Advanced Wheeler cap method for measuring the antenna efficiency

In 1959, Wheeler published the theory of the radiansphere, which is the boundary between the far and near field with a distance of λ/2π from an antenna. The inside field of the radiansphere is mainly involved in the loss power and stored energy, while the outside field is concerned with the radiation power. Wheeler's theory is that the radiation power can be removed from the antenna without disturbing the loss power and stored energy when it is shielded by placing the conducting shell on the radiansphere. The conventional Wheeler cap method predicts radiation efficiency by comparing the input resistances (or conductances) in free-space and in the cap. This involves the assumption that the antenna under test (AUT) operates as a simple RLC circuit at the operating frequency. However, efficiency cannot be measured accurately by simply comparing input resistance (or conductances) when the AUT has a more complicated operating principle. To solve this problem, we applied the cascade parallel circuit to the Wheeler cap method. This approach is superior to the conventional Wheeler cap, but it can be only applied to a microstrip patch with a probe feeding structure.