The concept of creating thin planar microwave antennas from microstrip substrates is rapidly establishing itself as an important new antenna design technique, but the extraneous radiation that is emitted from the antenna/feed transition region is now recognized as a severe constraint on the achievable side-lobe level performance of these antennas. Consideration is now given to the determination of the extent of this limitation and to how the extraneous radiation loss can be reduced. Two types of microwave feeds, the coax/microstrip and the triplate/microstrip, are analyzed using a variational method, and approximate field representations are introduced to obtain estimates of the radiation associated with these feeds. The computed results for polyguide and alumina substrates are examined in the light of the limited amount of independent experimental evidence available and the usefulness of the estimations endorsed.
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