Microstrip filters - dimensions

Hi, i'm a beginner on microstrip design and i was trying to design a microstrip filter. Using some equations, i almost did it, but i don't know about the "lenght" that i must use between the filter itself and his feedin' ports! i searched it in a lot of books, and nothing. Some help? Thanks.

Design your feedlines for the correct width = correct impedance and the length does not matter.

Microstrip use TEM mode of microwave, so increase the length between filter and input port, only increase the attenuation of the system. You can simulate this microstrip length, and minus its attenuation from the test results, that should be the S21 of your filter.

If your filter output is matched to 50 ohms and you place a 50 Ohm line at the end of it there will not be any transformations, only some loss depending on the length. However if your matching is not accurate the line will slightly change the impedance.

Only the width of the feedline matters which should be matched to 50 ohm impedance. Length of the feedline plays a minor role when you are using a dielectric substrate with very low loss tangent.

For example, for Rogers 6010, loss tangent is 0.0023 compared to the loss tangent of 0.02for FR4. And the loss introduced by the length of the feedline depends on the frequency of operation.

For simulation purposes, make the loss tangent equal to 0 and observe the frequency reponse and then simulate with the actual loss tangent and compare the difference in the frequency reponse.