Common Mode Chokes and Line Isolators in EFHW Antenna Systems
This article is an attempt to clarify more aspects about common mode chokes and line isolators used mainly in EFHW antennas (End Fed Half Wave). We will try to clarify what is the common-mode current and when this current can be a “problem" in RF systems like ham-radio antennas, what is a common mode choke and the difference between ferrite-based common mode chokes or air-wound common mode chokes. Further, we will refer to Common Mode Chokes as CMC and to Line Isolators as LI. Practical aspects (how to build, optimize and measure CMC/LI) are to be considered further in this next article, mainly for EFHW antennas.
1. What is really a common mode choke?
Often reffered to as a Feedline Current Isolator, a Choke Balun, a RF Choke, a 1:1 Guanella Balun (after its inventor) or a 1:1 Current Balun, the CMC is in essence a signal conversion/isolation device that suppresses (by its resistive behaviour) the current mode currents by forcing them to be balanced at its terminals (equal in magnitude and opposite in phase), even if the connected signal source or the output load are highly unbalanced. CMC exhibits high impedance to common mode currents/signals and does not affect (significanlty) the usefull differential signals transmitted through the feedline/coax. Because it forces the current at its output terminals to be balanced, the CMC are often named 'current baluns'. Because the CMC in discussion does not perform a a matching function, it can be named a 1:1 current balun.
Potentially, the current mode currents can appear both at receive and transmit, so installing a CMC in the feedline path will significantly reduce the received noise and improves antenna's system overall performance. At this point, there is a good idea to use CMCs for the power lines that supply the RF communication equipment (SMPS power supplies, linear power supplies etc.).
2. What is the purpose of a CMC? Why do we need it?
As explained above….(to be continued)
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