Re: BL2 -- Antenna Balun Usage

Posted by Bill Leonard N0CU on May 23, 2017; 4:58pm
URL: http://elecraft.85.s1.nabble.com/BL2-Antenna-Balun-Usage-tp7630889p7631044.html

I was going to respond to Jim’s comments about terminology, but Wes presented my thoughts better than I could have.

I would like to address two of the comments that were made in the responses.

First, EZNEC is powerful tool that can help understand theoretical behavior of ideal antenna systems. It can also, under certain conditions, predict the performance of real world antenna systems with reasonably good accuracy. However, much of the EZNEC Manual is devoted to discussing the model’s limitations. There are at least two (and probably more) that I am aware of that affect how well EZNEC can accurately predict how an antenna “system” will perform with, and without a balun.

First is the limitation with respect to ground, and the second is the inability to model unbalanced currents (ie, common mode currents) on a transmission line. Both limitations significantly affect the results for the balun problem. I am aware of two articles that use EZNEC to evaluate how a dipole’s performance is affected by the use of a balun. Both articles address these limitations the same way:
-an ideal ground is assumed to be at a known location
-the transmission line is eliminated and the source is moved up to the feed point of the dipole. The transmission line is modelled by simply adding a third wire coming off one of the antenna’s feed points.

The first assumption is a problem because none of us have an ideal ground, nor do we know how to quantify the location of our ground accurately in the model. The second one removes the component that has the most effect on the results, and tries to simulate it with a piece of wire. Although I have confidence in the behavior the model predicts, I seriously doubt that, for any real world antenna system application, whether EZNEC could be useful in deciding whether adding a balun would be worthwhile, and if so, where to install it and what the margin would be to the balun’s power dissipation limit.

I strongly disagree with Clay’s comment that the problem caused by common mode currents is nothing more than an impedance matching problem. If that were true, then all the articles, and EZNEC models that describe how a dipole that is feed directly with coax (also called a “Tripole”) can either have the pattern of a dipole, or the pattern of an inverted L antenna, depending on the common mode currents on the transmission line, are wrong. I agree that the impedance seen at the transmitter changes as the common mode current changes, however, I doubt that many, if any hams know what pattern their antenna actually has because they have no idea what magnitude of the common mode current is that is flowing on their transmission line since the predicted variation in the impedance between the two extremes is well within the 2:1 SWR range that most hams would consider normal.

Bill  N0CU