lengths in bandpass filter

hi everyone
i have designed a interdigital filter for 1 ghz first then the response was shifted to 1.5 ghz
then i designed a filter aqt 4.45ghz it is shifted to 3.5 ghz
my doubt is whether for low frequencies m,eans 1 ghz is it shifted to right and for 4.45 ghz it is shifted to left, is there any particular reason for this?

thank you

What is the electrical length (degrees) do you use for your design? You need to consider the self loading (capacitor to ground from the rod) when you make the design. For the same loading capacitor, the frequency will move down more at high frequency (4.45 GHz) than at low frequency (1 GHz).

People normally using tuning screws to adjust the center frequency.

Hope this helps!

thank you
i used for the 1 ghz case 1.948 inches and 4.5 ghz case 0.656 inches .

0.656 inches for 4.5 GHz is too long, maybe you can use 0.4 to 0.55 inches depending on your end gap and ground plane spacing.

1.948 inches is ok for 1 GHz (59.416 degrees), but then you need to use tuning screws to tune the frequency down. Or you can use longer resonators such as 2.5 inches (76.253 degrees).

thank you ,
how to calculate that how much length i have to reduce from calculated length can u tell meplz?
when i use 0.48inches as the resonator length then by tuning will get the exacttely 4.4 to 4.5 response?

iuse the formule for length is:
l(inches)=λ/4;
isit ?
ok thank you

you cannot use lamda/4 as the resonator length for filters, especially at higher frequencies as self loading capacitor has a cinsiderable contribution to the resonant frequency.

then can you tell me the exact equation for the length of the resonator?
thank you

It depends on end loadings (end gap), ground plane spacing, resonator diameter, center frequency, and filter bandwidth. Typically, 60 degrees should be good.

If you are talking about microstrip interdigital filter (this is smallest and least expensive realization), then 0.402" is good length for such filter. I include screen shoot from quick design that shows all dimension and S21 and S11 plots. This is schematic simulation only. EM simulation will provide response shifted down about 10% in frequency. Actual PCB will be very close to EM simulation. Substrate used for this example is 20mils thick Rogers 4003 with 1oz copper.

Best regards,
RF-OM