There are times where it is not possible, or practical to eliminate the source of RF noise.  Perhaps a recalcitrant neighbour has a plasma TV which wipes out large parts of HF spectrum with S9 noise.  This can be enough to render a HF station useless. 

There is a technique which can provide significant relief from nearby noise sources.   It involves setting up a section of wire at ground level as a ‘sense’ antenna for the noise source and a box that can mix this noise with your received signals out of phase with the noise.  The effect is that the noise can be almost completely suppressed and valid signals continue to arrive unimpeded.

There are kits and construction projects for this technology.  There are also units commercially manufactured units like the MFJ-1025 and MFJ1026.  The best way to understand this equipment is to view this short video by VK3MET on the QRM Guru website, where we can see the effect of both the source noise and the improved outcome.

Electronic Noise Cancellers are effective against power line noise, fluorescent lamps, computers and industrial processes.   The success of the technology relies upon the correct placement of the sense antenna and the scale of the noise source.  It is great where the noise emanates from a single point location, such as a faulty power supply or TV screen.  It is however less effective if the noise is arriving over a wide area, such as a long section of power lines that surrounds the home.

You can read about this equipment on the MFJ website here:

https://mfjenterprises.com/products/mfj-1026

A construction kit for noise cancelling technology by VK5TM:

https://www.vk5tm.com/homebrew/noisecancel/noisecancel.php

An article by VK3VB describes the construction and application of the VK5TM Noise Cancellation kit via the QRM.guru website:

https://qrm.guru/noise-canceller-kit-from-vk5tm/

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