I’ve been playing with the **DRA818V** modules that have been making quite a stir in the amateur radio world at the moment. I haven’t gotten one on a spectrum analyzer yet, but I have reason to believe that it will require a low pass filter to be RF legal. I’ll write more about that once I get a look at it, but figured I’d first built myself a low pass filter in case I need it (if not for these modules, but some other VHF project in the future).

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.

I’ve been playing with the

My process for building a low pass filter went as follows:

**DRA818V**modules that have been making quite a stir in the amateur radio world at the moment. I haven’t gotten one on a spectrum analyzer yet, but I have reason to believe that it will require a low pass filter to be RF legal. I’ll write more about that once I get a look at it, but figured I’d first built myself a low pass filter in case I need it (if not for these modules, but some other**VHF**project in the future).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.

Looking this filter up in a random copy of the

Looking this filter up in a random copy of the

**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**

**Read More: Designing and building a 2m low pass filter**