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Antenna Wave Tilt page 1

1.1) Antenna Wave Tilt

  Classical antenna theory includes the observation that when vertical radiation leaves an antenna it has a forward slope in polarisation which is termed wave tilt. There is also a small horizontal component that results in an elliptical polarisation. The presence of wave tilt is of interest because the detailed study of it contributes to the understanding of radiation mechanism. The study also re-enforces the knowledge gained in the previous radiation measurements by demonstrating multiple radiations including some with a forward slope in polarisation similar to the angle of radiation. Performing the mathematical analysis of the data obtained in the following measurements also enables the exact points of some of these radiations from the Tx Antenna to be determined.

  Wave tilt is described in Radio Engineers Handbook by F E Terman, McGraw-Hill, 1943. Terman has a description with a graph and a formula which gives wave tilt calculated from the earths surface conductivity and the transmitted frequency and it is probably one of the most descriptive explanations presently available. The following is an excerpt from the book, page 697.

  "Wave Tilt and Penetration into the Earth of the Surface Wave - The electric vector of the wave produced at the surface of the earth by a vertical antenna possesses a slight forward tilt. As a result the electric vector has, in addition to the normal vertical component, a slight horizontal component parallel to the surface of the earth and lying in the direction of propagation. These two components generally have a phase difference, so the the resulting electric vector is elliptically polarised. The magnitude of the forward tilt of the wave is independent of the height of the transmitting antenna.

  The magnitude of the wave tilt and the eccentricity of the elliptical polarisation depend upon the earth constants, and the frequency, and can be calculated with the aid of figure 23.


[Diagram of polarisation and graph of magnitude and phase]

Fig. 23. - Characteristics of wave tilt for different values of dielectric constant k and ground-conductivity factor x.

  Wave tilt is also mentioned in Antennas Theory and Practice by Sergei A Schelkunoff, John Wiley & sons, 1952. Schelkunoff is more concerned about the effects the conductivity of the earth has on removing the tilt over distance, but the point is that he does acknowledge the presence of wave tilt. The following is an excerpt from the book, page 211.

"7.6 Wave tilt at grazing incidence

  At great distances from a vertical antenna above a perfect ground, E is normal to ground. We have seen that finite ground conductivity has the effect of reducing E considerably. In addition it causes a tilt in the E vector. The component of E tangential to ground is utilised in the design of wave antennas ---"

On page 492 he continues with the wave antenna:-

  " Whereas the performance of all other antennas is impaired by the finite conductivity of the earth, the "wave antenna" operates better over a poor ground. Over a perfectly conducting earth, the wave antenna would fail completely."

  From page 493

  "Over an imperfect earth there is always a horizontal component of the electric intensity, due to the absorption of power by the earth."

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