Voltage baluns are making a comeback for HF antennas discussed the application of a common Ruthroff 1:4 (voltage) balun design to a slightly unbalanced theoretical scenario.
This article applies the same analysis to two reported measured dipole cases at 3.6MHz.
G3TXQ dipole system at 3.6MHz
(Hunt 2015) reported measurements of his dipole antenna system at 3.6MHz.
The measured and transformed values above were entered into the Simsmith model. He observed “My own doublet / ladderline installation looks quite symmetrical but measurements showed that to be far from true” (a lesson there).
Above the Simsmith model of Hunt’s antenna if it were used with a Ruthroff 1:4 balun comprising 7t windings on a FT240-43 equivalent toroidal core.
Total common mode component of current is 83% of the differential component, a really bad case of common mode current if that balun was used with that antenna system.
VK1OD G5RV system @ 3.6MHz
(Duffy 2023) reported measurements of a G5RV antenna system.
Za = 26.700 + 68.100i
Zb = 23.900 + 64.500i
ZC = 59 + 167i
z1 = 18.002 + 41.924i
z2 = 10.426 + 31.757i
z3 = 52.296 + 148.891i
Above both wye and tee equivalent circuits.
Above the Simsmith model of Duffy’s antenna if it were used with a Ruthroff 1:4 balun comprising 7t windings on a FT240-43 equivalent toroidal core.
Total common mode component of current is 23% of the differential component, a bad case of common mode current if that balun was used with that antenna system.
Conclusions
Measurements of real antenna systems show that symmetry should not be assumed, and common mode current can be relatively high using a common Ruthroff 1:4 (voltage) balun design.
References
- Hunt, S. May 2015. High performance common mode chokes in Radcom
- Duffy, O. October 2023. Working a common mode scenario – VK2OMD – voltage balun solution.