This is a fun little loop that I literally threw together the morning of Winter Field Day 2020. It uses an LMR400-type coax for the main radiating loop. It is an inexpensive, off-brand coax that is pretty stiff, which is actually a good thing as the loop is able to maintain it's shape without collapsing. I do still use a 3-piece PVC upright to keep it from twisting, and as a place to attach the coupling loop.
The air variable capacitor is a junk box part, but it has sufficient plate spacing to handle 20 watts SSB, CW and digital. With the single loop of coax, it will tune 30, 20, and 17 meters. I can add a second piece of LMR400 coax and tune it on 40 meters.
The neon bulb will fully light up at resonance, and is used when QSYing to a new band to quickly help me tune the capacitor.
The loop configuration for 30, 20, and 17 meters.