As Jim K9YC has pointed out in his writeup on baluns etc, there is a threshold effect when one applies a choke to reduce common mode current. The current is determined by driving voltage and total (complex) series impedance. The choke RF impedance will have a reactive as well as a resistive component, and so does the circuit before the choke is applied. The reactances may be of opposite signs and cancel out, in which case adding the choke may actually make it easier for the common mode current to flow. It is easy to think that one can add a marginal choke and look for a small improvement as a reliable indication that a good choke will be worth the investment or not. Wrong! Only then the choke tried has high enough impedance to dominate the circuit can one judge whether whether adding chokes helps. Also, adding the perfect choke near the antenna feed point is not likely completely to eliminate common mode RF current at the rig end of the cable. For common mode purposes, the perfect choke acts effectively to disconnect the outside of the coax from the antenna, but that metal is still there, close to the antenna, and it will be part of the antenna system as a parasitic antenna element, affect the radiation pattern, and carry substantial RF current. 73, Erik K7TV
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