Low-pass filters

 

"Station WRNY is equipped with the latest device known to the world of radio engineering, called a harmonic suppressor, the effect of which is the elimination of the signals of WRNY from every frequency except that to which it has been assigned by the Government, which means, in fan language, that the station may be tuned in on only one setting of the dial." Radio News magazine, August 1925

A low-pass filter attentuates the energy above a specified cut-off frequency. These filters are used to reduce the intensity of harmonics so that they don't interefere with other signals. Most of this filters on this page were designed for various ham radio gear operating in the bands from 1.8 to 14 MHz.

Unless otherwise noted these filters have 50 or 52 ohms input and output impedance. Capacitances are expressed in picofarads, inductances in microhenries. Most authors recommend using silver-mica capacitors.

for transmit frequencies
in the range of         C1     C2                           L1 
1500-2000 kHz    1800  1800   30 turns #26 wire on T-50-2 toroid 
3500-4500 kHz     680    680    21 turns #22 wire on T-50-2 toroid 
5500-7300 kHz     470    470    14 turns #22 wire on T-50-2 toroid 
5500-7300 kHz     820    820                           2.2

approx.
cutoff freq.      C1              C2            C3           L1, L2      
1200 kHz    .0039 uF    .0056 uF   .0039 uF       6.8 

1800 kHz    .0033 uF    .0043 uF   .0033 uF       5.6 

2000 kHz    1592 pF     3184 pF    1592 pF      3.98 
7300 kHz    470 pF       1000 pF     470 pF     42 turns
                                                                         #26 wire
                                                                        on T-50-2  

cutoff  frequency
(in MHz)             30 dB attenuation
                             point (in MHz)      C1,/C4    C2,/C3    L1,/L3    L2 
2.16                           4.0                     820         2200        4.44     5.61 
4.12                           7.3                     470         1200        2.43     3.01 
7.36                         12.9                     270          680         1.38     1.70 
10.37                       15.8                     270          560         1.09     1.26 

Based on an article by Ed Wetherhold published in UK Short Wave (Dec. 1983).