The Quadplexer for 4-Band antennas (10-15-20-40 meter bands) can be done inside one enclosure but there is a few drawbacks of such a design.

  • It is very difficult to achieve the maximum band Isolation because all bands are harmonics bands. The most difficult to deal with harmonics band combinations are: 20M-10M, 40M-15M and 40M-20M. It could be some coils coupling between those bands and harmonics will “leak” between bands and no extra BPFs can help with such a problem.
  • It is hard to locate all band sections inside one enclosure and the whole design in one box will be many trade offs between good parameters and limited band layout options.
  • As a result of all tight located bands in one box, band isolation will be lower, the same for Return Loss (VSWR will be a larger number) and  Insertion Loss will be not at the minimum point.

Some other way to achieve the best possible parameters was taken.  The Diplexer added to split low and high bands. Together with HF Triplexer installed after Diplexer  the Antenna input split into 10-15-20-40 meter bands.

The Diplexer schematics is more complex than that of 40-80 meter band Diplexer published here. It is not 3rd order but 5th order design with extra LC traps added to increase all band isolation.

  • The main goal of the design was a very low Insertion Loss and wide Return Loss (good VSWR) for all bands.

The simple diagram below shows the whole idea of the design.

4-band Installation Diagram

  • Diplexer split Antenna input into two frequency areas, below 10MHz and above 10MHz. Below 10MHz is where 40M BPF connected and above 10MHz is where HF Triplexer connected.
  • This approach with the added Diplexer allows to use 5-Band or 6-Band antennas.

Diplexer_10_40 001

  • Photo above shows Diplexer Work-In_Progress stage where you can see an aluminum shield installed between band ports to maximize the port isolation.

Below is the graph of the Diplexer 40M leg which is LPF (Low Pass Filter) below 10MHz. Two LC traps added to increase attenuation all upper bands.

  • Insertion Loss < -0.055dB
  • Upper Band Attenuation > – 50dB
  • VSWR < 1.1


Below is the graph of the Diplexer HPF (High Pass Filter) section above 10MHz. Two LC traps added to increase attenuation of low bands.


The important part for this Diplexer HPF section is to have minimum Insertion Loss for 20, 15 and 10 Meter bands. These are all numbers:

  • 20M Insertion Loss: < – 0.055dB
  • 15M Insertion Loss: < – 0.028dB
  • 10M Insertion Loss: < – 0.005dB

The same important requirements is to have a wide low VSWR for all 14 to 29 MHz frequency range. Please, see graphs above to see that VSWR is below 1.1 for the entire three band frequency range.

  • 40M Band Attenuation > – 45dB
  • VSWR < 1.1

Below is a graph showing band Isolations measured from both band ports with Antenna port loaded to 50 Ohm. The same if two radios connected to the Diplexer.


Please, see below the same graph but with Triplexer connected (No BPFs connected).

  • It was measured the same way as the graph above with VNA ports connected to 20 meter band port (Triplexer) and 40 meter band (Diplexer) ports with all other ports loaded to 50 Ohm.
  • You can see the same Isolation level between 20 and 40 meter bands (because Triplexer 20M leg is HPF and there is no 40M attenuation added), but 15 and 10 meter bands isolation is already below -100dB (no BPFs connected)


  • 20M HF Triplexer leg is LPF and has no extra isolation for 40M band (please, see HF Triplexer description), so all extra isolation must be delivered by 20M and 40M HP BPF. Another -67dB can be added by 20M BPF to -45dB delivered by Diplexer with a total isolation of 40M band better than -112dB.
  • The critical part is the isolation of 40M TX 2nd harmonics signal to 20M RX. 40M Diplexer leg delivers -50dB of isolation for 20M band plus at least -70-82dB (please, read 40M BPF webpage) delivered by HP 40M BPF with a total number better than -120-132dB of isolation.

Table with Isolation levels when Diplexer and Triplexer connected (Without BPFs)






20  – > -45
40 > -100 > -100 > -50

To be continued….