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Using Metal in Radomes

It is generally thought that metal should not be placed in front of an antenna aperture because metal will block RF energy. If this is the case, why then is it acceptable to use metal beams in some types of radomes and metallic fasteners in other types of radomes? Wouldn't it be better to use fiberglass of some other dielectric in front of the antenna so that signal blockage is minimized? Surprisingly, the use of metal in some parts of the radome results in better overall performance.

In considering a space frame radome, which can use either metal or dielectric beams as the load-carrying portion of the structure, it is readily noted that the member size for plastic or fiberglass beams will be larger. This is due to the fact that the mechanical properties of aluminum exceed those of materials such as fiberglass (i.e., the elastic modulus is 10 x 10 6 for aluminum versus 3 x 10 6 for fiberglass). The Induced Field Ratios (IFR's) for dielectric and metal beams of the same size are comparable. However, dielectric space frame radomes require larger beam cross-sections for equal structural strength than for metal beams resulting in higher transmission losses for dielectric space frame radomes.

In a dielectric space frame radome, microwave radiation passes through the dielectric space frame beams, but with a different phase than for the radiation passing through the thinner window region of the radome panel. This phase difference between the signals passing through the dielectric beams and the signals passing through the window region between the beams causes them to combine in phase or out of phase at different frequencies. This frequency dependent phasing results in the rapid fluctuations in transmission loss over frequency seen in dielectric space frame radomes (see graph).

By contrast, in metal space frame radomes the transmission characteristics are well behaved over wide frequency bands because the metal beams optically block incident microwave radiation preventing the frequency dependent phasing that occurs in dielectric space frame radomes.

These same phase interference effects occur in sandwich radomes, which use non-metallic fasteners. ESSCO uses metallic fasteners in most sandwich radomes to achieve the highest level of structural and electromagnetic performance.

The graph below compares predicted transmission loss for typical space frame radomes using dielectric and metal beams. As can be seen, for most cases, the transmission efficiency is better for the metal space frame radome.

Dielectric Space Frame vs. Metal Space Frame
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