My process for building a low pass filter went as follows:
- Select the type of filter and cutoff frequency desired
- Look up normalized coefficients in the ARRL Handbook
- Divide these coefficients by the cutoff frequency
- Convert the inductances into turns on some core and capacitors into the nearest values
- Build the filter.
Since I wanted this filter for 2m, the highest frequency I'm interested in passing is 148MHz, so I selected a cutoff frequency of 150MHz. In hind-sight, this was a poor choice, since a -3dB point only 2MHz above the band caused for a lousy insertion loss. A better choice would have been 10% higher than the top of the band, so 148MHz * 1.10 = 162MHz
I decided to build a 5 pole T configuration Chebyshev filter with 0.1dB of ripple.
ARRL Handbook (1981, but any recent one will do), it gives the component values needed for a 50 ohm filter at 1MHz. I'm also building this for 50 ohms, so all I need to convert is the frequency by dividing by 162MHz.
- L1 = 9.126uH / 162 = 56nH -- 3 turns on 1/4" air core
- L2 = 15.72uH / 162 = 97nH -- 5 turns on 1/4" air core
- L3 = 9.126uH / 162 = 56nH -- 3 turns on 1/4" air core
- C1 = 4364.7pF / 162 = 27pF -- 30pF on hand
- C2 = 4364.7pF / 162 = 27pF -- 30pF on hand
To convert the inductance values into solenoid designs, I have an old magazine article from the 70s that published a whole table of different diameters of air wound inductors.
For extra blog cred, I built the filter in an Altoids tin with SMA connectors on each side. The four solder pads were formed from a strip of copper clad with three hacksaw cuts.
Next step was hooking this up to a vector network analyzer to see how far off I ended up, which is when I realized that 150MHz was a poor corner frequency choice. Reforming the inductors and taking a turn out of L2 got me closer, but I would have ended up with better performance if I had designed it all correctly from the start.
Now once I get to the point of experimenting with keying up my DRA818V modules, I'll have a nice LPF on hand in case the harmonics do end up too high for on the air testing.