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[Balancing and closed loop antennas]

[Balancing antennas]  [Self balancing examples]  [Antenna common mode impedance]

Balancing antennas

Antennas need to be well balanced.

Well balanced means appropriate baluns at appropriate places on the feeder and/or appropriate feeder length with appropriate termination.

The common mode impedance of the antenna defines what is appropriate.

Low antenna common mode impedance

A half wave dipole antenna has a low common mode impedance. At the feed point, where the current is maximal, a current (choke) balun can be used.

The length and termination of the feeder should result in a high common mode impedance at the antenna feed point. When using a coaxial feeder, this can be accomplished by connecting the shield to a high frequency ground at n×¼ λ from the antenna feed point (n being an odd number).

High antenna common mode impedance

The quad, delta loop and folded dipole have a high common mode impedance. At the feed point, where the current is minimal, a voltage balun can be used.

The length and termination of the feeder should result in a low common mode impedance at the antenna feed point. When using a coaxial feeder, this can be accomplished by connecting the shield to a high frequency ground at n×¼ λ from the antenna feed point (n being an even number).

Last update: March 2, 2007

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