Re: non-destructive coupling of hoop loop to PL-310



In the photos section of this group, Gary has posted high-res pictures of the PL-310 loopstick. It has 80 turns and a reported inductance of 318 uH.

Revising Jim's calculation:
With a 15-turn coupling winding, the turns ratio is 80/15 = 5.33, transforming a 16 uH loop to 16*5.33^2 = 455 uH. The Si4734 receiver input will see 455 in parallel with 318, or 187 uH. That should be good for tuning over the whole MW band.

Sounds like a fine hoop-loop system with minimal intrusion into the radio. Drawbacks relative to a toroid transformer will be things like selectivity (lower Q), less deep nulling, and some added noise sensitivity. But if you don't have a lot of strong pests and local noise, you may not miss those attributes.

As Jim wrote, a ferrite loop with inductance in the vicinity of 16 uH will also work well. I've attached a quickie ferrite loop to my PL-380 with something like 11 or 12 loops of wire-wrap wire over a ferrite similar to the stock loopstick. I haven't done extensive tests, but basic RSSI tests over the band are comparable.


--- In ultralightdx@..., "jim_kr1s" <jkearman@...> wrote:


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.


Jim, KR1S <> <>

Join { to automatically receive all group messages.