TABLE OF CONTENTS A horn antenna is basically a transition from a transmission line to free space, taking into account the requisite impedance and field requirements. A brief description of transmission line theory will illustrate how this transition can take place for both coaxial and waveguide elements.
The following is an expository explanation of transmission lines in general and waveguides in particular. The explanations have been designed to be pertinent to the discussions on horns, biconical radiators, and polarizers and can be augmented by reference to many available texts (see Ref. 19).
Coaxial cable is perhaps the most useful type of RF transmission line since it exhibits a principal mode of propagation, called the transverse electric and magnetic, or TEM mode, capable of operating from DC to the high microwave frequencies. Figure 3-32 shows the construction of familiar coaxial cable, which consists of an inner conductor centered within an outer conductor, that may be braided for flexibility. A solid copper conductor is used for semirigid lines. The area between the two conductors is filled with a low-loss dielectric material, usually polyethylene or Teflon, which may be solid or made of beads. Flexible line has an additional outer polyvinyl chloride plastic protective coating, whereas semirigid line may be plated.
Energy propagates in the dielectric as the frequency increases. The E and H fields are traverse to the direction of...
