Transmitter Harmonic Filters
1. Low Pass Filters for 70 and 144MHz
The following 50ohm impedance filters are straight-forward to make and do not contain any high precision components. They may be constructed in die cast boxes (approx 4 x 2 x 1 inch) with internal screening sections, tin plate boxes (approx 4 x 1 x 1 inch) or boxes made from fibre-glass PCB material. The latter two options will require removable lids retained with tapped spacers and screws or right angle brackets and self tapping screws.
The design uses a four section constant-k structure with one section being 'm' derived to provide a steeper transition into the stop band. The capacitors shown are all 10% tolerance disc ceramic (not plate ceramics) or small wire ended or concentric silver mica types and have calculated values which may be made up of two (or three) similar value capacitors in parallel to reduce the lead inductance, increase the UHF attenuation and ease the problems of obtaining the final values. For example, 20pF may be made up of two 10pF or three 6.8pF units in parallel.
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
-3dB
-50dB
-65dB Notch
-55dB stopband
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 -50dB frequency performance.
Convention disk ceramic capacitors but not miniature or plate types should be good for at least 50 watts transmitter power. Concentric feedthrough capacitors mounted in the screens would be even better.