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E9C

ANTENNAS AND TRANSMISSION LINES

Practical wire antennas; folded dipoles; phased arrays; effects of ground near antennas

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E9C011 of 14

What type of radiation pattern is created by two 1/4-wavelength vertical antennas spaced 1/2-wavelength apart and fed 180 degrees out of phase?

Why With two elements, the pattern depends on the combination of physical spacing and feed phasing. Along the array axis (end-fire direction), the half-wavelength of extra travel adds another 180 degrees of phase, which cancels the 180-degree feed offset, so the signals arrive in phase and add. Broadside (perpendicular to the axis), the path lengths are equal, so the 180-degree feed difference cancels the signals completely. The result is a figure-eight with its lobes pointing along the line joining the two antennas.
Watch out A figure-eight broadside to the axis is what you get from half-wave spacing fed in phase, the opposite phasing; a cardioid comes from quarter-wave spacing with 90 degrees of phase difference.
Half-wave spacing: in phase = broadside, out of phase = end-fire. Quarter-wave and 90 degrees = cardioid.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
E9C022 of 14

What type of radiation pattern is created by two 1/4-wavelength vertical antennas spaced 1/4-wavelength apart and fed 90 degrees out of phase?

Why With 1/4-wavelength spacing, the wave from one element takes 90 degrees of extra travel time to reach the other. Feeding the elements 90 degrees out of phase makes those two 90-degree shifts cancel in one direction (fields add, 2x voltage) and add to 180 degrees in the opposite direction (fields cancel, deep null). The result is a heart-shaped unidirectional pattern with a single null off the back, favoring the element that is fed with the lagging phase.
Watch out The figure-eight answers describe other common combinations: two verticals fed in phase give a broadside figure eight, and 1/2-wavelength spacing with 180-degree phasing gives an end-fire figure eight. A single vertical, not a pair, is omnidirectional.
Quarter-wave spacing plus quarter-wave (90 degree) phasing equals cardioid: one front lobe, one back null.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
E9C033 of 14

What type of radiation pattern is created by two 1/4-wavelength vertical antennas spaced 1/2-wavelength apart and fed in phase?

Why With two verticals fed in phase, a point broadside (perpendicular to the line joining them) is equidistant from both elements, so the signals arrive in step and add for maximum radiation. Off the ends of the array, the extra travel to the far element is the full 1/2-wavelength spacing, which is 180 degrees of phase, so the two signals cancel and produce deep nulls. The result is a two-lobe figure-eight pattern broadside to the array axis.
Watch out The end-fire figure-eight is what you get from the same 1/2-wavelength spacing when the elements are fed 180 degrees out of phase, and a cardioid comes from 1/4-wavelength spacing with a 90 degree phase difference.
In phase equals broadside; out of phase equals end-fire. 1/4-wave spacing plus 90 degrees equals cardioid.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
E9C044 of 14

What happens to the radiation pattern of an unterminated long wire antenna as the wire length is increased?

Why A long wire carries standing waves of current with alternating phase every half wavelength. As you add more half-wave sections, the radiation from each section combines into more lobes, and the main lobes swing closer to the axis of the wire. A half-wave dipole radiates broadside, but by the time a wire is several wavelengths long the strongest lobes are only a modest angle off the wire itself, which is why long wires are aimed by pointing an end toward the target.
Watch out The idea that lobes get fewer and swing broadside is exactly backwards; broadside radiation is the behavior of a short wire near a half wavelength, and longer wires break that pattern up into many lobes.
Longer wire, more lobes, and they lean toward the wire: aim a long wire by its ends, not its sides.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
E9C055 of 14

What is the purpose of feeding an off-center-fed dipole (OCFD) between the center and one end instead of at the midpoint?

Why A center-fed dipole shows a low impedance (around 70 ohms) on its fundamental and odd harmonics, but the center becomes a current minimum on even harmonics, where the impedance jumps to thousands of ohms. Moving the feed point off center, typically about one third of the way along the wire, lands on a spot where the impedance works out to roughly 200-300 ohms on several harmonically related bands. That lets one wire plus a 4:1 (or similar) balun present a usable match on multiple bands such as 80, 40, 20 and 10 meters.
Watch out The choice about resonating over a wider frequency range is wrong because the OCFD is still a set of narrow resonances at harmonically related bands, not a broadband antenna. The common-mode choice is backwards: the asymmetric feed tends to increase common-mode current on the feed line, which is why a good choke is recommended.
OCFD = off-center for multiband match: feed near the 1/3 point, get ~200-300 ohms on several bands, use a 4:1 balun.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
E9C066 of 14

What is the effect of adding a terminating resistor to a rhombic or long-wire antenna?

Why An unterminated rhombic or long wire is a resonant, standing-wave antenna: current flows toward the far end, reflects, and comes back, producing lobes off both ends. Adding a resistor at the far end (typically several hundred ohms to ground or between the legs) absorbs that forward-traveling energy so nothing reflects back, turning it into a traveling-wave antenna with a single main lobe pointed away from the feed point. The cost is efficiency, since roughly a third to half the power can be dissipated in the resistor, but the gain in front-to-back ratio and bandwidth is usually worth it.
Watch out The choice about reflecting standing waves back to the transmitter is backwards: the whole point of the termination is to eliminate reflections, not create them. Polarization is set by wire orientation, and a termination resistor adds loss rather than reducing ground loss.
Terminate = travel one way. Resistor soaks up the reflection, so the pattern fires one direction only.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
E9C077 of 14

What is the approximate feed point impedance at the center of a two-wire half-wave folded dipole antenna?

Why A folded dipole is two parallel conductors joined at the ends, so the driving current splits between them and the feed point sees an impedance stepped up by the square of the number of conductors. With two equal-diameter wires the step-up factor is 2^2 = 4, and 4 times the roughly 72 ohm impedance of a plain half-wave dipole gives about 288 ohms, conventionally rounded to 300 ohms. That is exactly why 300 ohm twinlead was the classic feed line and building material for folded dipoles.
Watch out The choice of 72 ohms is the feed point impedance of an ordinary single-wire half-wave dipole in free space, which is the starting value before the folded geometry multiplies it by four; 450 ohms is a common open-wire ladder line impedance, not a folded dipole feed point.
Fold a 72 ohm dipole in two and you multiply by 2 squared: 4 x 72 = about 300 ohms, the twinlead number.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
E9C088 of 14

What is a folded dipole antenna?

Why A folded dipole is a half-wave dipole with a second conductor of the same length running parallel to it and joined to it at both ends, forming a long thin loop fed at the center of one side. The current divides between the two conductors, which steps the feedpoint impedance up by a factor of four, roughly 280 to 300 ohms instead of about 70 ohms, so it matches 300 ohm twinlead directly or 50 ohm coax through a 4:1 balun. The two-wire construction also gives it a wider usable bandwidth than a single-wire dipole.
Watch out The description of ends folded down 90 degrees is closer to an inverted V or a bent/shortened dipole, and forward gain describes a directional array such as a Yagi; a folded dipole has essentially the same figure-eight pattern and gain as a plain dipole.
Folded = two parallel wires joined at the tips; impedance x4, about 300 ohms, and wider bandwidth.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
E9C099 of 14

Which of the following describes a G5RV antenna?

Why The G5RV, designed by Louis Varney (callsign G5RV), is a 102-foot center-fed wire with a specific length of open-wire or ladder line (about 31 feet of 450-ohm window line, or roughly 34 feet of true open-wire) acting as a matching/transformer section. That stub transforms the feedpoint impedance so the antenna presents a workable load on several bands, and it is then joined to coax, usually through a balun, for the run to the shack. On 20 meters the flat top is three half-waves in phase, which is where the original design length came from.
Watch out The trap antenna answer describes a different multiband scheme, where LC traps electrically isolate sections of the wire; the shorted-coax wideband dipole fed with a 4:1 balun is the double bazooka, not the G5RV.
G5RV = 102 ft wire + ~31 ft ladder line + balun + coax. No traps, no coax radiators.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
E9C1010 of 14

Which of the following describes a Zepp antenna?

Why The Zeppelin, or Zepp, antenna is a half-wavelength radiator fed at one end rather than at the center, traditionally matched with a quarter-wavelength section of open-wire line. It got its name from the trailing wire antennas dangled from Zeppelin airships, where feeding from one end was the only practical arrangement. Because the feed point is at a voltage maximum (high impedance), it needs a matching section or tuner rather than a direct 50 ohm connection.
Watch out The choices about horizontal or vertical arrays that steer their pattern by changing phasing lines describe phased arrays, a completely different technique covered elsewhere in this group.
Zepp = Zeppelin's trailing wire: half wave, fed at the END.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
E9C1111 of 14

How is the far-field elevation pattern of a vertically polarized antenna affected by being mounted over seawater versus soil?

Why The far field of a vertical depends on ground reflection out to many wavelengths from the antenna. Seawater is a very good conductor, so it reflects the wave nearly perfectly and the direct and reflected waves add in phase at low takeoff angles; ordinary soil is lossy and absorbs much of the energy striking it at grazing angles. The result is that a vertical over saltwater shows noticeably stronger low-angle radiation, which is why shoreline and shipboard verticals are prized for DX.
Watch out The choices about extra lobes or lobes merging describe what happens when you change antenna height or stack elements in wavelengths, not what changes when you swap the ground constants under a vertical.
Saltwater is the world's best ground screen: verticals on the beach get a low-angle DX boost.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
E9C1212 of 14

Which of the following describes an extended double Zepp antenna?

Why The extended double Zepp is two collinear elements of about 0.64 wavelength each fed in phase at the center, giving a total length of roughly 1.25 to 1.28 wavelengths. That is the longest a collinear pair can get before the broadside lobe starts splitting, so it delivers about 3 dB gain over a half-wave dipole broadside to the wire. Because the feedpoint impedance is high and complex, it is normally fed with open-wire line and a tuner.
Watch out The 1.5-wavelength center-fed choice is a different antenna: at that length the broadside lobe has broken up into multiple lobes, which is exactly what the extended double Zepp stops short of. The end-fed options describe the original single Zepp idea, not the extended double version, which is center-fed.
Double = center-fed (two halves), extended = stretched past 1 wavelength to about 1.25 for extra gain.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
E9C1313 of 14

How does the radiation pattern of a horizontally polarized antenna vary with increasing height above ground?

Why A horizontal antenna's elevation pattern is formed by interference between the direct wave and the wave reflected from the ground, which acts like an image antenna the same distance below the surface. The peak of the lowest lobe occurs at roughly sin(theta) = lambda/(4h), so as the height h in wavelengths grows, that angle gets smaller. A dipole at 1/2 wavelength peaks near 30 degrees, at 1 wavelength near 15 degrees, which is why DX operators want their antennas high.
Watch out The beamwidth choices confuse planes: height above ground reshapes the vertical (elevation) pattern, while the azimuth pattern of a horizontal wire is set by the wire itself and changes little with height. The choice that says the takeoff angle increases has it backwards; that is what happens as you lower the antenna.
Higher antenna, lower angle. Half wave up gives about 30 degrees, one wavelength up gives about 15 degrees.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
E9C1414 of 14

How does the radiation pattern of a horizontally-polarized antenna mounted above a long slope compare with the same antenna mounted above flat ground?

Why The elevation pattern of a horizontal antenna is set largely by reflections from the ground beneath it, and the lobe forms relative to that reflecting surface. When the ground slopes away, the whole reflection geometry tilts downward, so the main lobe is aimed lower with respect to the true horizon in the downhill direction. A lower takeoff angle is exactly what favors long-haul DX, which is why hilltop stations with a clear downhill slope work so well.
Watch out The choice saying the takeoff angle increases has the tilt backwards: that is what happens looking uphill, where the rising ground pushes the lobe up. The beamwidth answers confuse the vertical pattern effect with horizontal azimuth beamwidth, which is set by the antenna itself, not the slope.
Slope tilts the ground mirror: downhill means the lobe points down, lower takeoff angle, better DX.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
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