As ARRL Antenna Handbook says:
” The common-mode filter or choke may be one of the best-kept secrets… “
As this picture shows, common mode filters/chokes are widely used in industry. Almost every power supply and modern appliance has one. But we are here talking about the one we need in our HAM radio applications. Do we need one and why?
Apart usual reason, to stop your own radiated power coming down the shield of your coax into your equipment, I personally prefer use of Common Mode Choke as Noise suppressor. Ever since I read an article from W1HIS on chokes my main goal with them is to make my station reception clean. For sure it worked wonders at my contest station as well as at many others I helped with my products.
It is very much known about these properties of CMCs but HAMs rarely use them for these purposes. To give you complete picture just read a quote from W1HIS paper:
In the HF hamshacks that I’ve visited, the background noise level heard on most HF bands (especially the low bands) could be reduced by more than an S-unit by means of common-mode choking. In some cases (which I could name but won’t, to avoid embarrassing my friends) I was able to reduce the received noise level by four or five S-units. I reduced my own received noise level on the low bands by even more. For years I’ve been a regular participant in CW nets on the 80- and 40-m ham bands, and in SSB nets on MARS frequencies near these bands. I hear better than anyone else in these nets. I am often the only participant able to copy every one of the dozen or two dozen stations in a net. Why can I hear so exceptionally well? My receiver is nothing special, nor is my antenna; it’s a wire just 23 to 40 feet high. Nor is my QTH very quiet. I have a small lot in a dense suburb, just two miles from Cambridge and three miles from Boston. I hear exceptionally well because I have good common-mode chokes in my antenna feedline, in the other cables connected to my radio, and also in the cables connected to the worst of the QRN sources in my house.
Well, I and thousands of other HAMs seen an improvement of their reception by using a Common Mode Filter / Choke in their feedlines.
73 Danny E73M / N4EXA