Three conductor transmission line loaded with a dense dielectric of its relative permittivity about 90 and shorted at its quarter wavelength makes a very small bandpass filter at about 900 MHz. A small cylindrical air cavity in the dielectric makes the propagation constants of the even and odd modes different and yields different resonance frequencies that makes the broad-band band pass filter. Various design parameters such as the diameter and the separation of the conductor lines, its length, the dielectric constants, and the coaxial line capacitors which connects the input and the output terminals are investigated numerically by calculating the transfer characteristics of the filter. The capacitance of the transmission line per unit length is calculated by using the boundry element method.
