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


Gary DeBock
 

Hi Chris and Jim,
 
Thanks to both of you for your suggestions, and information.
 
Chris, after removing the PL-310 stock loopstick and discarding the Chinese Litz wire, I did try many different 40/44 Litz wire coil turn configurations (and resulting inductances) on the PL-310 stock ferrite bar, in an attempt to get a uniformly large AM sensitivity boost across the band (as opposed to the selective boost on the frequencies up to 1300 kHz, when the stock bar was wound with 113 turns for 554 uh). Your suggested inductance of 350 uh was one of the coil configurations tried here on the stock PL-310 ferrite bar (made of a mystery mix), and it did allow tuning of the entire band on the PL-310 from 530-1700 kHz, as I recall. The reason why this 350 uh coil configuration was not considered a big success was because the AM sensitivity boost over the stock PL-310 was only around 5 dBu on the low band, and almost negligible on the high band. In general, as the inductance of the experimental coil was lowered to approach the 318 uh of the stock coil, the resulting AM sensitivity boost on the low band was also reduced, and became negligible. High band sensitivity was improved, however, finally becoming slightly better than the stock bar at an inductance around 325 uh (presumably because of the higher quality 40/44 Litz wire used in the experiments, compared to the Chinese Litz wire in the PL-310).
 
Since all these coil configurations were tried on the Tecsun stock ferrite bar (made of a mystery mix), the question is now how much sensitivity improvement we can expect when the (presumably) higher quality Amidon type 61 ferrite bars are used. Both Roy and I have ordered some shorter type 61 bars from Amidon for these testing purposes (and possibly Jim has, also). Nick Hall-Patch has been running similar tests on the PL-380 with type 61 ferrite bars he already had on hand, but I'll let him report his results directly (should he choose to do so).
 
What is definitely clear now is that the 554 uh inductance found to provide maximum AM sensitivity in the PL-310 model with the 7.5" Amidon bar and 81 turns of 40/44 Litz wire is not necessarily the optimum inductance value with other experimental loopstick parameters (ferrite bar size or length, ferrite mix, coil "Q" or DSP radio model). As such, both the PL-310 experimental loopstick photo album (on Ultralightdx) and the PL-380 7.5" External Loopstick Modification article (posted on dxer.ca and Ultralightdx) have been updated to provide this information. Each new experimental loopstick must be thoroughly tested to confirm its potential to provide optimum AM sensitivity with the Si4734 chips, and coil inductance is only one of several parameters that may affect the ultimate level of AM sensitivity provided by a loopstick.
 
When the shorter Amidon type 61 ferrite bars are tested with 40/44 Litz wire in the PL-310, we should know for sure whether an in-cabinet modified loopstick can provide a sizable AM sensitivity boost in the model. Maybe that's the reason why loopstick tinkering is so fascinating-- you never really know what will happen until you try out a new configuration :-)
 
73, Gary                       
 
In a message dated 1/1/2010 4:49:09 P.M. Pacific Standard Time, jkearman@... writes:
 



--- In ultralightdx@yahoogroups.com, "Chris Knight" wrote:
>
>
> I wonder what would happen if you went from 554uH down to 350uH in this case (PL-310)? If you have already tried it, please let me know. I'm also wondering what variations of stray PCB capacitance we can expect from these Chinese manufacturers.

Good analysis, Chris. I think the length of the leads from antenna to board probably contribute 3-5 pF by themselves. It appears the maximum capacitance the chip's varactor can achieve is between 325 and 375 pF. Let's say 350 pF. Scott was saying 270 uH was about the max for the G8. For the sake of discussion and uniformity across models, let's say 270 uH and 325 pF maximum C. That combination will resonate at 537 kHz. If you minimized stray capacitance then, a 330-uH coil as supplied by Tecsun could resonate across the US MW band. The problem with Tecsun antennas at MW is probably losses in the core material vs -61, rather than inductance.

At the low end of the band where it can be resonated, there is a clear technical advantage to using a larger inductance. The reactance of the coil increases with more turns, more rapidly than does the ohmic resistance. The "Q" is calculated by dividing reactance by resistance. If reactance rises faster than resistance as inductance is increased, then a larger coil will have higher Q. But to obtain the benefit of this Q, the coil must be resonated by the tuning capacitance. (Reactance and resistance are both measured in ohms; check Wikipedia or a copy of the ARRL Handbook for Radio Amateurs at your local library for more info on the differences.)

It doesn't appear that the range of capacitance available will allow a single antenna coil to resonate on both MW and LW. A 554-uH coil could resonate down to 375 kHz or so; as you note, however, it won't resonate at the high end of the MW band. A second coil and a tiny slide switch, to select one or both coils, could do the trick.

73,

Jim, KR1S

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