Lecture
A dielectric antenna (Fig. 7.27) consists of a tapered dielectric rod (a section of dielectric waveguide) that radiates electromagnetic waves along the longitudinal z axis.
When designing such an antenna, a transition is used to excite the lowest-order HE11 wave in it. This wave has no cutoff frequency,
i.e., it can propagate along the dielectric rod at all frequencies and at any rod diameter. The value of the phase velocity of wave propagation in a dielectric waveguide lies between the propagation values of the phase velocity of the same wave in the dielectric of which the waveguide is made. By reducing the rod diameter, the phase velocity of the dielectric-waveguide wave is brought closer to the velocity of a TEM wave in air.
Equality of these velocities ensures the absence of reflections (complete radiation of energy). It occurs when the diameter of the antenna tip equals:
Fig. 7.27. Dielectric rod antenna: 1 – tapered dielectric rod; 2 – transition
To obtain a good radiation-pattern shape, it is important that higher-order wave types do not propagate along the rod. Analysis shows that, to this end, the maximum rod diameter must satisfy the inequality:
The length of a dielectric antenna usually does not exceed l ≤ ( 6...7 ) λ0 , with the main-lobe beamwidth of the radiation pattern being 24 – 25° and the directivity 15 – 20 dB. Dielectric antennas are most often used as feeds for lenses and reflectors, as well as radiating elements of antenna arrays.
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