Bandpass filters are a must items on each station which are in SO2R category or any Multi-Operator category ( M/S, M/2, M/M). Filters help to minimize harmonics to allow a few bands simultaneous operation.

BAND
Insertion Loss (dB)
Return Loss (dB)
UP Band Attenuation (dB)
DOWN Band Attenuation (dB)
160 < 0.27 > 26  > 45
80 < 0.25  > 26  > 65 > 60
40 < 0.25  > 26   > 70  > 60
20 < 0.25   > 26  > 60 > 60
15 < 0.4   > 25  > 57 > 57
10 < 0.4   > 25   > 47 

A new boards designed to allow build filter types:

  • Cauer (Elliptic)
  • Chebyshev (W3NQN design)
  • 10M BPF (Chebyshev Type)
  • Cauer filter type is a better option compare to Chebyshev type for a higher adjacent band attenuation
  • Chebyshev filter type has a better attenuation for a nonadjacent bands compare to a Cauer type
  • Usually, for the same attenuation Cauer type BPFs have lower Insertion Loss compare to Chebyshev type

I plan to use a regular radial TDK capacitor (3kV or 6 kV) or to use custom made MICA capacitors from Charcroft. I used those Charcroft capacitors before with a success but the production lead time is up to 3-4 weeks and some other capacitor types was considered as a replacement.

Boards allow to use different capacitor types.

KF3B

Filters designed with trifilar type winding (except 15M band) to decrease the potential maximum voltage applied to capacitors  and as a result a better capacitor reliability.

As always, the goal was not to achieve the maximum possible band attenuation but to find a balance between good band isolation and low Insertion loss.

Return Loss (VSWR) should be an easy to achieve parameter as well.  -50-60dB of band attenuation is a good result which should meet a filter requirements for many different applications, especially together with Triplexer.

The filter Insertion Loss is a very important parameter to keep it low to reduce the heat dissipation and as a result a better filter reliability. Low Insertion Loss mostly defined by Q-factor of coils and a proper coil design should be made.

LP Band-pass Filter box enclosure is: 6″ X 3″ x 3″ ( 7″ long with connectors”):

15, 20, 40, 80 band BPFs are Cauer type, 10 and 160 band BPFs are Chebyshev type.

LP_BPFs

10M BPF can be built as a Cauer type for a better 15M band attenuation but drawback of that filter type is a lower attenuation of 2th harmonics from 20M band. The Chebyshev type I used has close to -90dB 20M band isolation.

  • The most efficient way to minimize the TX (transmission) harmonics is to install filter on a transmitting radio. It means if you have harmonics interference from 40M TX to 20M RX, the first thing to try is to install 40M BPF on a 40M TX radio.

160M BPF:

It is the 3rd order classic Chebyshev type band-pass filter.

You can find 160M BPF data in the article W3NQN_May_June_1998_QST but I recalculated the 160M BPF to be used with T130-2 toroid type and not T130-6 as it is in the original W3NQN design. Results are almost identical with a tiny difference for an Insertion Loss in favor of type 2 toroids.

  • Return Loss > 26dB (VSWR < 1.1)
  • Insertion Loss < -0.25dB
  • 80M Isolation > -45dB
  • 40M Isolation > -65dB

160_Chebyshev_Wide

160_Chebyshev_InBand


80M BPF:

It is the 3rd order Cauer (Elliptic) type band-pass filter.

  • Return Loss > 26dB (VSWR < 1.1)
  • Insertion Loss < -0.25dB
  • 160M Isolation > -60dB
  • 40M Isolation > -65dB (the 2nd harmonics)

80_Cauer_Wide

80_Cauer_InBand


40M BPF:

It is the 3rd order Cauer (Elliptic) type band-pass filter.

A very good 20M isolation to eliminate the 2nd harmonics interference with the 20M band.

  • Return Loss > 26dB (VSWR < 1.1)
  • Insertion Loss < -0.25dB
  • 80M Isolation > -60dB
  • 20M Isolation > -80dB (the 2nd harmonics)
  • 15M Isolation > -60dB (the 3nd harmonics)

40_Cauer_Wide

40_Cauer_InBand


20M BPF:

It is the 3rd order Cauer (Elliptic) type band-pass filter.

I already published 20M band Cauer type BPF on this page but this is a result with new PCB board design and some improved filter model.

  • Return Loss > 26dB (VSWR < 1.1)
  • Insertion Loss < -0.25dB
  • 40M Isolation > -60dB
  • 15M Isolation > -60dB
  • 10M Isolation >-60dB (the 2nd harmonics)

20_Cauer_Wide

20_Cauer_InBand

15M BPF:

It is the 3rd order Cauer (Elliptic) type band-pass filter.

  • Return Loss > 26dB (VSWR < 1.1)
  • Insertion Loss < -0.4dB
  • 20M Isolation > -57dB
  • 10M Isolation > -57dB

Cauer (Elliptic) type 15M BPF has lower 7MHz attenuation (around -42-45dB) what is typical for 3rd order Cauer type filter. The most efficient way to eliminate harmonics is to install a good filter on a transmission radio, in this case on 40M band ( Cauer 40M BPF published on this page has around -60dB of attenuation for 15M band).

  • Please, remember if 15 and 40 meter antennas installed too close to each other, a direct coupling between antennas can be an issue and no filters can help. 

15_Cauer_Wide

15_Cauer_InBand

10M BPF:

It is the 3rd order Chebyshev  type band-pass filter.

I already published more detais of 10M band BPF on this page

  • Return Loss > 23dB (VSWR < 1.15)
  • Insertion Loss < -0.4dB
  • 20M Isolation > -85dB
  • 15M Isolation > -47dB

10_Wide

10_InBand

Cauer type filters are not symmetrical and on a BPF label the recommended gear connections shown.

BPFLabels