August 30, 2016 at 10:34 am #1132
Ch. Zeshan YousafParticipant
Description of the APPT antenna:
The APPT type presents a regulated current-distribution antenna, possessing anti-fading radiation patterns over a wide wave range.
The antenna presents a mast, insulated in its base. Along the mast body, beginning from the base up to the certain height, vertical screen wires are stretched. The wires from the bottom parts of the antenna, and in conjunction with the mast, the section of the concentric vertical feeder, over which power is supplied to the antenna at a height, equal to that of the screen.
If one conceives the screen as an equivalent solid cylinder, the screen current from the feed point would flow out on its external surface (bottom part of the antenna), and the mast current would run along the antenna top part, formed by the mast body and the inclined wires.
Such a method of the antenna feed makes it possible to ensure cophasal drive of the top and bottom parts of the antenna in that zone of operating frequency range, where the antenna height is greater than half a wavelength.
Owing to the cophasal drive of the whole antenna, its radiated power is concentrated along the ground.
However, the power concentration along the ground does not exclude radiation at wide angles to the horizon.
For the purpose of optimum current distribution over the antenna bottom part, when undesirable radiations are suppressed effectively, the antenna bottom screen is insulated against the ground and connected to the current regulation stub.
The current regulation stub is formed with screen wires of the tuning and supply feeders and the ground. The stub length is assumed equal to 210 m.
To improve matching of the antenna to the feeder, as well as reducing the potentials on the antenna and the tuning elements, the surge impedance of the antenna top part is lowered owing to the suspension of inclined wires.
The antenna construction permits, depending on the assigned waves, to choose the screen height, equal to 0.33 H or 0.4 H (H = the antenna height)
To obtain better radiation patterns, the screen height over a wave range of 240 to 600 m is chosen equal to 0.33 H, and on waves shorter than 240 m is 0.4 H.
The APPT antenna can be tuned to the optimum anti-fading radiation pattern conditions or to the condition of maximum radiation along the ground.
Utilization of the antenna under the condition of maximum radiation along the ground is rational at the day hours, since the maximum value of the ground signal is ensured at that time.
However, on the operating wave, assigned for the Broadcasting Station, the difference in gain for both conditions is insignificant. Therefore, it is recommended, that the antenna should be used under the condition of optimum anti-fading radiation patterns both at the dark hours and at the day-time.
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