If the admi tta nce of a missi le, satellite, or drone-a ircraft antenna is mo ni tored a s the ve hi cle t ra vers es a n io n ized reg ion , it is possible to de te rmine t he free elect ron de ns ity and t he collision frequ ency of the region if theoretical re la t ions bet wee n t hese qua nt it ies are available. In t his pape r formu las a re developed that relate t he ad mittan ce of a n e lectricall y s hort cente r-driven d ipole or a base-dri ve n monopole when imm ersed in a cond ucting d ielect ri c to the e ffect iv e di electri c co nstant a nd conductivi ty of t he medium. F ro m wejl-known formulas re latin g these qu a nt ities to the free r lectro n density a nd the co ll ision frequ e ncy of an ion ized r egion, t hese latter ma y be d etermin ed d irectly fro m m easured ad mi ttances. The results obtain ed wh e n Lhe an tenna is t reated as a lumped capacito r a rc cons idered. It is shown t hat wh cn Lhe co ndu ctivity o f the mcd ium is in creased to a value t hat is st ill q ui t e sma ll, t he c ffect of radi a tio n on t he inpu t adm ittance becomes negli gible. The e lcct rica ll y s hort anLenna imlll ersed i n sea wate r is discussed brie Ry.

A class of protruding rocket antennas of low silhouette is analyzed. The approximate equivalent circuit of these structures is a shunt-driven transmission line terminated in reactors of arbitrary value at each end. Expressions derived for the currents in the circuit, as functions of the reactive terminations, are employed in calculating the radiation vectors of the antenna. The radiation resistance is obtained by integrating the Poynting vector over the surface of a great sphere enclosing the structure. The driving-point reactance of the antenna is determined from transmission-line formulas, which may be modified appropriately to include terminal zone effects.

A class of protruding rocket antennas of low silhouette is analyzed. The approximate equivalent circuit of these structures is a shunt-driven transmission line terminated in reactors of arbitrary value at each end. Expressions derived for the currents in the circuit, as functions of the reactive terminations, are employed in calculating the radiation vectors of the antenna. The radiation resistance is obtained by integrating the Poynting vector over the surface of a great sphere enclosing the structure. The driving-point reactance of the antenna is determined from transmission-line formulas, which may be modified appropriately to include terminal zone effects.