Proprietary websites may only use components that are produced by that company, but if your filter doesn’t have extreme requirements, you can often switch to equivalent parts by any company.

The supported filter designs can be active (op-amp based), passive (RC, RL, RLC), or both. Some are stand-alone software you install on your PC and others run on a website (usually a company’s website). There are quite a few software packages and services to choose from. Luckily for hams and other experimenters, there are plenty of free or low-cost programs to try (see the sidebar). There are several filter design software packages ranging from simple calculators to sophisticated CAD programs. Sounds like a job for some filter design software, doesn’t it? Practically, you’ll need to build the filter with standard-value components as well, and that will affect filter performance too.

There are tables and equations, but they are tedious to work with. So far, so good, but a filter that doesn’t attenuate signals very much above 1.8 MHz while attenuating them significantly in the adjacent broadcast band is not a simple thing to design. The usual solution is to install a high-pass broadcast-reject filter at the receiver input, attenuating the unwanted AM signals below 1.6 MHz while passing the desired 160 meter signals with little attenuation. Antennas for those frequencies pick up a lot of AM band RF, overloading the input circuits and creating distortion or false signals inside the receiver. Hams often experience fundamental overload on the 160 meter band (1.8–2.0 MHz) which is adjacent to the AM broadcast (BC) band (550 kHz–1.7 MHz). The AM signal is completely legal but just too strong, disrupting the function of the receiver or overriding the desired programming. The receiver might be a wireless telephone, a scanner, or even a TV or radio receiver. It occurs when a receiving device is functioning entirely properly but unable to reject a strong signal. If you’ve ever lived close to an AM broadcast station, you probably experienced the phenomenon known as fundamental overload. » Skip to the Extras Once you start, it’s hard to stop!

Software rf filter design