Performance of Short Antennas

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Abstract
The purpose of this paper is to present experimental data on the performance of vertical antennas having a physical height of less than one-eighth wavelength. These data cover many conditions of top loading performed on a 300-foot, self-supporting, tapered vertical tower with measurements of antenna resistance and reactance from 120 to 400 kilocycles. For these conditions, field-intensity measurements were made to determine the unattenuated field intensity at one mile over a frequency range from 139 to 260 kilocycles. Field-intensity measurements along eight radials were made to determine the horizontal pattern and root-mean-square field intensity. The best results are obtained when adequate top loading is used in conjunction with a good ground system. Such top loading increases the value of the radiation-resistance component and lowers the capacitive-reactance component of the driving point impedance. Since the loss resistance remains essentially constant with various types of loading, the radiation efficiency of the antenna is materially improved by raising the value of the radiation resistance. Increasing the radiation resistance and lowering the capacitive reactance both tend to lower the effective Q of the antenna circuit. In wide-frequency-band applications a low value of Q is very important. With short antennas having a small resistance and a large capacitive reactance, extra precautions should be taken to minimize base insulator losses. With high humidity, mist, fog, or rain the input loss resistance of a short unloaded tower may increase several times over its normal dry value.

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