Solving APRS Interference Issues with a Bandpass Filter and Coax Notches

John, DK9JC N1JJC wanted to set up an RTL-SDR APRS packet iGate. APRS stands for “Amateur Packet Reporting System”, and is a type of packet radio communications system used by Amateur Radio operators. They often use them to transmit short messages, weather sensor updates, and for vehicle tracking. An iGate allows APRS messages to be transmitted over the all world via the internet like so RF->iGate RX->Internet->iGate TX->RF.

When trying to receive the APRS packets John discovered a problem. He discovered that there was a very strong 100kW broadcast FM and 50kW DAB transmitter on a transmission tower in line of sight of his antenna. The strong signals were overloading the dongle and completely wiping out the APRS packets that he was trying to receive at 144.8 MHz.

First John tried a simple bandpass filter with 0.8 dB insertion loss and 20dB attenuation. The filter still wasn’t enough, so he went and made a several coax notch filters to take out each of the interfering signals. A coax notch filter is simply a length of coax connected via a “T” junction to the main coax cable. This creates a notch of attenuation at a frequency depending on the length of the notching coax. With these notches combined with the bandpass filter he was finally able to receive APRS packets.

A coax notch filter
A coax notch filter
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It is an effective filter, very economical and practical, it can be made with any coaxial cable, rg58, 59, 6, 174, 213, etc.
It is cut at 1/2 electrical wavelength at the frequency you want to attenuate.
Example: to attenuate 97 MHz
1.- 300/97 = 3.09 Meters (1 wave)
2.- 3.09 / 2 = 1.54 (1/2 wave)
3.- 1.54X0.66 = 1.02 meters (multiply by the speed factor of the coaxial cable, in this case 0.66. See manufacturer’s data, since it may be of another value, 0.82, 0.85, etc.)
4.- Connect one end of the coaxial in “T” as shown in the image.
5.- The other end must be short-circuited, that is, join the mesh with the central conductor.
6.- Ready, enjoy a simple attenuator! (a 1 meter long coaxial cable with one end shorted)


vote for a ready to use DAB Bandstopfilter in the range 175-230MHz from RTL-SDR.COM. The FM Broadcast can be notched with the FM Bandstopfilter.