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The question of whether a balun alter the tuning of an antenna system is often asked in online fora, and the answers offered are almost always wrong and almost never challenged, so the subject is clearly widely misunderstood.
This article explores the affect of a balun on feed point VSWR using two NEC models from the article Baluns in antenna systems.
They key characteristics of Model 1 are:
Fig 1 shows the magnitude of the conductor currents which contribute directly to radiated power. The common mode current path is the outside surface of the outer conductor of the coax, and is the vertical conductor in Fig 1.
The effect of the common mode current in the case of Model 1 is to:
Fig 2 shows the feed point VSWR curve from the model.
They key characteristics of Model 5 are:
Fig 3 shows the magnitude of the conductor currents which contribute directly to radiated power. The common mode current path is the outside surface of the outer conductor of the coax, and is the vertical conductor in Fig 3.
Note that the common mode current flowing on the coax is much less than in Fig 1. In this case, the coax contributes very little to radiation compared to the dipole legs.
Fig 4 shows the feed point VSWR curve from the model. The frequency of minimum VSWR has increased from 6.6MHz to 6.9MHz, some 5% higher.
Not only has the frequency of minimum VSWR changed, the impedance has changed. In this case, the dipole with no balun has better VSWR, but at the expense of the other effects of common mode current mentioned earlier.
Baluns may be quite imperfect at the upper or lower ends of their specified frequency range. This often takes the form of nonideal impedance transformation, not just in terms of the nominal ratio, but adding series and shunt reactance. This is a common problem with voltage baluns on the lowest bands, and also of voltage baluns on extreme loads at any frequency. This nonideal impedance transformation will affect feed point impedance, VSWR, and the frequency of minimum VSWR.
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