Here is the schematic for a standard 50Ω LPF for the 12-11-10m bands.
PCB Render:
Actual photo:
NanoVNA results of the PCB build:
Build Notes:
These values come from RobG (https://hackaday.io/)
Shunt C: 100pF
Series L: 12T on T37-6 core, 11T on T50-6, making about 485nH
Shunt C: 180pF
Series L: 13T on T37-6 core, 12T on T50-6, making about 580nH
Shunt C: 180pF
Series L: 12T on T37-6 core, 11T on T50-6, making about 485nH
Shunt C: 100pF
Winding wire size: 27 SWG (is non-critical)
Note: We needed to reduce 1T (from the values specified above) when using the T50-6 cores. As usual, use a DE-5000 LCR Meter (or better) to know the exact values.
We have reduced the cost of such filters by using cost-effective 1kV SMD C0G caps instead of hard-to-get and "probably-obsolete" Silver Mica caps.
Cost: Less than 500 INR per filter!
Time to build: Less than 30 minutes per filter
Tested input power: 5W to 10W. Can handle 25W to 30W it seems.
LPF for the 40-30m bands:
LPF for the 20-17-15m bands:
Note: We had to push the cutoff frequency to 25 MHz to get commercially-viable (available) high-voltage C0G caps.
References: