The Transverse Wavefront

When an antenna radiates an electromagnetic wave there is a spherical expansion (in general) to the expanding wavefront. Close to the transmitting antenna a receiving antenna would encounter a curved wavefront, since the sphere of expansion would grow outward to engulf the receiver.

Farther away, however, the surface of the expanding wavefront would appear essentially flat to the short relative length of the receiving antenna as shown below in Figure 4.5. If r (the radius of the propagation sphere) is sufficiently large, the small section of the spherical wave that actually

impacts the antenna may be considered as a plane. This small section of the surface of the expanding spherical wavefront is represented as a starting point (r, θ,φ) and then as the change in θ (dθ “delta theta” sometimes shown in equations as Δθ) and the change in φ (dφ “delta rho”). The “d” (or ) means “change in”. Figure 4.5 (below) shows an area on the surface of a spherical wavefront given as (r,θ,φ) and the two deltaʼs that make up the vertical and horizontal extent of the area. Refer to Appendix C if you are unfamiliar with rectangular, cylindrical, and spherical coordinate system representation of vectors.

Figure 4.5 Surface Area Defined On the Spherical Wavefront

An 802.11 wireless NIC sees the expanding spherical wavefront as if it were a flat, planar wavefront because the length of the 802.11 antenna is so short compared to the curvature of the actual spherical expansion. This view is represented below in Figure 4.6.

Figure 4.6 An 802.11 NIC Encounters a Flat, Planar Wavefront

The relationship between the perceived propagating planar wavefront and the interposed receiving antenna means that the electrical component of the wave and the magnetic component of the wave have a perpendicular relationship to each other. That is, we could think of the two aspects of wave

Copyright 2003 - Joseph Bardwell