A home-built low-pass filter for 4m
by Tony Hawker G4CJZ
This unit uses four capacitors, only two of which are variables (Jackson C804 type), and a single tapped coil. It is built in a die-cast box, and has been tested up to 80W input power.
The coil consists of 14 turns of 12-14 SWG copper wire, on a 13mm former, and spaced at 3mm/turn.
The fixed capacitors are 68pF silver-mica types, and the C804 variables are 75pF maximum, and their ground connections are made via solder tags, bolted to the die-cast box.
Alignment is a simple process: connect the filter between a transmitter and VSWR meter, and attach a dummy load to the filter output.
Set the TX to the highest frequency, then adjust C1 and C2 for lowest VSWR.
Alternatively, connect a power meter between the filter and dummy load, and adjust C1 and C2 for maximum output power.
Simulation by Bo OZ2M
I have tried to simulate the above filter using the following values
"left to right":
- 57 nH
- 30 pF
- 102,4 nH
- 68 pF
- 151,5 nH
- 102,4 nH
- 30 pF
- 57 nH

As it can be seen the 70 MHz attenuation is 0,1 dB, but more can be expected
in a real circuit. The second harmonic is attenuated about 19 dB and third
harmonic 60 dB.
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