Traditional HF ATU designs have often incorporated a 4:1 voltage balun. As more users become aware that voltage baluns are quite lossy for high VSWR loads in the region of differential voltage maxima, the choice of an appropriate transformation ratio is being revisited.
In the article Feeding multiband dipoles with open wire feeder and balun I discuss the effects of a 4:1 balun on the performance of an ATU.
The reasons often proposed for use of a 4:1 transformation are:
In the article Optimum length of ladder line , I present an analysis of typical ATU losses when used with and ideal 1:1 current balun and 400Ω transmission line operating at a range of high VSWRs. The graph above is from that article, and the model is detailed in the article.
Above is a graph of the same scenario, but using an ideal 4:1 balun between the transmission line and ATU. A practical 4:1 current balun should exhibit similar behaviour to the ideal balun used in the model. A 4:1 voltage balun will depart from significantly from the ideal where the differential voltage is high (away from the current minimum at 90° in the graph), balun losses alone could be several dB near voltage maximum, and this model would grossly understate losses.
There has been some extension in the ability to match the 400Ω transmission line operating with VSWR=80 at any line length, and some small reduction of ATU loss in the region of voltage maximum (0° and 180°), but at the expense of a large increase in loss near current minimum (90°) for high VSWR cases.
One is not always better than the other, but my choice for general purpose use would be a 1:1 current balun for better ATU loss near current maximum.
© Copyright: Owen Duffy 1995, 2017. All rights reserved. Disclaimer.