Abstract: A radio frequency transmission line filter has an insulative substrate substantially in the form of a plate of dielectric material, a ground plane formed by metallization on one face of the substrate, capacitor plates formed by areas of metallization on the other face of the substrate to form, in conjunction with the ground plane, capacitances, and coupled to the capacitances, respective metallized short circuit transmission line stubs forming inductances. The inductances and capacitances together form transmission line resonators. To allow the designer a wide choice in capacitance and/or inductance values, the substrate is recessed in selected areas to bring the ground plane to within predetermined distances of the capacitor plates or inductance stubs. The ground plane may be trimmed in the recessed areas to adjust component values.