Noise Sources, RFI and their Suppression

With the advent of new technology in TV's, power supplies/chargers, new lighting products and other modern devices, there is an unfortunate downside from the electronic noise many of these items generate. To that end, we receive numerous emails and calls seeking a remedy to the ever increasing noise floor in our receivers.

No question that baluns are one of the answers, but they are not the sole answer and in some cases are not the answer at all. To better understand this statement it is important to understand the different sources of noise and how they can be dealt with.

First is noise generated by common mode currents which can be induced on the shield of a coax feedline if it is exposed to strong RF such as those from a nearby high power AM station. The shield is also subject to common mode if it runs close to or through electrical fields created by AC lines in your home or electrical equipment (like switching power supplies) located in close proximity to the feedline. Common mode that distorts your audio creates "mic bite" occurs when the coax feedline passes through the radiated RF near field from your antenna.  Common mode currents can also be created by a large impedance mismatch at the feedpoint of an antenna.  It should be noted this is only an issue when coax is being used and not with open wire or ladder line feedlines.

The good news is that high quality 1:1 baluns (aka feedline, isolation and or choke baluns) can usually eliminate this type of noise or RFI when installed in the feedline close to your equipment.  Note the term "high quality" which means if you think the price is a good deal, the chances are the balun is not! Additionally, having one installed at or near the feedpoint of your antenna is not only beneficial to your transmitted signal, but can also be a first line of defense in the war against noise.  This type of balun should generate high levels of choking impedance on the bands where the noise is the is most noticeable while having negligible insertion loss to any incoming signal.  

Be aware of baluns claiming extreme levels of choking impedance!  To achieve these levels they typically use cores that are not well suited for use on HF frequencies and also have an elevated high insertion loss. These baluns should be considered as suspect and avoided especially if from unknown sources.

It should be noted the air wound coaxial chokes sometimes called "ugly baluns", have very limited effectiveness and tend to have a very narrow peak of choking impedance.  Also, baluns wound on low permeability cores such as iron powder types are basically worthless when used for a 1:1 balun.  Conversely baluns wound with high permeability cores to artificially increase the measured choking impedance also run hotter when in operation.

The second type of noise comes from a differential source and is exactly the same as a received signal whether it be SSB, CW, RTTY, AM or any of the other modes.  These sources can be atmospheric in origin or worse from the dreaded faulty power line on above ground transmission poles.  This type of noise cannot be suppressed by a balun and  can usually only be minimized using DSP (digital signal processing) whether in your transceiver or external processing like the excellent line of Hear It  products from BHI.

RFI whether in your stereo, telephone, smoke detectors or your neighbors electronics require a different approach using ferrite toroids or clamp on ferrites.  Mix 31 or as some call it 31 material, is excellent when used to suppress this type of RFI and is highly recommended. QST has published a very good short article that will help you understand how radiated RF enters your electronics and how they should be suppressed.  It can be accessed using this link.

20th Mar 2017 Bob, KZ5R

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