40/80 Band Diplexer

This page dedicated to a low and high power diplexer design and implementation.

I would start with low power diplexer.

The design is a combination of LPF and HPF circuits with LC traps for 40 and 80 meter bands to increase band isolation.

When you have LPF and HPF type Diplexer, the only BPFs will provide band isolation for other bands because Diplexer has no attenuation above and below 80 and 40 meter bands.
As an example, Diplexer has no 20M band attenuation for 40M band port (HPF) and Diplexer has no 160M band attenuation for 80M band(LPF). The only attenuation for those bands, in the example above, is from connected BPFs.
A better 5th order Diplexer can be built which has two LC traps and as a result a better band isolation. Please, see Split Diplexer as an example.

Some numbers I achieved for 200 watt diplexer:

Insertion Loss (IL) < 0.1dB.
Band Isolation > -35dB for 40M band
Band Isolation > -30dB for 3.5-3.8 MHZ (lower isolation above 3.8MHz)
Return Loss (RL) > 27dB
100 watt output power with 100% cycle
200 watt output power PEP

The triplexer board can be used to build the Diplexer leaving the middle section empty.

Please, see schematics below – a Cauer (Elliptic) type.

Please, see schematics below – a Chebyshev type.

Please, see Diplexer low power graphs below built based on a Cauer type.

The difference between filter types is not a significant to discuss but two schematics allows to chose more suitable capacitor’s values.

More complicated with more LC traps for different bands diplexer can be build for 40/80 meter bands. It is 5th order Diplexer which can be built using the low power triplexer board.

Please, see schematics below: