Transmitter Harmonic Filters
1. Low Pass Filters for 70 and 144MHz
The following 50ohm impedance filters are straight
The design uses a four section constant
The tables below show the values and measured performance for each band. Values may be scaled for other bands of interest.
Values: 






Band 
C1 pF 
C2 pF 
C3 pF 
C4 pF 
L1 
L2 
70MHz 
40 
64 
20 
80 
5t 0.375inch ID 
3.5t 0.375inch ID 
144MHz 
20 
32 
10 
40 
4t 0.25inch ID 
3t 0.25inch ID 
All inductors are wound with 18swg tinned copper wire with turns spaced approximately one half the wire diameter. ID = Inner Diameter.
Attenuation Results:
Band 
<0.2dB 
 
 
 
 
70MHz 
75 
80 
100 
103.5 
To 500MHz min 
144MHz 
150 
160 
192 
205 
To 500MHz min 
All frequencies in MHz
The notch figures are typical of a production run and are determined by the adjustment of L2.
Each filter should be adjusted for minimum passband ripple and minimum loss at the frequency of interest by small variations of the turns spacing of each inductor. L2 may be adjusted to obtain the 
Conventional 500v disk ceramic capacitors but not miniature or plate types should be good for at least 50 watts transmitter power output. Concentric feed