Measured relative responses toward the zenith of short-whip antennas on vehicles at high frequency

The response toward the zenith of an untuned 16.5- foot vertical whip antenna mounted on the rear comer of a jeep, relative to the response of a half-wave horizontal dipole at 15 feet above ground, was measured at high frequency with an ionospheric sounder. At 4 MHz, the relative response of the whip was -52.5 dB, and it increased with frequency at a rate greater than 12 decibels per octave (i.e., greater than frequency to the fourth power). The absolute power gain of the whip at 4 MHz was estimated at about -37.5 dB, ±6 dB relative to an isotropic radiator, and the variation with frequency was approximately a fifth-power law i.e., Gp≈ -70 + 55 log10(fMHz) dB] in the frequency range of 4 to 8 MHz. The short-path sky-wave signal received by the untuned whip can be enhanced by about ±10 dB, ±4 dB relative to that received with a vertical whip, by bending the whip away from the vehicle at an angle of 30° or more. When the same whip is bent forward and tied down over the hood of the vehicle, the received sky-wave signal is enhanced by only +3 dB, ±4 dB. These results should apply regardless of whether the vehicle is traveling in open or wooded terrain.