CWAZ Low-Pass Filters

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One of the first technical things a new Ham learns is that an RF signal coming from an oscillator or amplifier is rarely just a simple sine wave. A signal at, say, 14MHz (20 meters) also contains some “energy” at 28MHz, 42MHz, and even some at 56MHz and higher multiples of 14MHz. Good engineering practice and national radio-spectrum authorities (such as the FCC) require these harmonics be suppressed below a certain level relative to the main signal (the “fundamental”) before they can be allowed on the airways.

Enter, then, the low-pass filter (LPF). The LPF attenuates the “spurious” harmonic output of a transmitter to an acceptable level. The filters MDRF now offers are based on the “CWAZ” (Chebyshev with a zero) design by Jim Tonne, WB6BLD, and popularized by Ed Wetherhold, W3NQN.

In addition to deep attenuation of harmonic frequencies, other virtues of this design include that it uses standard-value capacitors, and that it provides extra attenuation of the second harmonic without increasing the filter’s SWR within the band for which it was designed.

MDRF offers the filters for the 160, 80, 60, 40, 30, 20, 17, 15, 12, and 10 meter bands. Each can be used for power levels up to 50 Watts. The filters are available as a kit of PCB, toroids, NP0 capacitors, and enameled wire, or as a fully-assembled and tested unit (SMA connectors not included).