FM Band notch filter with coax cable

Usually the case is that we are close to a radio station transmitter(for repeaters) transmitting with some kilowatts, or near an airport and this is causing problems!!! In such case you need to have a filter cutting out these bands. The filter described here is for the VHF Band (2m) and it is really easy to construct. You will need the following:

Notch filter block diagram
  • Coax cable (good quality LMR-240,400,600)
  • T connector ,bnc or n-type
  • Signal generator & spectrum analyzer so you can tune your filter to the desired frequency
  • A plastic electrical box to use it as enclosure (if you wish)
I personally wanted to create a Notch Filter for a near by Radio Station that transmits on 94.5Mhz. So after some quick calculations:

The Wavelength of 94.5Mhz is 3.17m
LMR-240 VF = 84.0

So for the 1/4λ coax stub: 1/4*3.17*84 =  66.57cm

For the 1/2λ coax section between the 2 stubs: 1/2*3.17*84 = 133.14cm = 1.33m

Personally i constructed a one stub filter which has great results with low insertion loss! Please see the plot from the Spectrum Analyzer below

2 Markers @ M1=93.0810Mhz & M2=145.100Mhz

Lets write down the results!

I was sweeping a span of 120Mhz with center frequency the one that i want to eliminate(94.5Mhz). Tx power from the Signal Genearator was at -30dBs. After i tuned the filter at 93.810Mhz (this was ok for me to test) the attenuation at this point is at -32.98dBs!!! The insertion loss(@145.100Mhz) is calculated at -1.1dB which is fair, especially if the FM radio transmitions are causing you problems and you cannot hear a thing!!! In general this simple filter attenuates the FM Broadcast band at the edges approx -20dBs which again is ok especially if you do not have 1st or 2nd order spurious. Obviously you can improve the efficiency of the filter by adding a second and maybe a third stub(open coax).

Test setup

When you finish your filter you can finally put it in an enclosure for protection and that's it.

73s

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