CoaxNotchFilter
Introduction
Coax notch filter is a bandstop filter that can be made of segments of coaxial cable soldered together and ended by two coaxial connectors, or could be made by joining coaxial segments that have connectors soldered using a T adapter. Building them is surprisingly easy and getting all needed parts is easy too.
You need custom band stop filters to practically receive anything with RTL SDR dongle because it gets overloaded with many strong signals in your neighborhood. For me nearby signals at 144.800 MHz can be received with no problem but the 420-450 MHz band is unusable. Checkout my more detailed overview of problems while receiving weak to medium strength signals using SDR here: RF_stuff
First experiment
notch filter was tested for cca 66MHz frequency (random only for test). Odd multiples of frequencies have been attenuated. (1x66MHz 3x66MHz 5x66MHz...)
First rtl sdr's antenna connector was left disconnected. sample rate was set to 2048ksps. Gain was set to 0dB, so both amplifiers in rtl sdr should not amplify the signal at all. Signal from noise generator is strong enough. 60dB noise floor was observerd on spectral plot with radio software. Noise generator was connected to 30dB attenuator signal to protect rtl sdr dongle input to be destroyed by large voltage. Attenuator output wss first connected directly to the antenna input of rtl sdr. -40dB floor was observed with noise generator powered on. this is 20dB more noise reception on frequencies from 40MHz up to cca 400MHz tested.
At this point we can add filters and observe which bands are filtered.
After adding T adapter between attenuator and rtl sdr, with additional 77 cm long stub connected to adapters third port we have observed a dip in noise strength at cca 66MHz. The signal dropped back to -60dB at the bottom of the dip. The dip reapperared at odd frequencies. cca 198 MHz, 330 MHz.
References:
[1] This tells how to calculate dimensions of coaxial segments.
[2] Various coax filters and their measurement. The article suggests that you can use these stubs to remove unwanted harmonic frequencies from your transmitter that can interfere with other radio equipment.
[3] Story of success with receiving APRS traffic near strong DAB transmitter.
[4] Nice view of attenuation chart of coax notch filter in the end of video.
