Band-pass filters are items to have on each station which is 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)
UPPER Band  (dB)
DOWN Band (dB)
160 < 0.27 > 26  > 45
80 < 0.25  > 26  > 65 > 56
40 < 0.25  > 26   > 70  > 60
20 < 0.25   > 26  > 60 > 60
15 < 0.4   > 25  > 57 > 57
10 < 0.4   > 25   > 45
  • 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

Table below shows the difference between popular W3NQN and Cauer type BPFs for adjacent bands (minimum values)

BAND
W3NQN (dB)
Cauer (dB)
Difference (dB)
80 > – 42 > – 56 14
40 > – 46 > – 60 14 
20 > – 37 > – 60 23
15 > – 34 > – 57 23

Band-pass filter boards is 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.

BPF boards are to fit into Hammond 1411MU enclosure. Hammond make it with only four holes for the box cover (two on each side). Three more added on each side of the box looks much better.

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 > -56dB
  • 40M Isolation > -65dB (the 2nd harmonics)
  • 20M Isolation > -55dB

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 > -84dB
  • 15M Isolation > -45dB

200W_10

200W_10_InBand


 

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

BPFLabels