Re: more on PL-380 loop antennas

jim_kr1s <jkearman@...>

--- In ultralightdx@..., "nhpdxr" <nhp@...> wrote:

Haven't yet tried to track down whether there is a series capacitor between the SiLabs chip varactor pin and the loop antenna, but there is one in the SiLabs suggested circuit. If that is too low a value, presumably the capacitance created by the varactor and the capacitor in series with it will require a larger resonating inductance than recommended in the SiLabs spec sheet. However, my understanding is that the varactor's value should be automatically adjusted to compensate for the series capacitor, at least up to a point...
A series cap will reduce the available tuning range with the on-chip varactor, too. When you connect two capacitors in series, the total capacitance is less than the smaller of the two. Because the series capacitor has a constant value, the higher capacitance of the varactor, used to tune the low end of the band, is modified by a higher percentage than the minimum capacitance used to tune the high end of the band. That could explain why even your large inductor couldn't be resonated at the low end. If the series capacitor is small enough, the varactors won't have enough range to tune the inductor across the band.

Somewhere on that board is a tiny chip cap. Intuition says it ought to be 0.1 uF or greater. Wonder what they actually used? It might be possible to bridge it with a leaded cap; 0.1-uF ceramic caps are tiny. But a high-Q chip cap of the correct value is only about $1.50, though they are hard to handle, being very small.


Jim, KR1S

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