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:
|Insertion Loss||< -0.6dB||< -0.42dB||0.18dB|
- 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.