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The page provides detailed information on coax stub filters for multi-transmitter setups, including specific lengths for different bands and types of coax. It also references additional technical notes for further analysis.

A range of easy-to-build RFI filters by John Regnault, G4SWX

Coaxial stub filters used at W7GJ

Cutting out the noise using a simple coax filter to improve the poor front end filtering

Will additional coax stubs improve Band Pass Filter performance?

Fix VHF TVI problems usign a quarter wave stub filter

When operating contest in a multi transceiver environment interferences between the 40m and 20m are guaranteed. An easy and inexpensive way to reduce interferences is to add coax stub filters

The Quarter-wave stub can be used for many purposes. If it is left with an open end it can be used as a notch filter to attenuate certain frequencies. A quarter wave length of a transmission line can also be used as an impedance transformer, to know more about the Quarter-wave impedance transformer

This project documents the construction of a coaxial 50 MHz notch filter to eliminate inter-band interference between 50 and 70 MHz transceivers. Using RG-213 coax and based on quarter-wave stubs, the filter achieved a 44 dB attenuation at 50.060 MHz while maintaining low insertion loss on 70 MHz. A dual-stub design broadened the notch response and minimized attenuation on 70 MHz to 0.2 dB. Fine-tuned using an FA-NWT network tester and Elecraft XG3 signal source, the filter effectively resolved interference for seamless dual-band operation.

The article discusses the construction of a UHF band-stop stub filter to protect an APRS receiver from potential damage during a balloon launch. The author, who communicates using a 441 MHz transmitter, needed to ensure that the RTL-SDR dongle receiving at 144 MHz wouldn't be damaged by the transmissions. The solution involved creating a quarter-wavelength open stub filter using coaxial cable, which attenuates the 441 MHz signal while allowing the 144 MHz signal to pass through. The filter's design is based on the principles of constructive and destructive interference, with careful measurement and trimming to achieve the desired frequency response. The final filter provided 34.8 dB of insertion loss at 441 MHz and minimal loss at 144 MHz, effectively protecting the receiver.