Topband: Balun Discussions

[John Kaufmann]

Sinisa Hristov wrote:

> > Note that each time the number of turns is doubled, the reactance is multiplied by a ratio of four.
> 
> The trend stops rather quickly and it is easy
> to have too many turns on higher bands.
> 
> After parallel resonance is reached impedance drops.
> 


You can actually see the impedance go through resonance at some frequency by connecting to an impedance analyzer and sweeping the frequency.  Below resonance, the choking impedance increases with increasing frequency, and as you get near resonance it goes up very sharply.  Just on the high-frequency side of resonance, the impedance starts to drop very sharply and continues to decrease (although less sharply) with increasing frequency.

You can use the swept-frequency measurement to tune the choking impedance to maximum by adjusting the number of turns until you have maximum impedance at the frequency of interest.  Assuming you are interested in single-band operation, you can't do better than that.  For multi-band operation, you have to be a bit more careful, because you obviously can't have maximum impedance everywhere, so you choose number of turns for best "overall" choking impedance over the bands of interest.

I've used my AEA Bravo analyzer in the swept-frequency mode to optimize the number of turns for coax baluns for 160m as well as other bands.  (The AEA CIA-HF analyzer can also be used in the same way, while the MFJ analyzer can be manually swept to do this).  On the AEA's LCD display, it's very easy to see resonance.  At resonance the choking impedance is so high that it's way off scale on my analyzer, although I can clearly see the frequency where the impedance goes through maximum.  I would "guesstimate" many kilohms there.  I don't have any other instrumentation that can accurately measure RF impedance in the range of kilohms.

73, John W1FV