Re: BITX40 LPF mod

Arv Evans


I first came across that parallel cap and inductor trick when Dan Tayloe N7VE used it in the
BITX20A, and later the BITX17A.  It worked well there so I decided to try it with my early
version BITX40 from HFSignals.  No reason why it should not work with other transceivers
that use an LPF connected to the antenna. 

From LTSpice simulations this trick seems to work best with the inductor closest to the
antenna.  I did do several simulations with the parallel capacitor on the inductor closest
to the finals, but results were a bit discouraging.  My guess is that if the parallel LC is
on the RF PA side of the LPF then unwanted RF gets reflected back toward the finals and
does strange things.

This morning I posted more information for using an LCR series trap on the output to suck
out 2nd harmonic energy.  This too seems to work, at least in the LTSpice simulator.  Will
probably be trying this in the real circuit sometime in the next week or so. 


On Mon, Aug 6, 2018 at 11:39 PM Jerry Gaffke via Groups.Io <> wrote:
Don Cantrell ND6T documented this adjustment to the Bitx40 LPF in January of 2017:

On Mon, Aug 6, 2018 at 09:14 PM, Arv Evans wrote:
With all the filter discussion regarding uBITX I decided to take a look at design of
the output filter on the BITX-40.

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