Re: Mixed bag with the Air Core Antenna add-on to Tecsun PL310

Nick Hall-Patch

--- In ultralightdx@..., D1028Gary@... wrote:
Recent experimentation here (and by Nick Hall-Patch, and others) indicates
that any change in an experimental loopstick parameter (ferrite bar size
or length, ferrite mix, Litz wire, coil "Q" or Si4734 Radio model) can
change the optimum loopstick coil inductance required to match the DSP chip
varactor for peak AM sensitivity.
As Gary mentioned, I tried a 3" long, 0.375" diameter Amidon type 61 rod with the PL-380, so here are some preliminary results.

I initially used Amidon's 32mH/1000t approximate AL value for this rod, to calculate a 125 turn coil of 10/44 Litz to create Gary's "secret sauce" 554uH winding. However, this number of turns formed about a 750uH coil instead , but I did some quick tests with it anyway, by disconnecting the leads of the stock ferrite antenna and soldering in the leads from the new coil instead. With the coil placed in approximately the same position as Gary's 7.5" rod relative to the PL-380, the maximum strengths of several semi-locals at midday were noted relative to the PL-380 stock core which had been observed a few minutes earlier prior to the changeover to the new coil. Some examples
stock 700uH coil
CFTE-1410 50dBu 57 dBu
KOMO-1000 41dBu 49 dBu
CKNW-980 50dBu 59 dBu
CHMJ-730 49dBu 60 dBu
KIRO-710 35dBu 56 dBu

all of these had a 25 dB S/N ratio using either coil....except KOMO, which dropped to 18dB using the 700uH coil (interesting; strength increase but S/N decrease...remember, big local CFAX-1070 is in the same direction). I tested other weaker signals, but these were showing fading, so I'm not sure how valid the comparisons were. KPOJ-620 and KXL-750 both showed a 5 or 6 dB improvement in signal strength with the 700uH coil, with S/N going from zero to a few dB, but they were fadey, so hard to claim accuracy without a proper A/B comparison. The semi-locals noted above had showed an increase in signal strength as much as 7dB at the top of the band to as much as 11 dB at the bottom of the band.

Now, I reduced the windings to approximately 100 turns, which gave about 550uH on an SRS LCR bridge, all frequencies tested also showed increased signal strength from the stock bar, and the increase was still more impressive at the bottom end of the MW band. Increased strength ranged from 2dB in the X-band through 4 or 5 dB from 1510 down to 1000kHz, and more like 9dB below 1000kHz (12 and 15dB increase was noted on the two stations from Portland, but propagation may have changed in the few minutes between changing one set of ferrite rod leads to the other). These are less than Gary's 7.5" rod improvements, hardly surprising, given the 2.5x difference in the length of the long rod, and its larger diameter. What is of interest, is that they are also somewhat less than the signal strength differences noted using the 700uH coil on the same rod.

What was quite important however, was position of the rod relative to the radio. If the rod was snugged up against the back of the radio (this is sort of approximate), the signal strength dropped, if it was placed above and away from the radio like the 7.5" loopstick was, signal strengths were at their best. I suspect this may have to do with effect of proximity of the radio's circuitry to the loop, which could drop the "Q", and therefore the voltage swing available from the loop. The RLC bridge I used to measure my new coil has a "Q" indication, and even though the bridge itself has a plastic case concealing its circuitry, if I left the coil on the case next to the test leads from the instrument, Q was about 70. If I propped the coil up a couple of inches above the case, Q went up to above 100 (Q is measured at the 100kHz clock frequency of the bridge, I believe, so not entirely relevant at MW, but the ratio of 100 to 70 is probably significant). As comparison, the PL380's stock rod in place measured 227uH with a Q of 23.5, so its location probably is not helping it any so far as Q is concerned.

So, Gary might want to move his rewound PL-310 core away from the radio (or even move the original one) and see if he gets improved signal strength!

I also had another 3" 61 type core left over from some other project with a 200uH coil scramble wound on it (all Litz used was 10/44), and tried it with the PL-380, and there were gain improvements over the stock rod even with that (but with the 3" rod away from the radio), and the improvements weren't wildly different from that observed with the 554uH coil. Makes it look as if the length and diameter of the rod had more effect than the inductance.

My results seem to imply that the Q of the coil is an important parameter, not necessarily just the inductance. (or at least the Q as influenced by the core's position relative to the radio; it doesn't seem the inductance changes with position), At the moment I don't understand that, as the Q is most important in a resonant circuit, and I don't see how a 750uH core can be resonant with this radio at the high end of the band. It makes sense that too high an inductance will not allow the varactor to peak the coil; but maybe the extra turns deliver enough extra voltage to overcome the lack of peaking???

I'm looking forward to Gary's slider experiments with a 3" rod.

best wishes,


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