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G9D

ANTENNAS AND FEED LINES

- Specialized antenna types and applications

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G9D011 of 12

Which of the following antenna types will be most effective as a near vertical incidence skywave (NVIS) antenna for short-skip communications on 40 meters during the day?

Why NVIS works by firing energy almost straight up so the ionosphere reflects it back down over a circle roughly 0 to 300 miles across, filling in the skip zone. A horizontal dipole mounted low, about 1/10 to 1/4 wavelength above ground, has its main lobe pointing nearly straight up because ground reflection reinforces the straight-up direction. On 40 meters by day that means roughly 10 to 30 feet up, which is easy to put up and gives reliable regional coverage.
Watch out A horizontal dipole at 1/2 wavelength is the classic DX height: the pattern splits into lower-angle lobes with a null overhead, which is exactly what you do not want for short skip. Verticals are worse still, radiating at very low angles toward the horizon.
NVIS = Never Very-high Install Skyward: keep the horizontal wire low, 1/10 to 1/4 wavelength, to shoot straight up.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G9D022 of 12

What is the feed point impedance of an end-fed half-wave antenna?

Why On a half-wave wire the current is maximum at the center and drops to nearly zero at the ends, while voltage is maximum at the ends. Impedance is voltage divided by current, so feeding at an end where current is near zero gives a very high impedance, typically a few thousand ohms. That is why end-fed half-wave antennas need a matching device such as a 49:1 transformer or a parallel tuned circuit to work with 50 ohm coax.
Watch out Approximately 50 ohms describes the same half-wave wire fed at its center, where current is high and voltage is low; that is the classic dipole feed point, not the end.
Ends of a half wave are voltage peaks, current nulls: high V over low I equals high Z. Center feed low Z, end feed high Z.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G9D033 of 12

In which direction is the maximum radiation from a VHF/UHF "halo" antenna?

Why A halo is simply a half-wave dipole bent into a near-circle with the ends close together but not joined. Because the dipole is horizontal and curved back on itself, the usual figure-eight pattern is smeared out into a roughly uniform pattern all around the horizon, giving horizontal polarization with no need to rotate the antenna. That is why halos are popular for VHF/UHF SSB and CW mobile work, where horizontal polarization is the convention.
Watch out Radiation broadside to the loop plane describes a full-wave quad loop or a driven loop array, not a halo; a halo has relatively little radiation along its axis.
Halo = bent dipole, horizontally polarized, hears all around the horizon. Think halo over your head, radiating outward in its own plane.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G9D044 of 12

What is the primary function of antenna traps?

Why A trap is a parallel LC resonant circuit inserted in an antenna element. At the trap's resonant frequency it presents a very high impedance, effectively disconnecting the wire beyond it so the antenna is electrically short for that band; at lower frequencies the trap acts mostly as a loading inductance and the full length radiates. That lets one physical antenna resonate on several bands from a single feed line.
Watch out The talk of resonant circuits makes filtering answers like notching spurious frequencies tempting, but the trap is inside the radiating element to set its electrical length, not in the feed line to clean up the transmitter's output. Traps also do nothing to stop you from transmitting out of band.
Trap = tuned circuit that "traps off" the rest of the wire, so one antenna covers many bands.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G9D055 of 12

What is an advantage of vertically stacking horizontally polarized Yagi antennas?

Why Stacking antennas in a given plane sharpens the pattern in that plane. Two Yagis spaced vertically form a broadside array in the vertical dimension, so their combined pattern is compressed in elevation, adding roughly 3 dB of gain and pushing more energy toward the low takeoff angles that favor DX. The individual elements stay horizontal, so the polarization does not change.
Watch out Narrowing the main lobe in azimuth is what you get from stacking side by side horizontally, not one above the other. Selecting or combining polarizations requires separate vertically and horizontally oriented driven elements, which stacking identical horizontal Yagis does not provide.
Stack the direction you want to squeeze: vertical stacking squeezes elevation, side-by-side squeezes azimuth.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G9D066 of 12

Which of the following is an advantage of a log-periodic antenna?

Why A log-periodic array uses elements whose lengths and spacings scale by a constant geometric ratio, so different portions of the array become the active radiating region at different frequencies. That makes the feedpoint impedance and pattern stay usable over a very broad frequency range, often covering several ham bands in one antenna. The trade-off is that only a few elements are active at any one frequency, so gain is modest for the number of elements used.
Watch out The choice about gain per element is the opposite of the truth: in a Yagi all elements work together on one narrow band, so a Yagi gives more gain per element than a log-periodic. Harmonic suppression is also backwards, since a broadband antenna radiates harmonics readily rather than rejecting them.
Log-periodic = Lots ofPtterns... simpler: LP means 'Lots of bandwidth, Puny gain per element.'
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G9D077 of 12

Which of the following describes a log-periodic antenna?

Why A log-periodic dipole array is built from a series of dipole elements whose lengths and spacings scale by a constant ratio as you move along the boom, so the dimensions grow logarithmically. Each element is resonant near a different frequency, and the active region shifts along the boom as you change frequency. That geometry is what gives the LPDA usable gain and SWR over a very wide frequency range, unlike a Yagi that is tuned to one band.
Watch out The choices about impedance, gain or SWR varying periodically or logarithmically with frequency invert the idea: it is the physical element dimensions that follow the logarithmic progression, and the result is that performance stays roughly constant across the design range.
The 'log' in log-periodic is in the boom, not the performance: lengths and spacings scale logarithmically.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G9D088 of 12

How does a "screwdriver" mobile antenna adjust its feed point impedance?

Why A screwdriver antenna gets its name from the small DC gear motor (like one from a cordless screwdriver) that drives a threaded shaft, moving a coil in and out of contact with a sliding connection at the base. Changing how many turns of that base coil are in the circuit changes the loading inductance, which cancels the capacitive reactance of the short whip and brings the antenna to resonance on the chosen frequency. A short mobile whip is electrically much shorter than a quarter wave, so it looks capacitive, and adjustable base loading inductance is what makes one antenna cover many bands.
Watch out Extending and retracting the whip is how a telescoping or center-loaded adjustable whip is tuned, and a capacitance hat is a fixed top structure that raises the current distribution; neither is what the screwdriver motor is doing.
Screwdriver motor turns a screw that moves a COIL: motorized variable base inductance.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G9D099 of 12

What is the primary use of a Beverage antenna?

Why A Beverage is a very long wire (one wavelength or more) strung just a few feet above ground, usually terminated at the far end with a resistor equal to its characteristic impedance. That layout gives excellent front-to-back ratio and low noise pickup off the end, which is exactly what you want for weak-signal listening on 160, 80 and 40 meters. Because so much energy is absorbed by the lossy ground and the termination resistor, it is very inefficient as a radiator, so it is a receive-only antenna in practice.
Watch out The transmitting choice is the trap: the same ground losses and terminating resistor that kill noise also waste most of your transmit power, so hams put up a Beverage to listen and switch to a vertical or dipole to talk.
Beverage = "Be Very Long, Very Low, and only Listen."
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G9D1010 of 12

In which direction or directions does an electrically small loop (less than 1/10 wavelength in circumference) have nulls in its radiation pattern?

Why A small loop acts as a magnetic dipole: current is essentially uniform around the loop, and it responds to the magnetic field component that threads through the loop. Radiation and reception are strongest in the plane of the loop (a figure-8 pattern edge-on), and drop to sharp nulls along the axis perpendicular to the loop, which is the broadside direction. Those deep nulls are why small shielded loops are prized for direction finding and for nulling out a local noise source.
Watch out The choice naming the plane of the loop describes where the small loop's signal peaks, not its nulls; that answer would be right for a large full-wave loop, which does radiate broadside.
Look through the hole of a small loop and you hear nothing: nulls along the axis, peaks edge-on.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G9D1111 of 12

Which of the following is a disadvantage of multiband antennas?

Why A single-band resonant antenna acts as a crude bandpass filter: at the second or third harmonic of its design frequency the feedpoint impedance is far from a match, so harmonic energy from the transmitter is reflected rather than radiated. A multiband antenna is deliberately designed to accept and radiate on several bands, many of which (for example 7 MHz and 21 MHz) are harmonically related, so it happily radiates any harmonic output the transmitter produces. That makes good transmitter filtering and clean operation more important, since 97.307 still holds you responsible for spurious emissions.
Watch out Needing an antenna tuner is not the drawback being described, and many multiband designs such as trapped dipoles or fan dipoles present a usable match without one; open wire line is only required for certain designs like the doublet.
Multiband means multi-harmonic: the antenna that loves several bands will also radiate your harmonics.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G9D1212 of 12

What is the common name of a dipole with a single central support?

Why If you hang a dipole from one central support, typically a single mast or tree, the feedpoint sits at the apex and both legs slope down toward lower end supports, forming the shape of an upside down V. This is called an inverted V. Because the legs droop, the antenna behaves like a dipole with a slightly lower feedpoint impedance (often near 50 ohms) and a somewhat broader, less directional pattern than a flat-top dipole, and the ends can be much closer to the ground.
Watch out A sloper also uses one support, but it is a single slanted radiator run down from the top of a tower, not two legs drooping from a center feedpoint; an inverted L is an end-fed vertical section with a horizontal top wire, and a lazy H is a stacked pair of driven elements needing a wider supporting structure.
Picture the wire: center up, ends down, the letter V turned upside down. One support in the middle equals inverted V.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
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