B-006
Subelement B-006
Section B-006-012
Section B-006-012
If you made a half-wavelength dipole antenna for 28.150 MHz, approximately how long would it be?
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10.66 metres
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2.53 metres
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5.33 metres
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Correct Answer5.08 metres
Key words: half-wavelength DIPOLE. Wavelength (lambda) in metres IN FREE SPACE is 300 divided by frequency in megahertz. A 'dipole' is approximately 95 % of one half wavelength in free space = '(300 / 2) * 0.95' divided by frequency in megahertz. In this example, the dipole must be (300 / 28.15 MHz / 2) * 0.95 = 5.06 metres. The frequency is in the 10-metre band of 28.0 to 29.7 MHz, a dipole there must necessarily be 5 metres long.
Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.
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What is one disadvantage of a random wire antenna?
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It must be longer than 1 wavelength
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It must be installed in a straight line, without bends
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It usually produces vertically polarized radiation
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Correct AnswerYou may experience RF feedback in your station
Because the 'random wire' and 'long wire' antennas frequently originate right at the back of the antenna tuner in your station, stray RF (radio frequency) can be a problem.
Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.
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What is the three-dimensional radiation pattern of a half-wavelength dipole in free space?
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Spherical (equal radiation in all directions)
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Correct AnswerA torus (donut shape) around the antenna
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A major lobe off each end of the antenna
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Major lobes at 45-degree angles to the antenna
Picture a horizontal dipole viewed from above. If you plotted radiation all around it in a single horizontal plane, the plot would look like a "number eight": peak radiation at 90 degrees (broadside) from the antenna, negligible radiation from the ends. Now, imagine that pattern in successive planes around the antenna, you get a donut shape.
Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.
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What is the impedance at the feed point of a half-wave dipole in free space?
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300 ohms
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450 ohms
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52 ohms
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Correct Answer73 ohms
Ignoring ground effects, what is the radiation pattern of a horizontal half-wave dipole installed with the ends pointing North/South?
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Radiates equally in all directions
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Correct AnswerRadiates mostly to the East and West
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Radiates mostly to the South
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Radiates mostly to the South and North
Picture a horizontal dipole viewed from above. If you plotted radiation all around it, the plot would look like a "number eight": peak radiation at 90 degrees (broadside) from the antenna, negligible radiation from the ends.
Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.
Tags: none
What is a major advantage of an end-fed half-wave antenna (EFHW)?
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Matching network not needed to feed with coaxial cable
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Correct AnswerCapable of multi-band operation
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High resistance to local noise pickup
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Develops minimal common-mode current
In addition to working as a half-wave antenna on its lowest frequency, the antenna may present suitable impedances at other frequencies and become a usable compromise antenna.
Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.
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What is a disadvantage of using an antenna equipped with traps?
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Correct AnswerIt may radiate harmonics more readily
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It can only be used for one band
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It picks up more noise than a simple dipole
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It is too sharply directional at lower frequencies
An antenna with traps is a multi-band antenna (i.e., resonant at more than one frequency). If the transmitter leaks harmonic energy (multiples of the operating frequency), this harmonic energy may be more readily radiated by a multi-band antenna. For example, traps are inserted in an antenna for 80 metres to permit operation on 40 metres; if your transmitter puts out 'harmonics' while you operate on 80 m ( say, 3.5 MHz ), the second harmonic falls in the 40 m band. The antenna is also resonant at that frequency and would freely radiate the harmonics.
Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.
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What is an advantage of using a trap antenna?
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It has high directivity at the higher frequencies
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It minimizes harmonic radiation
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Correct AnswerIt may be used for multi-band operation
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It has high gain
The only reason why antenna traps (parallel resonant circuits) are useful is to permit operation on more than one band from the same physical antenna. Through their high impedance at resonance, traps shorten the antenna by making the antenna sections beyond them inaccessible.
Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.
Tags: none
If you were to cut a half-wave dipole for 3.75 MHz, what would be its approximate length?
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40.00 metres
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Correct Answer38.13 metres
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80.00 metres
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62.40 metres
Wavelength (lambda) in metres IN FREE SPACE is 300 divided by frequency in megahertz. The dipole is approximately 95 % of one half wavelength in free space = '(300 / 2) * 0.95' divided by frequency in megahertz. In this example, the dipole must be cut to (300 / 3.75 MHz / 2) * 0.95 = 38 metres. [ 3.75 MHz is in the 80-metre band of 3.5 to 4.0 MHz, a DIPOLE there must be below 40 metres long ].
Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.
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