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Re: Question about LPF mod, adapting BITX40 to 20m #bitx40help

 

Scott

You can do pretty much what you suggest, as long as the output impedance
matches that of your antenna.? Insertion loss in the simulation will tell you if
things don't match up.

There is an interesting opportunity with this type of LPF.? While playing with
the simulation, try adding a capacitor in parallel with L8 and/or L9 to cause
a dip in 2nd harmonic energy.? Usually you do not have to change anything
with the existing LPF.? Simulation runs will tell you when you get it right.

Arv
_._


On Mon, Dec 4, 2017 at 5:07 PM, Xcott Craver <xcottcraver@...> wrote:
Hello,

I just ordered a BITX40 with the goal of converting it to 20m, based on the amazing wealth of info on this group.? I ordered a 20m BPF kit and LPF kit from QRP labs, and while waiting for delivery I simulated the new LPF in KE7ER's LTSPICE model for the PA.

But here comes the possibly dumb question:? futzing with the LTSPICE model, I checked what would happen if instead of installing a new LPF I just altered the BITX40's LPF windings from 17 turns to 11 (from 1.1uh to 436nh), leaving the capacitances as they are on the board.? This appears to admit the 20m band, and the Smith chart tells me it's a match for 50 ohms at 14.2MHz.

Would it be a bad idea to simply perform surgery on the existing windings as I've described?? Certainly it's not optimally flat over 0-14MHz, but it appears to work over the band, and limits the higher frequencies.? Is there some big sacrifice I am missing?

I've attached a gif of the modeling for the PA with QPR Labs filter (top) and my uncouth reduction of the windings (bottom).

[Side note:? while I was writing this post I went to the door and found a package from DHL sitting there.? Wow!? So much for the simulation phase!]

Thanks Scott K2CAJ
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