Re: Mixed bag with the Air Core Antenna add-on to Tecsun PL310
Chris Knight <chris@...>
I found a website which has simplified resonant frequency formulas manipulated to easily figure out capacitor and inductor values: http://www.electronics-tutorials.com/basics/resonance.htm . In an earlier email response to John B. (dated 12/20) you mentioned a break-even point (1340 kHz.) where below that there was noticeable improvement over the stock coil. Since the coil you used has an inductance of 554uH, I placed 1.34M and 554uH into the formula to solve for capacitance and came up with 25pF (at 1340 kHz). Since we now know the si4734 cannot provide less than 7pF of capacitance, I subtracted 25pF-7pF = 18pF. This is the stray capacitance present in your PL-310. In the si4734 datasheet there’s a note saying “Stray capacitance on antenna and board must be < 10 pF to achieve full tuning range at higher inductance levels.” That would imply these ULR circuit boards are being manufactured with higher-than-optimum stray capacitance. Next, I used 1.7MHz which is where you should need 25pF so the si4734 can tune through the high end of the band. The result was 350uH inductance. Plugging 350uH at .54 MHz into the formula, I got 248pF. Assuming the si4734 can provide 248pF on its internal varactor, you should be able to tune the entire AM BCB with a fixed coil inductance of 350uH and maybe have enough left over to cover a nice chunk of the longwave band.
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.
From: ultralightdx@... [mailto:ultralightdx@...] On Behalf Of D1028Gary@...
Sent: Friday, January 01, 2010 12:56 PM
Subject: Re: [ultralightdx] Mixed bag with the Air Core Antenna add-on to Tecsun PL310
>>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.<<