The classic W3NQN 10M band BPF is not good….at least for me -25dB Isolation for 15M band looks like not a good number at all. In my previous modifications to Cauer (Elliptic) type BPF for this band I had -50dB for 15M band isolation, but 20M band isolation was a smaller number (-35dB only) This is specific to a Cauer type graph shape. 

  • 5B4AGN BPFs  are classic well-known W3NQN design 3rd order Chebyshev type BPFs.

I wanted to have better attenuation for 20M band as the second harmonics could be a problem, especially if tribander is a main antenna. So, the 3rd order Chebyshev capacitor-coupled BPF is the good one to meet the requirements.

Below is a short comparison with 5B4AGN BPF:

Parameters W3NQN Modified Difference
Insertion Loss   < -0.6dB   < -0.42dB    0.18dB
Reject 15         >25dB >41dB  16dB
Reject 20  >47dB    >73dB   26dB 
  • 16 dB difference is 40 times better band isolation in watts
  • 26 dB difference is 400 times better band isolation in watts


Please, see schematics below. This is 3rd order Chebyshev capacitor-coupled BPF. Many different values could be calculated, if required.



The graph below shows BPF Insertion Loss, Isolation and Return Loss.


The graph below shows BPF  In-Band Insertion Loss and VSWR.


I used T130-0 iron powder toroids as their Q-factor is higher than those of T106-0 size cores. 

If you can use a larger enclosure, air coils could be a better  choice as with their higher Q-factor you can get Insertion Loss below 0.4 dB. This is a very good number for a 10M band.

This schematics gives me -0.25 dB Insertion Loss for my High Power 10M BPF. Air coil must be made right for a maximum Q-factor.

Be aware that those LC shunt capacitors (grounded) stressed by a high current close to 3-5 A for 100 watt output power and good quality capacitors highly recommended. For the same reason, air coils are better than those made on toroids.