I would like to share some of my learnings of tuning BPF after constructing 15mtr and 20mtr. This is in addition to website's published tips. This may help some one i think.
- First follow instructions of inductor tuning as per PI4CC website
- Once L value has been accurately measured inside the enclosure, install proper C value i.e. Place all tank circuits and other inductor coils on the board with shield separators
- Do not interconnect resonance tanks yet with other parts of the circuit.
- It should only be looking at series or parallel tank circuits only (else you will find multiple interacting resonance points)
- I start with Cauer filter (series resonance first stage) tuning as it is easiest to tune.
- Using L and C values provided, calculate the resonance frequency again and probe using either dip meter or VNA to check the circuit for resonance.
- Adjust this resonance to be nearest to theoretical value
- Important: try to place VNA probe in alignment direction with the coil and at some distance such as 10mm from the coil
- Now continue to other tank circuits and check resonance frequencies.
- If you see too much interaction while measuring, you may plan to place additional shield plates between coils of the sections as well ( for me this worked as it clearly showed me exact one frequency of the tank at one time)
- Now connect all interconnecting points between boards and required shorts on the boards as per entire circuit
- In the next steps, you should not try to tweak resonance tanks as they are already tuned.
- You should now see a shape of BPF on VNA or Scope (connect input of BPF to TX port and output to RX port of VNA). But probability of double hump or high IL would be there.
- These issues will go off once you start adjusting the series coils of Cheby 2, 3 and 4
- Always start with Cheby 2 coil to get the best possible shape with minimum IL and low SWR
- Then move to Checy 2 coil and cheby 3 coil in that order
- Cheby 3 coil has bit more impact on IL as per my observation (it is more sensitive)
- One may have to go back and forth in this sequence to tweak the shape
- Ensure that, all these measurements are done with lid closed (normally curves will move 200 to 400 Khz upwards depending on proximity of the lid, side wall etc)
- I could not tune these BPF for IL lower than -0.36dB and also i found that the IL shape is very sharp. So ensure that you are at mid of your intended operating frequency range. Edges of the band will have higher IL (max -0.5 dB) and SWR(max 1.4) readings.
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