4 CELLS PASSBAND FILTER FOR HF(in italiano)
I'm presenting a calculator for a four cells passband filter easy to build (see bibliography):
L = R (F2-F1) / (2.pi.F1.F2)
Cp = (F2 / F1) / (2.pi.R. (F2-F1))
Cs = (F1 + F2) / (4.pi.R.F1.F2)
F1 and F2 in MHz, F2>F1, L in Henry, Cp and Cs in Farad, R in Ohm
For example, we want to calculate a filter for 7-7.2 MHz, 40 meters band for Europe.
If we calculate the filter for an impedance of 50 Ohm, we get the following values:
L = 0,032 μH, Cp = 16370 pF, Cs = 448 pF.
As you see, the values of L (too small) and C (too high), make very impratical the construction of such filter.
It is better to calculate the filter for an higher impedance (e.g. 800 Ohm), then to transform this impedance to our requireded values.
With the impedance of the example (800 Ohm), we get the more pratical values of:
L = 0.505 uH, Cp = 1023 pF, Cs = 28 pF.
Then we can transform the 800 Ohm impedance in our example value of 50 Ohm, creating a tap at (square root of 50/800)%, or at 1/4 of the total turns of L1 and/or L4, starting from ground.
This calculation can be made either for the input or output of the filter.
The general formula for the tap position is given by the square root of the required impedance divided by main impedance %.
I have adopted as default value of impedance for the filter, 1500 Ohm (suitable for a NE602), obviously user modifiable.
Then, when necessary, the user may input the required values of input and output impedance.
You can study in deep the original article, cited and linked in bibliography,
Check your inputs: max frequency 30 MHz, hi frequency higher than lo frequency, input and output impedance less or equal to main impedance.
Andrè Ducros (F5AD), Tête HF dècamètrique à forte linèaritè. Radio REF 4, 1977 (in French)