Here is another good information source on baluns and specifically the
type of material and how it performs under various conditions.
http://www.karinya.net/g3txq/chokes/
As it is said "I have no dog in this fight".
73
Bob, K4TAX
On 9/2/2018 8:28 PM, Bill Frantz wrote:
This is one of the clearest explanations I have seen of the issue. Thanks Erik.
73 Bill AE6JV
On 9/2/18 at 5:53 PM, ebasil...@cox.net (Erik Basilier) wrote:
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
-----------------------------------------------------------------------
Bill Frantz | There's nothing so clear as | Periwinkle
(408)356-8506 | a design you haven't written | 16345 Englewood Ave
www.pwpconsult.com | down. - Dean Tribble | Los Gatos, CA 95032
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