This duplexer was made so that seperate 6m and 80m antennas could be combined so as to feed the single antenna socket of an ICOM IC-706. It is a near standard design with one bodge capacitance added to improve the 80m match.

Circuit:

The 680p capacitor is a leaded silver mica, with all others being ATC700 or equivalent chip types. Coils are self-supporting, and the whole built onto a pcb, shown below as a guide:

Performance is as below [all views, 5MHz/div horiz, 10dB/div vert]:

 a) Return loss

b) Through response, low side

c) Through response, high side

d) Isolation between 80m and 6m ports

ie, 55/60dB between bands