In this program you give the center frequency of the RF band and then you can look around how far the match is satisfactory and how attenuation increases while changing the circuit Q. For many narrow band cases this basic 3 element circuit is all that is needed. The program has also some use in more demanding cases - with broad bands, high impedance transform ratios, high attenuations. With it one can calculate starting values for cascaded blocks, and then put the circuit into a network analysis program for evaluating and optimizing. These are often thought as different cases, but actually are closely related. The first two can be regarded as special cases of the third.
Terminations are resistive.
When using the program the main principle is to vary the circuit Q to find out the best combination of bandwidth and out of band attenuation with the component values available. Q tunes down to the minimum value at which the match is still possible. Tuning towards high Q values the circuit begins to be more like bandpass with more or less unrealistic component values. Attenuation and also return loss are calculated for one variable frequency fx, which can also be tuned by the arrow keys. In the Butterworth design the series coil has higher inductance and the capacitors are smaller, and the best return loss is at zero frequency.