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.