Electrically-small antennas

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Abstract
Few engineers would choose to use an electrically-small antenna on purely electrical grounds, except for the important characteristic of selectivity in connection with the short notch. There are, however, many reasons for using small antennas. There is no consensus on the definition of 'electrically small': some workers adopt a maximum dimension from the feed point of λ/30 but Schelkunoff and Friis suggest λ/8. Now it is perfectly possible to achieve large instantaneous bandwidths with a top-loaded λ/8 monopole or with Josephson's open folded monopole so this seems rather too large an upper limit. The A/30 criterion was probably chosen because it permits the use of two approximations: constant current in the small loop and linear current in the short monopole or dipole. In general the characteristic expected of an electrically short antenna is low radiation resistance and large reactance; hence very small instantaneous bandwidth with respect to the impedance of normal radio equipment. The short notch has a high radiation resistance at the mouth but may nevertheless have a reasonably high Q.

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