Antenna design principles
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The general concept, design parameters, and experimental techniques in developing dielectric-loaded antennas using flat, copper-clad, or copperplated dielectrics are discussed in Refs. 1, 2 and 3. These techniques are very useful in designing the basic antenna element for incorporation into the radome.
Keep in mind, however, that dielectric loading of a waveguide reduces its wavelength and phase velocity and also lowers the cutoff frequency. Thus, a smaller waveguide volume can transmit energy over a given frequency band, the cross-sectional area of a dielectric-loaded guide therefore is considerably smaller than the conventional air-filled guide for a given frequency.
Waveguides using solid dielectric as the propagating medium experience dielectric αd and copper αc losses in addition to the other transmission characteristics. These losses are small and sometimes can be neglected in waveguide antennas. But, at any rate, the losses are given by:
a and b = the waveguide dimensions,
ϵr = relative dielectric constant,
tan δ = loss tangent of the material,
fc = operating frequency.
Characteristics of longitudinal slots in dielectric-filled waveguides also are given in Ref. 3. Included are data on slot impedance and admittance as a function of slot length, as well as a discussion of slot scattering coefficients when the waveguide is excited in the TE10 dominant mode. The resonant length of a slot in dielectric-filled waveguide lies between half free-space wavelength and a half wavelength in the dielectric and is given by
where λ0 is the free-space wavelength.
Dielectric materials and plating techniques
The radome material first used in our experiments was a glass-cloth laminate impregnated with silicone resin. A variety of other dielectric materials also have been copper-plated and used for dielectric-loaded slot antennas. Table 1 summarizes their electrical characteristics. Each material has some significant advantages. Important selection criteria are homogeneity, electrical characteristics, and behavior in a particular environment. Results of standard tests on these materials agree closely with data quoted by suppliers. Teflon-glass laminate and some of the silicone-based materials are particularly stable and well suited for waveguide fabrication. The adherence of the plated copper was very strong, and in most cases could withstand temperatures beyond 350° F without delamination. Metal is plated on the bare dielectric surface by an electroless process using no electrodes or electrical equipment. Objects of any desired shape or size can be plated. Surfaces are smooth and uniform without excess metal on the edges. Electrolytic plating is used thereafter to obtain desired metal thickness.