Hi all, by way of a Christmas present to the group, I have attached a
diagram of an L-Pi LPF design. The idea is that the filter offers both low
pass filtering and also will match a PA output impedance (quite often in the
4 to 10 ohms range) to 50 ohms as well. This approach can avoid one
transformer and one more variable in the tune-up process. If anyone wants
to play the design is done using the disc in Wes Haywards "Intro to RF
Design" with the program ZMAT
I have used it in a successful Class E stage lash-up, so it does work. It is
suggested that air cored coils and pulse rated capacitors are used. I used
ordinary 400v caps and iron-dust cores and they did get warm at 150 watts.
Also a Smith Chart simulation confirms that a few switched caps on the
output (C2) could help to tweak it for best match to an aerial that is not
exactly 50 ohms.
Happy Chrismas de Alan G3NYK
[email protected]
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