Band Pass Filter Board

Updated 6 May 2024 MW

Sections of the BandPass Filter board

The bandpass filter board is in some regards might seem like a simple thing. Don’t be fooled. It is a central and extremely important part of any radio design. See it as a funnel, two directional for RX and TX. If there are any obstructions in this tunnel it will reflect significantly on the performance of both receiver and transmitter. When testing or finding one particular band, say, is not working well (low output power, or poor performance on receive), suspect the BPF first!, and re-test it. For this reason it is worth a lot of effort to get this board right, and indeed there have been quite a few prototypes and a lot of time spent in perfecting this board.

  • Up to 16 BandPass filters:
    • 1800 & 630 Meters LF Bands in bays 1 & 2
    • 160 through 10 Meters HF Bands in bays 3 – 13
    • 6, 4 & 2Meters VHF Bands (Not all countries are allowed the 4M band) in bays 14, 15 & 16
    • 8 Mtrs or any other bands can be can be added.
  • Each selected with 4 bit parallel TTL level logic (0000 – 1111 Binary) supplies to the board. This board is therefore suitable for non HamPi Radio projects.
  • Switching can be done with EITHER 16 PIN diode switches OR the option to use very small OMRON G6K relays. A choice of a small performance gain vs cost. (Note: This is under review. The latest V6 board being tested only has PIN switching. The loss of a PIN diode is insignificance in the scheme of things).
  • A LED indicator is there for each BPF bays to indicates which Band is selected. It is extremely helpful (and less frustrating) to be tuning the correct coils!
  • 14dB or 18dB Attenuator (or any other, depending on the 3 resistors fitted)
  • 18dB Pre-amplifier (remember one S unit is 6db below S9, and 10dB above S9 on HF)
  • 2 x notch filters for tuning out Local Oscillator leakage. Very important on TX.
  • 2 x Relays for selection of external LF and VHF modules.

Understanding Band Pass Filter and setup in a nutshell

Note in the picture above. There are three tuned circuit stages to each Band Pass Filter. Each stage has a transformer and capacitor that resonate together. In this case IFT2 & C3, IFT8 & C15, and IFT14 & C33.

BPF Considerations:

Here are a few things I have learnt in designing and building filters.

  • Aim to have all capacitors as low in value as possible, so as to achieve the best performance.
  • If the capacitance is too low in value, tuning DOWN to to the desired frequency will not be possible, and a higher value cap will be needed. If the capacitance is too high, performance will suffer, shown by too much loss.
  • Coupling Capacitors C9 & C27 couple the stages and also set the bandwidth of the circuit. The aim is to have the lowest overall loss, while also having a wide enough – but not too wide bandwidth. The larger the capacitance the wider the bandwidth, but the loss lower loss. The lower the capacitance the sharper the notch and narrower bandwidth, but then loss will be higher. The aim again is to cover the particular band (Say 14.000 to 14.350mHz for 20 Meters) with a relatively flat top, and to see a sharp roll-off as the signal gets out of band, while filtering out as much out-of-band interference as possible (see below).
  • Resonance C3, C15 & C33 are the capacitors in parallel with the individual transformers. If the capacitance is too high there will be a triple peak and the pass-band will be too wide. If the capacitance is too small the circuit will not tune DOWN to the desired frequency, and the ferrite slug in the transformer will be out of the top or bottom of the core.
  • With a 10mm TOKO / HamPiRadio type Ferrite, as a rule of thumb, if the ferrite slug is close to the top of the core but not protruding past the top this will be the best performance.
  • In simple terms see the capacitors as slugging, or dampening the ferrite transformer. The higher the capacitance the more dampening. A bit like putting a finger on a piano string near the end. Too much pressure and the string cannot freely vibrate.
  • See the image below. This is an example on 20 Metres and the loss here is 1.79dBm. Note that at 1mHz each side of the centre frequency the filter has achieved a 10dB attenuation, and at around 1.5mHz it is -20dBm attenuation.
  • Tune all three transformers to achieve the shape below. The centre transformer, IFT8 only changes the frequency and not the amplitude of the filter. The outer two transformers adjust the amplitude mostly, and not much change in frequency.
  • It is desirable to have a Spectrum Analyser with a tracking generator option to do this. The one below does a splendid job, but costs around NZ$2500. If you do not have one or you have no access to one (another HAM?, Radio Club?), a NANO VNA will also do the job, and are much cheaper. It can be done with a signal generator and a good oscilloscope too monitoring the RF and noting the loss, peaking the sign wave.
  • For selecting parts and tuning use the band marked ’12’, and fit sockets for easy part substitution. This band is at present unused (11mtrs) and is covered by the 10m band 13. See picture below.
1.79dB loss on 20m. A good result.
For tuning each band before soldering them in place, use the unused position 12 with sockets for testing and tuning (Note: These photos are of an older PCB version)


NumBandCoil typeC1,3,5C2,4Loss dBNote
11800(160k)500uH2nF820p3Red core 400-800uH
480(3.6)HamPi 4356560p56p2,2
560(5.36)HamPi 4356330p33p2,2
640(7.1)HamPi 4356220p22p2,4
730(10.125)HamPi 3245120p12p1,8
820(14.175)HamPi 3245100p12p2,5
917(18.1)HamPi 222456p4p72,6Toko 3335
1015(21.175)HamPi 222433p3p32,6Toko 3335
1113(24.9)HamPi 222427p3p33Toko 3335
1211 N/F

1310(28.5)HamPi 222427p4p72,7Toko 3335
146(50.2)HamPi 10244p71p1,1
154(70.25)HamPi 102410p1p2,5

Notch2 IF notch filtersHamPi 324547p

See notes below
  1. All loss figures in the following table are measured on the board including the PIN Through circuitry, which accounts for a small loss in itself (0.4dB).
  2. Band 1, 1900m. Will need a 4-800uH coil (red), found in old Medium Wave transistor radios. It was hard to get these manufactured.
  3. HAMPI 4356, 3245, 2224 & 1024 coils are custom wound 10mm transformers like the now obsolete TOKO type. If you want to self wind old TOKO type cans please contact me for turns details. The reason for going to the trouble of getting them custom wound was I couldn’t get good enough performance from ones available.
  4. The 11mtr band is unused, and works on the 10mtrband coils. For broadband receive short circuit this BPF
  5. 6, 4M bands are ‘OK, but need BW and loss improvement and to be done with open wound coils.
  6. 2M band not done yet. This will need open would coils.
  7. Note: Preamp is a supplied 12V directly from the CPU board, and the Through and Attenuator run on 5V sourced on this board – hence the different resistor values to maintain a similar current through the PIN diodes.
  8. There is an added complexity when using a 10.7mHz IF (Or other IF frequency close to a HAM band) in that the skirt of the two notch filters on this board will affect the bandpass filter for 30 meters (10.1mHz). Since this is a low power band it won’t matter on TX too much as there will be enough drive, but in receive it could add extra loss tot he filter. Tune these two notch filters exactly on the LO frequency, 10.7, or 9mHz etc. This stops any stray IF signal affecting the transmitter and also stops any stray IF being seen at the antenna and radiated on receive (and upsetting your old AM/FL radios in the house).

Other Transformers

  1. T1 (preamp) BN43-2402 2t : 8t ct 0.2mm
  2. T2 (preamp) BN43-2402 10t ct : 2t 0.2mm
  3. Other transformer ferrite options have been banded around on the internet for similar pre amps including BN61-202 (0.5dB Less NF) and BN-73-202 . I have used various ones larger than the BN43-2402 and they work fine, generally using the type 43 material. When experimenting remember to test for coverage on all the bands you want the pre-amp to work on. In general a pre-amp will not be needed below 10mHz on any reasonable antenna. On higher bands and less than ideal antennas a pre-amp can be of help when the band is not fully open, but it will add noise.

Attenuator Choice

  1. Attenuator can be changed as desired. -14db 75/50/75 R -18db 39/12/39 R. Remember each step on the S Meter is 6db up to S9 and 10dB over S9