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

Gary DeBock

Hello Gregory (and Chris),
Thanks for reporting the results of the 7.5" loopstick transplant in your PL-310 model, and congratulations on the new stations you have logged with it.
Since this 7.5" loopstick transplant procedure was developed for the PL-380 model (not the PL-310), the 530-1700 kHz performance testing was only done here on the PL-380 model. Using the exact same material and procedure as described in the transplant article, the PL-380 model here had a very great AM sensitivity boost on all frequencies from 530-1700 kHz (averaging 20 dBu), with no issues on the high band like you describe.
Concerning whether this PL-380 transplant procedure developed for the PL-380 will work with equal effectiveness in the PL-310, as far as I know, you are the first hobbyist to try it in the PL-310 model. I have a PL-310 model here hooked up to the same Amidon loopstick with an 81-turn Slider coil, and no matter how I tune the Slider coil, the PL-310's AM sensitivity performance does not equal that of the finished PL-380 7.5" transplant model. It comes fairly close on most frequencies when the Slider coil is peaked for optimum inductance match with the DSP chip, but it is still about 5 dBu less than that of the PL-380 transplant model on most AM frequencies. The reason for this is a mystery-- it may possibly be due to improved circuitry in the PL-380 model installed by Tecsun to compensate for the smaller stock loopstick. In any case, the testing here indicates that the PL-310's performance with the 7.5" Amidon loopstick transplant seems slightly inferior to that of the equivalent PL-380 model, although there were no high-band issues on the transplant-model PL-310 here, like you described.
The symptoms you describe DID occur here on the PL-310 experimental loopstick composed of 113 turns of 40/44 Litz wire on the stock ferrite bar, which was tested here as a possible replacement for the stock PL-310 loopstick. This experimental loopstick (documented in a photo album on Ultralightdx) had a coil inductance of 554 uh, and did boost up AM sensitivity significantly on all AM frequencies up to about 1300 kHz. Above 1300 kHz, however, it was inferior to the stock loopstick in performance, with symptoms exactly like you describe. Presumably the coil "Q" of this experimental loopstick (practically covered with Litz wire) was unacceptable at the high AM frequencies.
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. The loopstick parameters in the PL-380 7.5" loopstick transplant article were developed and tested on the PL-380, but may not be directly applicable to the PL-310 model, Gregory. In any case, if you wish to optimize the AM sensitivity in your PL-310 transplant model on all frequencies, you may need to ensure that the 554 uh coil inductance is optimal for your particular experimental loopstick. The easy way to try this would be to add or subtract a few turns of Litz wire from the coil, to see if the high band issue would be corrected or not. You are also welcome to borrow the 81-turn Slider coil here, if you wish to perfectly fine-tune your new loopstick for optimal AM sensitivity.
73, Gary DeBock      
In a message dated 1/1/2010 9:25:01 A.M. Pacific Standard Time, gmosherat@... writes:

A few days ago I completed a Air Core Add-on to my Tecsun PL310, essentially I performed the 7.5 Inch Loopstick Transplant as described by Gary DeBock (BTW Gary - thants for all your work on this stuff - much appreciated!).

I can report mixed results, read on.

Antenna replacement consists of a Amidon 7.5" Type 61 ferrite bar wound with 81 turns of 40/44 Litz wire starting at 43mm from one end of the bar. Wire is wrapped around the Ferrite Rod first covered by J&J waterproof tape (Sticky side out). The one difference between my antenna and the one described in Gary's article is that I used 1 inch width J&J tape rather than the 2-inch width variety (I can't find the 2-inch stuff locally). Also, the original antenna was left in the radio, albiet, disconnected.

I only performed one test between the old and new antenna, at 1180 Khz (local 50Kw station). The new antenna gave me a 40 dBu increase over the stock antenna.

Pervious to this modification I had logged 115 stations barefoot. In the past two days (using the new antenna) I have added 15 new stations, not bad! I suspect not all of these can be attributed to the new antenna, but on the whole the radio is more sensitive.

Why do I say it's a mixed bag? It seems something is wrong at approx above 1370-1400khz. dBu seems ok, even on frequencies where there I hear nothing it hovers around 49-50. The problem is dB S/N... It stays at 00 for anything other that local stations (1370 and 1460). For example, On 1520 khz is a 50kw station 65 miles away. I logged this station prior to the antenna mod, It came in loud and clear. I can hear it on the car radio, and other radios in the house just fine. Now after the antenna mod when I tune in 1520kHz on the PL310 I have to raise volume to the highest level and use headphones to hear the station. Reading are: dBu: 49, dB S/N is 00.

So.. It seems that above approx 1370-to 1400 Khz dB S/N ratio stays at 00. Even with this phenomenon five of my new loggings are from w/in this frequency range.

Has anyone else performed this mod on their PL310 or PL380 and if so are you seeing the same results?

Any of you out there have an explination for what I am seeing?


Gregory Mosher


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