*This article describes a Low Pass Filter by Rick NM3G in the Ultimate3S kit. Capacitor values are the same as those in the 2m LPF design (22pF at the ends, 43pF in the middle - by parallel 33pF and 10pF). *

*NOTE: this 222MHz LPF design is now available in the shop. It is supplied as "2m" and the coils are wound a little shorter. *

## Filter coil construction

Measure three pieces of enameled wire (from 6 meter filter kit) 1.43" long. Strip and tin ends 1/4", wind two turns around a 0.15" (5/16") drill bit.

Note, to strip wire, apply some solder to your soldering iron tip, place supported enameled wire in this melted solder, and add a bit more solder to heat the enameled insulation and float it away from the surface. Additional flux MAY be needed. Remove excess solder from wire and soldering iron tip and solder coil onto board. After installation, reshape coil into a roughly solenoid shape … don't worry about absolute shape, at 222 MHz, the wire length determines the inductance much more than the number of turns or the coil diameter/length.

I ran the first approximation by leaving the caps in the model and let the computer do the heavy lifting.

With both the 2 meter and 222 beacons, the output is approximately -1.7 dBm (or about 0.67 mW). I suspect this is because the filter is designed as a 50 ohm input/50 ohm output, and the pair of transistors is not 50 ohms at either frequency.

By the way, because of the frequencies involved, the coil shape (diameter, length, number of turns) really doesn't change the filter tuning much at all ... it's largely a function of wire length. I made the coils look like traditional coils because people expect that!

## Test results

50 Ω test results

221.5 MHz (fo), 1.44 dB insertion loss

443.5 MHz (2Fo), 43.55 dB insertion loss

665.5 MHz (3Fo), 49.99 dB insertion loss

Beacon transmitter tests

221.5 MHz, -1.74 dBm output

443.5 MHz, -57.6 dBm output

666.5 MHz, -25.32 dBm output