http://www.robkalmeijer.nl/techniek/electronica/radiotechniek/hambladen/qst/1994/06/page27/index.html
AOR 7070 roofing filters 45.1 MHz: http://www.arrl.org/files/file/QEX_Next_Issue/Jul-Aug_2013/QEX_7_13_Horrabin.pdf
https://martein.home.xs4all.nl/pa3ake/hmode/index.html
Ete na, ca sa vezi! http://www.hfsignals.com/index.php/ubitx-circuit-description/
Ete na, ca sa vezi! http://www.hfsignals.com/index.php/ubitx-circuit-description/