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

jim_kr1s <jkearman@...>

--- In ultralightdx@..., "Chris Knight" <chris@...> 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.


Jim, KR1S

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