Re: Mixed bag with the Air Core Antenna add-on to Tecsun P...
Thanks for your comments, and I'm happy that you are joining in the loopstick experimentation for the PL-310/380 DSP models.
There is of course a formula for determining the inductance of a conductor, namely
L= 0.0002 [ 2.3 log ( ---- -- 0.75)] uH
where "L" is the inductance in microhenries, "l" is the length of the conductor in cm, and d is the diameter of the conductor. This formula is for a single wire, and does not apply for ferrite-wound coils (which undoubtedly will have a more complex formula). If you wish to calculate inductance this way, I certainly admire your dedication :-) Almost every experimenter I know simply uses their LCR meter to check coil inductance.
In the PL-310/380 loopstick experimentation, my own focus has been to empirically test different combinations of ferrite cores, Litz wires, coil 'Q" and inductance to find a combination that works well with the 4734 DSP chips. As I have shared with the group, the only combination found so far that was really had superb performance on all AM frequencies (in the PL-380) was the 7.5" Amidon type 61 ferrite bar wound with an 81-turn coil of 40/44 Litz wire (at 554 uh), as documented in the PL-380 external loopstick transplant article. This configuration has the near optimum 1:3 coil-to-bar length suggested by Ben Tongue for maximum coil "Q" (in the article linked by Jim Kearman in a recent message) and has the additional advantage of being mounted outside the cabinet (found by Nick to increase coil "Q" in his PL-380 experiments). The quality of the ferrite bar mix and Litz wire apparently also make a difference in this case. Consequently, the Si4734 chip in the PL-380 seems to provide great AM sensitivity across the AM band with this particular loopstick configuration, even though the inductance of 554 uh may not be within the Silicon Labs' printed chip specifications.
A shortcut to finding optimum coil inductance during loopstick testing is to use a "Slider" coil, after an experimental loopstick has been made and connected to the PL-310 (or PL-380). The coil is slid in search of a sharp audio reception peak on weak stations, which indicates the inductance providing maximum AM sensitivity in the DSP radio. As Roy has cautioned, the tuning must be shifted back and forth to confirm this peak, and the RSSI and S/N readings must be averaged out before any conclusions are made. The big challenge now is to find a shorter loopstick configuration that will work well as an internal loopstick replacement for the PL-310/380 stock loopsticks. Like you and other tinkerers, I have ordered a serious shipment from Amidon toward this end, as well as one from Dave Schmarder. It's great that this loopstick experimentation is proving so popular-- the excitement almost seems like something contagious :-)
73 and Good Luck, Gary
In a message dated 1/2/2010 10:35:13 A.M. Pacific Standard Time, email@example.com writes: