Roll Your Own RF Filters


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Notches for AM

Another spot where a notch filter is often needed is in front of an AM receiver. It's not uncommon to have a stronger, nearly adjacent AM carrier interfere with the desired signal in an EAS receiver. What can you do about that? One look at the formula for 1/4 wavelength in inches will tell you that using coax for this type of filter is impractical. You can still roll your own notch filter for the AM band though. Instead of using coax, we can use lumped constants otherwise known as simple capacitors and inductors. A quick search on the Web reveals at least one online resonance calculator (1728.org/resfreq.htm).

Another spot where a notch filter is often needed is in front of an AM receiver. It's not uncommon to have a stronger, nearly adjacent AM carrier interfere with the desired signal in an EAS receiver. What can you do about that? One look at the formula for 1/4 wavelength in inches will tell you that using coax for this type of filter is impractical. You can still roll your own notch filter for the AM band though. Instead of using coax, we can use lumped constants otherwise known as simple capacitors and inductors. A quick search on the Web reveals at least one online resonance calculator (1728.org/resfreq.htm).

You'll connect these components together in a series fashion, and literally connect it between the two input terminals on your AM receiver. See Figure 2. Alternatively, you could wire them in parallel, and then put that circuit in series with the input of the AM receiver.

lumped constants to make a filter

Figure 2.


Put the components in a small metal project box, and connect the box itself for the ground reference of the radio receiver.

How will you tune up this filter? Connect the filter to the input of the receiver, and then tune the radio to the undesired frequency, and tune the filter for the weakest signal in to the receiver. Use your ears for this. Use care when tuning a small variable inductor like the one mentioned: you can easily break the slug in you aren't careful.


Irwin is transmission systems supervisor for Clear Channel NYC and chief engineer of WKTU, New York. Contact him at doug@dougirwin.net.


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