Optimal Zo for Guanella balun sections

A Guanella balun may have several sections, and they may be connected in parallel on one side and series on the other side so as to achieve nominal impedance transformation ratios other than 1.

The question is often asked, what is the optimal Zo for these line sections?

Several answers exist in ham lore, but the answer is relatively simple and revealed by the most basic understanding of transmission lines.

If you do not want standing waves on a line section and its associated impedance transformation, then make sure that Zo=V/I… easy as that.

(Guanella 1944) explains it with examples:

Screenshot - 09_07_16 , 15_55_57

Note above that he refers to coil systems. He did not describe for instance (b) on a single core, a shared magnetic circuit which would be a single core system, but he states clearly two coil systems. (Sevick 2001) and lots of other hams say otherwise, but they are wrong.

So back to the issue, if the load impedance of the balun is known or there is a specific nominal load impedance, then choose Zo for each line section to be equal to V/I in that section.

Note, V/I might not be the same in some TLT configurations, one needs to analyse the configuration rather than lazily sprouting some Rule of Thumb (RoT).

In that vein, here is a semi-schematic from PA0V to entertain your analytical capability.

Clip 206

The pair of tabs to the left are driven by FET drains, the upper pink centre conductor is grounded, the lower end connecting to C1 is the output to a nominal 50R load. The network shown near OUT is for find load adjustment. There are two coax sections making this TLT, shields bonded all the way around and the centre conductors connected as shown. What is the optimal value of Zo for each the coax sections?

References

  • Duffy, O. May 2008. A review of the Guanella 4:1 balun on a shared magnetic circuit. VK1OD.net (offline).
  • Guanella, G. Sep 1944. New methods of impedance matching in radio frequency circuits. The Brown Boveri Review.
  • Sevick, J. 2001. Transmission line transformers 4th Ed. Noble Publishing Co. 9-15.