I've built 2-turn magnetic loop RX antenna using 5 meters of 6.5mm copper tube. I was able to tune it with a variable air-spaced capacitor in the range of 1.8-10Mhz, but I noticed that as tuning frequency goes to 7MHz and above - selectivity gets less significantly, reception peak gets wider and much less visible (around 300-500kHz wide, which is extremely wide for magloop as far as I understand). Tried 2 different variable capacitors - result is the same. Everything is soldered.
I assume there is an optimal length of the loop / number of turns to get the highest selectivity. But how to identify this optimal number of loops / length of 1 loop?
What causes this drop of Q-factor / selectivity at higher frequencies, where capacitor value gets smaller than ~20pF?