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QHB #9 -- November 2002

 

Low Pass Filters for QRP Transmitters, by Phil DeCaire, WB7AEI


The expected power output of a class C power amplifier can be calculated from a simple equation:
Po = VCC^2 / 2*RL


3 element Butterworth filter with a cutoff of 9 MHz 

5 element design below shows a loss of only 0.4 dB with an input impedance of 37 +j21 ohms at 7.04 MHz

If you need more harmonic attenuation for a given cutoff frequency, add more elements.  
Shown below is a
7 element 0.1 dB ripple Chebyshev low pass filter for 40M

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Last Modified: January 2003