resonance is very sharp, and if you scan the frequency range coarsely, you will easily miss the point.
Not very bad for a first trial, isn’t it?
Some of the stations received with FT8 while the antenna is still in the shack.