Overview

From APPLIED MICROWAVE & WIRELESS, VOL. 8, NO. 4, NOVEMBER/DECEMBER 1996

This design note describes a high performance low-pass filter that can be adapted for cutoff frequencies from 500 MHz to a few GHz. The filter is constructed from common hardware store items and only requires a soldering iron and hand saw for assembly.

The design begins with an L-C low-pass filter with element values determined by conventional techniques [1]. A 5th order Chebyshev low-pass with 0.1 dB pass band ripple and a cutoff of 800 MHz is shown in Figure 1 (4.56 pF, 13.6 nH, 7.86 pF, 13.6 nH and 4.56 pF).

Figure 1: Equivalent circuit and mechanical dimensions of the hardware store low-pass filter.

The shunt capacitors are realized as low impedance lines and the inductors are realized as high impedance lines using:

Z ₀ is the characteristic impedance of the line that replaces a lumped element. Then the impedance is low for capacitive lines and the impedance is high for inductive lines, the electrical length of the line ( ?), is short and the best performance results.

#24 AWG wire (diameter=19 mils) is used for the coaxial high impedance lines. The outer conductor is a 9/32-inch brass tube (ID=0.25 inches) from K&S Engineering, stocked by many hardware and hobby stores. The resulting line impedance is 155 ohms. A brass-plated door hinge pin with a diameter of 230 mils is cut to the desired lengths to form 5-ohm low impedance lines.