A very short High Power 5th Chebyshev type Bandpass filter specification:

Please, see BPF specifications and graphs here.

BAND
Insertion Loss (dB)
Return Loss (dB)
UP Band Attenuation (dB)
DOWN Band Attenuation (dB)
160 < 0.12 > 26  > 65
80 < 0.4  > 26  > 70 > 65
40 < 0.32  > 26   > 70 > 60
20 < 0.32   > 26  > 60 > 65
15 < 0.37   > 26  > 57 > 60
10 < 0.3   > 26  > 65 

I designed my BPFs to be used after amplifier with kilowatts of applied power and  this requirements defined the design criterias:

  • as low Insertion Loss as I can achieve
  • low in-band ripples
  • low VSWR in a wide band-pass
  • the minimum (not maximum) required adjacent band isolation
  • mechanical stability
  • if any capacitor failure happens, the Isolation level should be still up to protect the 2nd radio in SO2R configuration
  • build BPFs in two different enclosures for a wall or  for a “tower” (one box over another) installations

A good BPF is a balanced solution of all parameters shown above.


I am using  C coupled or L coupled resonator Chebyshev type BPFs.

  1. Capacitor coupling graph and schematics are below and you can see that this can be used for 10M band only as higher bands will have less isolation. I added HPF filter to this schematics and get a good 10M BPF.
  • All schematics below is just an example of C or L coupled BPFs and not a real implementation values.
  • If you need a real schematics I built my BPFs from, please contact me directly. 

c_10

capacitance

2. Inductor coupling graph and schematics are below and you can see that this type has an opposite reaction. All lower bands have less isolation. 

l_10

inductance

3.We need something of both and I used a combination of capacitor and inductor coupling to have graph almost symmetrical, like graph below.

mix

So, if you have more capacitive than inductive coupling your higher band isolation going down and if more inductive than capacitive than lower bands isolation going down.

The coil design is important to consider as extra self-capacitance (stray capacitance) between turns decrease higher band isolation….but increase lower band isolation


I would like to start explaining what happens if some capacitor gets broken (electrically or mechanically). The BPF I build is very good for this matter. If any capacitor failure happens the band Isolation is still high but Insertion Loss and VSWR would go up right away.

  • It means 2nd RX radio still protected and if you have a SWR protection built-in into your amplifier you will knew the problem right the next second. If you do not have such a protection it is highly recommended to use some SWR/Power meter with SWR  using PTT line protection, like this one : PowerMonitorIII_v4
  • You can buy it directly from website below :  http://ikr.ru/catalog/izmeriteli-moschnosti-i-ksv
  • The achievable BPF  adjacent band isolation could be a very high number and it sounds like “the more isolation the better” but the price for it will be a higher Insertion Loss and more difficult to get a low VSWR.

Insertion Loss is a heat which dissipated on BPF parts, such as capacitors and coils. Q-factor of coils much lower than that of capacitors that is why the most heat will be dissipated on coils. Larger wire/tube diameter and a fan cooling should be planned for a high power BPF above 1000-1500 watts of output power.

  • It is a nice idea to use a radio band decoder, like this one from RemoteQTH.com or any other with a relay output, to turn ON a required High Power BPF cooling fan. This approach will decrease a fan noise and will allow all cooling fans last longer. With SO2R setup only two BPF’s cooling fans will be running simultaneously.

It is hard to melt a copper wire to destroy the filter but with temperature goes up the coil changes its dimension if not built on a ceramic core. Yes, it is a very good idea to use ceramic cores for coils inside the BPF, but it is an expensive and hard to find part today.

I ran a few tests with an extreme hot and cold temperature to see how much BPF’s parameters would be changed. The graphs shifted left/right when temperature changed with band attenuation up or down 3-5dB. As BPF’s VSWR below 1.1 is quite wide by frequency range, it’s never been above 1.2 with an extreme temperature change tests. It is an acceptable result but a room temperature considered as a normal usage requirements.

-0.4 dB of Isolation Loss is 10% loss of applied power. For 1500 watt output power this is 150 watt dissipation inside the BPF with 100% cycle. Of course, our CW/SSB TX cycle is not 100 % . We can consider this 150 watt dissipation  an average between 65-100 watts. The -0.3 dB loss brings that level below 50 watt.

BPFs I designed can work without air cooling for 1000-1500 watts depending on a band. For a low bands, such as 40-80 meter bands, all coils have more wire turns than those for  a high bands, especially for a 10 meter band.

The longer the coil wire the easier to cool a coil wire off. If we compare 40 meter band BPF with 10 meter band BPF, the first could be used without cooling up to 1500 watt CW/SSB but with 10M small 3 turn coils this power level will make coils very hot and that is why a lower output power without fan cooling recommended.

  • BPF Impedance should be as close to 50 Ohm as possible. If BPFs used with triplexer, both have to have as close Impedance to 50 Ohm as possible. 

I built all my BPFs better than -55dB of adjacent band isolation and can see some improvements could be done for the whole set design by getting  that isolation level lower to -50 dB to improve the Triplexer with BPFs  as a whole set performance.

If BPFs connected between Amplifier and Antenna than it makes some sense to increase Isolation level if required. That is why I build my 40 and 80 meter band BPFs with higher adjacent band isolation and 10-15-20 meter bands used with triplexer with lower numbers.

  • The total band isolation of the 3000W triplexer with BPFs is better than -80 dB and this level is a sufficient level of band isolation up to 5000-6000 watts to the tribander antenna.

Military specification doorknob capacitors  have a specified nominal reactive power. All capacitors used at least two in parallel with minimum reactive power of 10 Kvar. Those capacitors should handle a power up to 4000 watt level without any problems.

Mechanical air coils stability is an important factor to consider and it is easier to achieve good mechanical stability with larger inductance coils. The minor a few millimeters changes in a large coil relatively would change the whole inductance less that in coils with 3 turns and low inductance. The ceramic core is a way to go, but I was able to get good results with a large diameter bare copper wire and a proper way to secure the coil. The factor that change inductance the most is a temperature change, but if BPF used not outside of the building than it should be no concern for BPF performance.

Please, see all detailed information about BPFs on a related webpages.