SELF

32

S.B. Karavashkin and O.N. Karavashkina

While in the near field, the wave front is not a circumference, because the distance to the receiver is comparable with the dipole size, and the field strength at the point of measurement is formed by two charges. This is well seen in Fig. 17 by the oblate shape of the first maximum of wave front. In the near field the wave front is rather an oval slowly transforming into a circle in the far of dipole. So, should we try to measure the real pattern in the near field, we would have, moving along the oval, to vary not only the azimuth of measuring dipole but also its distance from the source. The situation shown in Fig. 18 follows from this.

Fig. 18. The construction showing the distortion of measurement with the measuring dipole moving along the arc of circumference in the near field

We see from this construction that when the dipole moved exactly along the circumference, it captures other equiphase lines of field; due to this, there occurs the inversion of the field strength on the periphery of the radiation region. This just causes the feature that the inversion is present in the vector diagram but absent in the amplitude diagram. By the conditions of plotting the vector diagram, we moved the measurement dipole exactly along the circumference, whilst in plotting the amplitude diagram we retained only the orthogonality between the axis of measurement dipole and radius-vector. So the amplitude diagram showed the real position of wave fronts in the dipole measurement, since it showed only the field strength under the given orientation of measuring dipole at each point of the studies region. On the other hand, should we, when plotting the amplitude diagram, give exactly the trajectories of measurement, along these trajectories we would get the inversions, similar to those which are present in the vector diagram.