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Per the subject, what is the capacitors purpose on a balun?

Used a lot on wire wound ferrite toroidal baluns.

Balun

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  • $\begingroup$ Looks like this is not really a balun, but an unun. Most likely for feeding an end-fed antenna. $\endgroup$ – Mike Waters Sep 4 at 1:21
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Adding a capacitor to a balun like that is not standard practice and is probably a bad idea.

A bulun is supposed to be a somewhat frequency independent device which cancels out common mode current and a soon as that capacitor is added the unit becomes a tuned circuit with a limited bandwidth.

My guess is that someone added the capacitor to adjust the SWR, which might fix a matching problem but will also limit the bandwidth of whatever antenna system this is used with.

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    $\begingroup$ See my comment above. $\endgroup$ – Mike Waters Sep 4 at 1:22
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    $\begingroup$ @MikeWaters A very good point, I think my answer also applies to an unun. $\endgroup$ – Andrew Sep 4 at 13:06
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Without having the ability to measure this particular one, I expect that it's to counteract the inductive reactance introduced as a side-effect of the balun design/implementation.

Edited to add: check out this semi-related answer. In constructing his balun, he used a prototype with a variable capacitor to find the value with lowest SWR for his application, which implies a matching function.

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