Figure 3-4. Horizontal Antenna Mounted for Short-Range Sky Wave.

TM 11-5985-379-14&P

Horizontal Antenna Mounted for Short-Range Sky Wave.

Figure 3-4.

the direction off the dipole or wire ends. Due to the

The required antenna gain is computed to provide

p r o x i m i t y of the antenna to ground, this latter

a given field strength at a receiver site from the

mode radiates like a lossy transmission line and

t o t a l propagation path loss, the operating fre-

i t s efficiency is generally poorer than a whip.

quency, and the transmitter power. The total path

loss is found to be 101 dB (NBS Report 72491).

T h e shape of the radiation pattern of the hori-

The power delivered to the antenna by the AN/

zontal dipole is essentially constant for heights

PRC-47 is assumed to be 25 watts, taking into

n o t exceeding one-quarter wavelength, The dir-

account coupler losses. The equation for computing

ective gain of these horizontal antennas is 7 dB

the required antenna gain is shown in figure 3-9.

above an isotropic. Half-power beamwidths of the

T h e gain is 2 dB above an isotropic radiator.

v e r t i c a l l y radiated lobe are 80 degrees in the

plane of the dipole and 100 degrees in the plane

T h e required effective height of the antenna is

normal to the dipole axis.

found by considering the following.

For a fixed height above ground, the amount of the

W h e n a horizontal antenna is close to ground,

input power radiated proportionately in each of

e n e r g y is radiated in two modes. The desired

these modes is a function of the relative percentage

dipole mode produces radiation with a maximum

of the antenna input resistance characterizing each

in the vertical direction. The undesirable Bever-

mode. Each of these, in turn, is a function of the

age mode creates a vertical electric field between

height above ground. Figure 3-10 shows the total

t h e conductor and ground, producing vertically

input resistance and that portion due to the dipole

polarized ground-wave signal with a maximum in

mode as the dipole height is varied. As the height

Haydon, G. W., Lucas, D. L., and Harrison, R. A., "Technical Considerations in the Selection of

Optimum Frequencies for High Frequency Skywave Communication Services," p. 45, NBS Report 7249,

U. S. Dept. of Commerce, Boulder, Colorado.

3-5