While I'm waiting for the new round board with drills for the air trimmers and two independent loops I built a fully assembled version of the first two boards but with the trimmer instead of fixed caps:
This works really well:
I wrote a little utility which sends packets at different frequencies across the spectrum and use the gateway to capture RSSI. That way I can adjust the trimmer without even attaching the VNA and with a fully live setup.
Both boards work well, close to the performance of Moteinos, Tinos etc. The rectangular model has smaller bandwidth than the round design. It might be the vias in the round one that degrade its Q and thus its performance. That had been a problem also when I measured it with the VNA. But it's still as good overall because the larger loop means more radiation resistance.
What's best is - and this is what I wanted to test with these boards - these loops are really incredibly stable to environmental changes. I've had them on wood, on the floor, on the balcony, in hand etc with very, very minor shift in resonant frequency.
For me this proves that these high Q loops are viable without active tuning if you use air trimmers.
Joe