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I am curious as to what happens to a symmetrical dipole center-fed with balanced feeder-line. Let's assume the dipole is arranged as a sloper - that means it's not parallel to ground (horizontal). Additionally there are several buildings, trees or other conductors or in short, the dipole might be mechanically symmetric but electrically it is unbalanced. The balanced feeder-line is made of 12cm spacers and travels along a path between two surrounding buildings (each 2m distance) until it reaches the symmetrical coupler (ATU). The setup is a crowded city so this scenario should show a bad condition.

According to theory and books there shouldn't be expected any issues related to imbalance and common-mode. But is that true in reality? Let's go further and assume our balanced feeder-line is mechanically not symmetric, for example one lead is 5 or 10cm shorter/longer than the other one. It is self-explanitory that a ham radio operator wouldn't do that intentionally. Is it earrated to use a choke as shown here? (see attachment)

http://www.wolfgang-wippermann.de/balun1zu1undefklein.pdf

This is a 21 winding Reisert balun scheme (it reverses the windingopposite for lower capacitive coupling). He says that placing this choke between a symmetrical feeder line and a coupler will prevent imbalance (common mode current) through the wires. It will not prevent imbalance occured through stray emmission. Makes sense for me but I would like to hear your comments. Will this choke placed right after the output of a symmetrical coupler prevent imbalance currents, that are caused by the example mentioned?

http://www.dg0sa.de/balunatuhl.pdf

73 Nick!

electrical not symmetrical! balun for undefined impedances

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  • $\begingroup$ What's a "symmetrical coupler"? $\endgroup$ – Phil Frost - W8II Sep 19 '18 at 16:33
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    $\begingroup$ Tuner, ATU, matchbox or call it whatever you like. It is a matching / impedance transforming device. There are asymmetrical and symmetrical ones. $\endgroup$ – Nick Sep 19 '18 at 16:38
  • $\begingroup$ @PhilFrost-W8II I believe that "symmetrical" = "balanced". I have a balanced antenna "coupler" tuner that I only use for center-fed dipoles fed with balanced line. $\endgroup$ – Mike Waters♦ Sep 19 '18 at 17:53
  • $\begingroup$ I can't read German. However, if this broadband ferrite core balun has low loss combined with sufficient self-balancing characteristics, then it doesn't have the drawbacks that my legal-limit homebrew tuner does. This tuner can tune a 75m dipole on any amateur band from 80-10m to a perfect 1:1 VSWR, but it is not broadband. $\endgroup$ – Mike Waters♦ Sep 19 '18 at 18:07
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    $\begingroup$ yes, Mike is right. "symmetrical" does mean balanced. I am sorried for my bad english. However, hope you understood my question. $\endgroup$ – Nick Sep 19 '18 at 18:14
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You could look at this many ways:

Theoretical: if the system is balanced throughout, there's no need for a choke.

Practical: the antenna's surroundings, slope of the antenna, and other variations will unbalance the system somewhat, so a choke may be a good idea.

Pragmatic: a little common-mode current isn't a problem per se. Especially if the antenna is strung between buildings in a crowded city, the antenna might pick up just as much noise as the feedline, so there's little to gain by reducing common-mode current.

Lazy: just install the choke: it can do no significant harm.

Empirical: measure the common-mode current and find out.

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Any imbalance in the source, the transmission line or the load can give rise to common mode currents. I make this point because we often see only the antenna (the load) as the culprit when in fact the "balanced" tuner and the routing or construction of the transmission line can be culprits as well.

The addition of a common mode choke, such as the one depicted in the linked pdf, will generally be helpful in reducing common mode currents. However, if the differential to common mode impedance ratios become extreme, the balun may not function as intended. High SWR and high common mode current can also lead to destructive failure modes of the balun.

Contrary to the "undefinierte Impedanzen" claim in the paper, poorly matched baluns can cause non zero reflection coefficients independent of its efficacy as a common mode current suppressor.

A balun that is connected to an impedance that has a significant capacitive reactance may cause odd impedance transformations due to the formation of a parallel LC circuit.

If you are using balanced transmission line and you wish to measure the balanced and common mode currents, you can construct a simple instrument such as this:

enter image description here

The details of this instrument are discussed by the author on his website. Do exercise caution as this author has previously published articles that are, in my opinion, grossly wrong and even his referenced web page contains errors. But the meter and his descriptions for how to apply it to measure tuner balance, etc. are correct in the context of this topic.

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