Re: non-destructive coupling of hoop loop to PL-310
Congrats on the Hoop Loop! The right number of turns you wind on the ferrite rod depends on how many turns Tecsun used, and the inductance of the rod by itself. Has anyone measured the inductance of a PL-310 internal antenna? Squaring the ratio of the number of turns on the original antenna divided by the number of turns you add, and multiplying by 16 (the inductance of the 23-inch Hoop Loop with 3 turns), gives you the transformed inductance. That inductance is in parallel with the inductance of the internal antenna. Here we're less concerned with the positioning of the winding as the turns ratio. You determine the total inductance by
Lt = L1 X L2 / L1 + L2
Not knowing the specs of the PL-310 internal antenna, I winged it and assumed 56 turns and 330 uH.
56 / 15 = 3.7, the turns ratio. The inductance is transformed by an amount equal to 3.7 squared, or about 14. A 16-uH Hoop Loop is transformed to 16 X 14 or 224 uH. The parallel combination of 224 and 330 is 133 uH, which is below what the chip is specified to tune. Yours does drop off a bit below 560 uH. If you used 10 turns, the antenna inductance would be transformed to about 500 uH. In parallel with 330 uH, you'd have 200 uH, which is safely within the chip's specified range. From what Scott has told me, I'd try to get at least 160 uH, to make sure it tunes down to 530 kHz.
Even better, though I suppose soldering inside the radio defeats "Barefoot," would be 5 bifilar turns arranged as shown on the Hoop Loop page, with the center tap connected to ground. It shouldn't affect the Barefoot classification because the extra winding hanging free will probably degrade the already lousy internal antenna's performance!
If you can get the coupling loop closer to the antenna winding, it will make a big difference. You can't remove an antenna from one of these radios without messing up the Litz wire, unfortunately. The glue they use to keep the antenna from rattling gets onto the wire and shreds it when you lift out the antenna. It is better to put the coupling loop near the grounded end of the internal antenna winding if possible. If the -310 is like the -380, that won't work. If I cared about Barefoot, I'd buy a second PL-380, but so far I have resisted the temptation. :)
Your question about using a ferrite-rod antenna sent me back to the bench. On the -61, 7.5-inch rod I picked up, 13 turns came to 17.5 uH, which I decided was close enough to 16 uH for government work. Connected to a MW matching transformer, signals were weaker than with the Hoop Loop, but it worked better than expected with so few turns on the rod.
If you have a way to measure inductance, start with 15-16 turns spread over about half the length of the rod, and work back until you get close to the same inductance as your Hoop Loop. Go a little higher rather than lower. The extra turn can only help.