We studied a simple retractable monopole antenna for cellular portable phones using computational and experimental analysis. This antenna configuration uses an antenna element, consisting of a straight conducting wire in both the extended and retracted positions, by changing the feed point on the element. We found that the optimization of some parameters, such as the element length and the feed point, enable the input impedance of the retracted position to be adjusted to that of the extended position. This characteristic guarantees a low mismatch loss in both the extended and retracted positions. We also discuss the effect of the terminal height and the gap width between the element and the terminal. The numerical simulations and experiments using a simplified model prove that this antenna configuration has an excellent return loss characteristic above 20 dB, even in the retracted position, and that also it has a high efficiency of 85% in the extended position including the matching circuit loss. Since this antenna configuration also provides a spatially efficient and mechanically durable structure, we used it in a cellular portable phone.
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