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G9B

ANTENNAS AND FEED LINES

- Basic dipole and monopole antennas

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G9B011 of 12

What is a characteristic of a random-wire HF antenna connected directly to the transmitter?

Why A random wire is an end-fed antenna with no balanced feed line, so the transmitter chassis, power leads and ground system act as the other half (the counterpoise) of the antenna. That means real RF current flows on the equipment itself, which can cause RF burns, distorted audio, and erratic operation of accessories. The cure is a good RF ground, a counterpoise, or a matching unit that isolates the radiator from the station.
Watch out Length is not the issue: random wires work at many lengths, and a wire an odd number of quarter wavelengths long is often a good choice, so the claim that it must exceed one wavelength is false.
End-fed wire = the shack is the other half of the antenna. Expect RF in the station.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G9B022 of 12

Which of the following is a common way to adjust the feed point impedance of an elevated quarter-wave ground-plane vertical antenna to be approximately 50 ohms?

Why A quarter-wave vertical over perfectly horizontal radials shows a feed point impedance near 36 ohms, which is a poor match for 50-ohm coax. Drooping the radials downward, typically around 45 degrees, raises the feed point impedance to roughly 50 ohms and gives a good match. The sloped radials also make the radiation pattern a bit more like a half-wave dipole fed at the center.
Watch out Sloping the radials upward moves the impedance the wrong way, lowering it further below 36 ohms; radial length and coiling are used for tuning resonance or for trap/loading tricks, not for setting the 50-ohm match this way.
Radials droop down to bring 36 ohms up to 50. Down for up.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G9B033 of 12

Which of the following best describes the radiation pattern of a quarter-wave ground-plane vertical antenna?

Why A vertical monopole over a ground plane is symmetrical around its vertical axis, so it radiates equally well in every compass direction. The pattern only varies with elevation angle: strongest at low angles toward the horizon, with a null straight up off the end of the radiator. That azimuthal symmetry is exactly what "omnidirectional in azimuth" means, and it is why verticals are popular for DX and for mobile use where you cannot rotate an antenna.
Watch out Isotropic and hemispherical both imply equal radiation in all directions including straight overhead, but no real antenna is isotropic and a vertical has a deep null at the zenith. Bi-directional in azimuth describes a horizontal dipole, which has two main lobes broadside to the wire.
Vertical = all compass directions, nothing straight up. Horizontal dipole = two lobes broadside.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G9B044 of 12

What is the radiation pattern of a dipole antenna in free space in a plane containing the conductor?

Why A half-wave dipole radiates most strongly broadside, at right angles to the wire, and has deep nulls off the wire ends because the current elements cancel in that direction. In the plane that contains the conductor, plotting relative field strength versus angle traces two lobes perpendicular to the wire, the classic figure-eight. In the plane perpendicular to the wire, the same antenna is omnidirectional, which is why the question specifies which plane you are looking at.
Watch out The figure-eight off the ends is backwards: the ends are where a dipole radiates least. The circular pattern is what you see in the plane at right angles to the conductor, not the plane containing it.
A dipole shouts sideways and whispers off its ends: figure-eight broadside, nulls at the tips.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G9B055 of 12

How does antenna height affect the azimuthal radiation pattern of a horizontal dipole HF antenna at elevation angles higher than about 45 degrees?

Why A horizontal dipole's classic figure-8 azimuthal pattern, with deep nulls off the wire ends, only really shows up at low elevation angles and when the antenna is well above ground. When the dipole is lower than about 1/2 wavelength, ground reflection dominates and most of the energy goes up at high angles, where the pattern fills in and is nearly the same in all compass directions. This is exactly the NVIS condition used for short-skip regional work on 80 and 40 meters.
Watch out The idea that end radiation is eliminated is backwards: a low dipole fills in the end nulls rather than deepening them, and height certainly does affect the pattern, so the no-effect choice is wrong too.
Low dipole = cloud burner: under 1/2 wavelength high it radiates straight up in all directions (NVIS).
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G9B066 of 12

Where should the radial wires of a ground-mounted vertical antenna system be placed?

Why A ground-mounted quarter-wave vertical needs a low-loss return path for the antenna's ground currents, and those currents flow in the soil right under the antenna. Laying many radial wires on the surface or a few inches down puts copper in the path of that current so the earth's resistance doesn't eat your RF power. Buried radials are not resonant elements, so their exact length matters less than having lots of them; 16 to 32 or more is typical.
Watch out Radials raised well above ground describe an elevated ground-plane antenna, which is a different design that uses only a few tuned quarter-wave radials rather than a buried ground screen; the question specifies a ground-mounted vertical.
Ground-mounted means ground-level radials: on the dirt or just under it, and the more the better.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G9B077 of 12

How does the feed point impedance of a horizontal 1/2 wave dipole antenna change as the antenna height is reduced to 1/10 wavelength above ground?

Why A horizontal dipole interacts with its reflected image in the ground, which is 180 degrees out of phase. As the antenna gets very low, that out-of-phase image increasingly cancels the radiation, so the radiation resistance and hence the feed point impedance drops steadily, heading toward near zero at zero height. Above roughly 1/5 wavelength the impedance wiggles around the free-space value of about 73 ohms, but below that it falls off continuously, so a dipole at 1/10 wavelength shows a noticeably lower impedance than 73 ohms.
Watch out The choice about a peak near 1/8 wavelength confuses low heights with the higher region, around 0.3 to 0.4 wavelength, where the impedance actually rises above 73 ohms. The claim that height has no effect ignores ground reflection entirely, which also controls the takeoff angle.
Low dipole, low impedance: the ground image cancels it. Only vertical monopoles like being near ground.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G9B088 of 12

How does the feed point impedance of a 1/2 wave dipole change as the feed point is moved from the center toward the ends?

Why On a half-wave dipole the standing wave puts maximum current at the center and maximum voltage at the ends. Since impedance is voltage divided by current, the feed point impedance rises steadily as you move away from the center, going from roughly 70 ohms at the middle to several thousand ohms near the tips. That is exactly why an off-center-fed dipole uses a 4:1 or 6:1 balun, and why an end-fed half wave needs a very high impedance matching transformer.
Watch out The idea that it decreases has the current and voltage distribution backwards; there is also no peak partway out, the increase is monotonic all the way to the ends.
Center = high current, low Z (~70 ohms). Ends = high voltage, high Z. Move out, Z goes up.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G9B099 of 12

Which of the following is an advantage of using a horizontally polarized as compared to a vertically polarized HF antenna?

Why A vertical monopole works against ground: the return currents flow through the soil near the base, and lossy earth absorbs a significant part of the power unless you install an extensive radial system. A horizontal dipole is a self-contained, balanced radiator that does not rely on earth as part of the antenna, so ground conduction losses are much smaller (though ground still affects the pattern and takeoff angle). That is why a horizontal antenna at a decent height often outperforms a ground-mounted vertical on HF despite the vertical's low-angle pattern.
Watch out Feed point impedance and radiation resistance actually go the other way: a quarter-wave vertical shows about 35 ohms versus roughly 70 ohms for a halfwave dipole, so the horizontal antenna has the higher values. Radials are a vertical-antenna feature and are not part of a horizontal dipole at all.
Verticals borrow the dirt for their other half; horizontal dipoles do not, so they lose less in the ground.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G9B1010 of 12

What is the approximate length for a 1/2 wave dipole antenna cut for 14.250 MHz?

Why For a half-wave wire dipole the standard practical formula is length in feet = 468 divided by the frequency in MHz. That gives 468 / 14.250 = 32.8 feet, which rounds to about 33 feet. The 468 figure already includes the roughly 5 percent shortening from the velocity factor of wire and end effects, so it is a bit less than the 492/f free-space half wavelength.
Watch out About 16 feet is half that answer, which is a quarter-wave length, the size of one leg of the dipole or of a quarter-wave ground-plane vertical.
468 over f in MHz for a half-wave dipole; 234 over f for a quarter-wave vertical or one leg.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G9B1111 of 12

What is the approximate length for a 1/2 wave dipole antenna cut for 3.550 MHz?

Why The standard rule of thumb for a half-wave wire dipole is length in feet = 468 divided by frequency in MHz. At 3.550 MHz that gives 468/3.550 = 131.8 feet, about 132 feet total, or roughly 66 feet per leg. The 468 constant already accounts for the velocity factor and end effects of real wire, so it comes out about 5 percent shorter than a free-space half wavelength.
Watch out The 263-foot answer is what you get from 936/f, which is a full wavelength, not a half wave; 42 feet would suit a much higher frequency band.
468 over f(MHz) for a half-wave dipole; 234 over f for a quarter-wave vertical. 468/3.55 is about 132 ft.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G9B1212 of 12

What is the approximate length for a 1/4 wave monopole antenna cut for 28.5 MHz?

Why A quarter wavelength in feet is approximated by 234 divided by the frequency in MHz (this is the 468/f half-wave formula cut in half, already including the velocity factor of wire). For 28.5 MHz, 234/28.5 is about 8.2 feet, so roughly 8 feet of radiator above the ground plane.
Watch out The choice near 16 feet is the half-wave dipole length (468/28.5 = 16.4 feet), which is twice what a quarter-wave monopole needs.
234/f for a quarter wave, 468/f for a half wave. 234/28.5 = 8 ft.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
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