Common-Mode Chokes Explained - W1HIS

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Common-Mode Chokes Explained - W1HIS

Common-Mode Chokes Explained - W1HIS

  System Dependencies
  Minimum PowerSDR Version:     N/A  
  Applicable Hardware:     Any transceiver  

Content provided by: Chuck, W1HIS
Reprinted with permission of the author.

A lot of RFI problems are due to common mode RF currents flowing through the antenna feed line back into the shack due to imbalances in the antenna system. Once these currents are back in the shack, they radiate out and are the source of RFI in other equipment, particularly if the other equipment is not properly grounded with a good low impedance ground at the frequency of operation. Feed line common mode issues are indicated by problems on one or two bands, problems that move around when the feed line is moved, cases where different equipment is effected each band and where the problem changes based on power level.

One solution is to use common-mode chokes to prevent currents from entering the shack. The reference to the following article by Chuck, W1HIS, Common-Mode Chokes, is a detailed technical explanation of the use and construction of common-mode chokes for RF applications.

Another benefit of using common-mode chokes is that they remove common-mode noise from the feed line reducing the QRN picked up by the receiver allows you to hear weaker stations if your QTH is in a noisy location.

Author's Note:
"When I wrote that draft, few distributors carried the ferrite components, made by Fair-Rite Products, Inc., that I suggested using in common-mode chokes; and *very* few distributors would sell these components in small quantities. Fortunately, the availability of these components has improved greatly. Now, when someone asks me where to buy these components, I just tell them to Google for the ten-digit Fair-Rite Products part number(s). If you do so, Google will find for you the web pages of several distributors that stock the components; and, if you visit these websites, you will usually find more than one distributor that sell small quantities of the part you want, via their websites, at reasonable prices and with good customer service."

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Common-Mode Chokes by W1HIS

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Last Modified:Sunday, April 18, 2010
Last Modified By: Administrator
Type: Document
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