Feedlines and Antenna Systems
vertical antennas - types, dimensions, characteristics
How do you calculate the length in metres (feet) of a quarter-wavelength vertical antenna?
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Divide 468 (1532) by the antenna's operating frequency (in MHz)
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Divide 300 (982) by the antenna's operating frequency (in MHz)
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Correct AnswerDivide 71.5 (234) by the antenna's operating frequency (in MHz)
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Divide 150 (491) by the antenna's operating frequency (in MHz)
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If you made a quarter-wavelength vertical antenna for 21.125 MHz, how long would it be?
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3.6 metres (11.8 ft)
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Correct Answer3.36 metres (11.0 ft)
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7.2 metres (23.6 ft)
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6.76 metres (22.2 ft)
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If you made a half-wavelength vertical antenna for 223 MHz, how long would it be?
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Correct Answer64 cm (25.2 in)
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128 cm (50.4 in)
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105 cm (41.3 in)
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134.6 cm (53 in)
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Why is a 5/8-wavelength vertical antenna better than a 1/4-wavelength vertical antenna for VHF or UHF mobile operations?
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A 5/8-wavelength antenna has less corona loss
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Correct AnswerA 5/8-wavelength antenna has more gain
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A 5/8-wavelength antenna is easier to install on a car
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A 5/8-wavelength antenna can handle more power
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If a magnetic-base whip antenna is placed on the roof of a car, in what direction does it send out radio energy?
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Most of it is aimed high into the sky
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Most of it goes equally in two opposite directions
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Correct AnswerIt goes out equally well in all horizontal directions
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Most of it goes in one direction
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What is an advantage of downward sloping radials on a ground plane antenna?
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It increases the radiation angle
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It brings the feed point impedance closer to 300 ohms
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Correct AnswerIt brings the feed point impedance closer to 50 ohms
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It lowers the radiation angle
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What happens to the feed point impedance of a ground-plane antenna when its radials are changed from horizontal to downward-sloping?
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Correct AnswerIt increases
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It decreases
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It stays the same
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It approaches zero
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Which of the following transmission lines will give the best match to the base of a quarter-wave ground-plane antenna?
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300 ohms balanced feed line
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75 ohms balanced feed line
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300 ohms coaxial cable
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Correct Answer50 ohms coaxial cable
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The main characteristic of a vertical antenna is that it will:
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Correct Answerreceive signals equally well from all compass points around it
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be very sensitive to signals coming from horizontal antennas
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require few insulators
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be easy to feed with TV ribbon feeder
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Why is a loading coil often used with an HF mobile vertical antenna?
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Correct AnswerTo tune out capacitive reactance
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To lower the losses
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To lower the Q
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To improve reception
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What is the main reason why so many VHF base and mobile antennas are 5/8 of a wavelength?
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The angle of radiation is high giving excellent local coverage
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Correct AnswerThe angle of radiation is low
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It is easy to match the antenna to the transmitter
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It's a convenient length on VHF
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