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Can a balun alter the tuning of an antenna system

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.

Analysis

Model 1 (no balun)

They key characteristics of Model 1 are:

Fig 1:

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:

  1. distort the pattern from that of the dipole alone, possibly reducing antenna gain;
  2. result in higher field intensity immediately adjacent to the feed line, and increased risk of Electro Magnetic Compatibility issues (ie interference to other electronic equipment in the premises and in neighboring properties);
  3. couple more noise and other unwanted emissions from power wiring and equipment in close proximity to the feed line to the receiver; and
  4. to cause very high charge at the transmitter (ie very high voltage wrt ground).
Fig 2:

Fig 2 shows the feed point VSWR curve from the model.

Model 5 (with balun)

They key characteristics of Model 5 are:

Fig 3:

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:

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.

Other effects

Baluns may be quite imperfect at the upper or lower ends of their specified frequency range. This often takes the form of non-ideal 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 non-ideal impedance transformation will affect feed point impedance, VSWR, and the frequency of minimum VSWR.

Conclusions

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