G9 Antennas and Feed Lines

A. The distance between the centers of the conductors and the radius of the conductors

The characteristic impedance of a parallel-conductor line depends on its geometry: the spacing between the conductor centers and the conductor radius (along with the insulating material). For air-spaced line, Z0 ≈ 276 log(D / r), so wider spacing or thinner wire raises the impedance.

• B. Spacing matters, but the length of the line does not change its characteristic impedance.
• C. Conductor radius matters, but the characteristic impedance of a practical line does not depend on the signal frequency.
• D. Characteristic impedance depends on neither frequency nor length; it is set by conductor spacing, radius and insulation.

B. High SWR increases loss in a lossy transmission line

With high SWR, standing waves create points of high current and high voltage along the line, and power travels back and forth. That raises the resistive and dielectric losses, so a lossy line wastes more power than it would when matched.

• A. SWR and loss are related: standing waves increase the loss in a line that has any loss at all.
• C. High SWR does not make line loss hard to measure; it makes the loss larger.
• D. High SWR increases the effect of line loss rather than reducing it.

D. 450 ohms

Window line is parallel-conductor line whose plastic insulation has openings ("windows") cut along its length. Its nominal characteristic impedance is about 450 ohms, much higher than coaxial cable.

• A. 50 ohms is the impedance of common amateur coaxial cable such as RG-8 and RG-58, not window line.
• B. 75 ohms is the impedance of coaxial cable such as RG-59 and RG-6, used for television and cable systems.
• C. 100 ohms is not a standard impedance for amateur parallel line; window line is about 450 ohms.

C. A difference between feed line impedance and antenna feed point impedance

Power is reflected wherever a wave meets an impedance different from the one it is traveling in. If the antenna feed point impedance does not equal the characteristic impedance of the feed line, part of the power reflects back toward the transmitter.

• A. Operating at resonance makes the feed point impedance resistive, which usually reduces reflection; resonance alone does not cause it.
• B. Too much power can damage an antenna, but the fraction of power reflected depends on impedances, not on the power level.
• D. Feeding a balanced antenna with unbalanced line can cause common-mode current on the feed line, but reflected power comes from an impedance mismatch.

B. Attenuation increases

Coaxial cable loss rises with frequency, because skin effect forces current into a thinner layer of the conductors and the dielectric absorbs more energy. A cable that is fine on 80 meters can be quite lossy at UHF.

• A. Coax loss depends strongly on frequency; data sheets list different loss figures for each frequency.
• C. Loss increases with frequency, not decreases.
• D. There is no "Marconi's Law of Attenuation"; coax loss is explained by conductor and dielectric losses.

D. Decibels per 100 feet

Feed line loss is a power ratio, so it is expressed in decibels, and US manufacturers specify it per 100 feet of line at a given frequency. Loss in dB scales with length, so 50 feet has half the dB loss of 100 feet.

• A. Ohms per length describes conductor resistance, not the loss of RF power along a line, which is a ratio expressed in decibels.
• B. Decibels are the right unit, but the usual reference length is 100 feet, not 1,000 feet.
• C. Ohms measure resistance or impedance, not the loss of RF power along a line.

D. The antenna feed point impedance must be matched to the characteristic impedance of the feed line

Standing waves come from power reflected at the load. When the antenna feed point impedance equals the characteristic impedance of the feed line, nothing is reflected, so no standing waves form and the SWR is 1:1.

• A. Having the feed point at DC ground (as with some matching methods) does not determine whether RF is reflected.
• B. An odd number of quarter wavelengths transforms the load impedance seen at the input, but the standing waves on the line remain.
• C. A line that is a multiple of a half wavelength (electrically, not physically) repeats the load impedance at its input, but the standing waves on the line remain.

B. 5:1

The SWR on the feed line is set by the mismatch between the line and the antenna at the far end. A matching network at the transmitter end only transforms the impedance the transmitter sees; the 5:1 standing waves between the network and the antenna are unchanged.

• A. The transmitter sees 1:1, but the line between the tuner and the antenna still carries the 5:1 mismatch.
• C. The 5:1 SWR is already measured relative to the line's characteristic impedance, and adjusting the tuner does not change it.
• D. Reflected power at the transmitter end does not change the line SWR, which is set by the antenna mismatch.

A. 4:1

For a resistive load, SWR is the ratio of the larger impedance to the smaller one: SWR = 200 ohms / 50 ohms = 4, written 4:1. SWR is always written with the larger number first.

• B. 1:4 is the right ratio written backwards; SWR is expressed as a number of 1 or more, followed by :1.
• C. A 2:1 SWR on a 50-ohm line would come from a 100-ohm or 25-ohm load, not 200 ohms.
• D. 1:2 is both the wrong value and written backwards.

D. 5:1

For a resistive load, SWR is the larger impedance divided by the smaller: SWR = 50 ohms / 10 ohms = 5, written 5:1. A load lower than the line impedance causes a mismatch in the same way as a higher one.

• A. A 2:1 SWR on a 50-ohm line would come from a 25-ohm or 100-ohm load, not 10 ohms.
• B. 1:2 is both the wrong value and written backwards.
• C. 1:5 is the right ratio written backwards; SWR is written with the larger number first.

A. Higher loss reduces SWR measured at the input to the line

Reflected power is attenuated twice, once going to the antenna and again coming back, so a lossy line makes the SWR measured at the transmitter look better than the SWR at the antenna. For example, with 3 dB of line loss and an open or shorted far end, the reflected power arrives back 6 dB weaker (3 dB each way), and the meter reads about 3:1 instead of an infinite SWR.

• B. Loss reduces, not increases, the SWR measured at the input, because it weakens the reflected wave.
• C. Loss makes the input reading less representative of the antenna, so it does not improve accuracy.
• D. Loss does affect the reading: it hides part of the mismatch at the antenna.

B. Station equipment may carry significant RF current

A random wire connected directly to the transmitter (usually through a tuner) has no separate feed line or counterpoise, so the station equipment becomes part of the antenna system. RF current can flow on equipment cases and cables, causing RF burns, "hot" microphones and interference in the shack.

• A. A random wire can be any convenient length; it does not need to be longer than one wavelength.
• C. A random wire usually runs at least partly horizontal, so its radiation is not only vertically polarized.
• D. Effectiveness depends on the wire length compared with the wavelength; a typical random wire is electrically shorter on the lower bands, so it is not more effective there.

B. Slope the radials downward

With horizontal radials, an elevated quarter-wave ground plane has a feed point impedance well below 50 ohms. Sloping the radials downward raises the impedance, and a moderate droop brings it close to 50 ohms for a good match to coax.

• A. Sloping the radials upward lowers the feed point impedance further, moving it away from 50 ohms.
• C. The radials of an elevated ground plane are about a quarter wavelength long; making them longer than a wavelength is not how the impedance is adjusted.
• D. Coiling the radials turns them into inductors and spoils their function; it is not a way to set the impedance.

D. Omnidirectional in azimuth

A quarter-wave vertical radiates equally well in all compass directions (omnidirectional in azimuth). Its radiation is concentrated at low elevation angles, with little straight up.

• A. A bidirectional azimuth pattern belongs to a horizontal dipole, which radiates broadside in two opposite directions.
• B. An isotropic radiator, equal in every direction including up and down, is a theoretical reference; no real antenna has that pattern.
• C. A vertical radiates little straight up, so its pattern is not a hemisphere.

A. It is a figure-eight at right angles to the antenna

In free space, in a plane containing the wire, a half-wave dipole radiates most strongly broadside (at right angles to the wire) and has nulls off its ends. The result is a figure-eight pattern with lobes on both sides of the antenna.

• B. The ends of a dipole are where its nulls are, not its lobes.
• C. A circular pattern appears only in the plane perpendicular to the wire, not in the plane that contains it.
• D. Unequal lobes on the two sides describe a directional antenna such as a Yagi; a dipole is symmetrical.

C. If the antenna is less than 1/2 wavelength high, the azimuthal pattern is almost omnidirectional

A horizontal dipole's deep nulls lie along the wire near the horizon. At elevation angles above about 45 degrees, a dipole less than 1/2 wavelength high radiates almost equally in all compass directions, which is why low dipoles work well for short-range, high-angle (NVIS) contacts.

• A. Height changes the pattern in predictable ways (higher antennas favor lower elevation angles); it does not become unpredictable.
• B. Height above ground strongly affects a horizontal dipole's pattern because of reflections from the ground.
• D. Low height fills in the high-angle radiation off the ends; it does not eliminate it.

C. On the surface or buried a few inches below the ground

A ground-mounted vertical uses radials to give its return current a low-loss path through the lossy earth near the base. These radials are laid on the ground surface or buried a few inches deep, running outward from the base.

• A. Radials raised high above ground describe an elevated ground-plane antenna, not a ground-mounted vertical.
• B. Radials run outward from the base, roughly perpendicular to the vertical element, not parallel to it.
• D. Radials connect at the base and spread outward; they are not placed at the center of the antenna.

B. It steadily decreases

A horizontal half-wave dipole has about 73 ohms feed point impedance in free space. Close to the ground, its own ground reflection interacts with it, and as it is lowered from about 1/4 wavelength toward 1/10 wavelength the feed point resistance falls steadily, ending below the free-space value.

• A. The impedance falls, not rises, as a dipole is lowered toward the ground.
• C. There is no peak at 1/8 wavelength; the impedance keeps falling over that range of heights.
• D. Height above ground affects the feed point impedance, especially at low heights.

A. It steadily increases

On a half-wave dipole the current is highest at the center and falls to near zero at the ends, while the voltage does the opposite. Since impedance is voltage divided by current, it rises steadily from about 73 ohms at the center to several thousand ohms near the ends; off-center-fed antennas use this to get a higher feed point impedance.

• B. Moving toward the ends lowers the current and raises the voltage, so the impedance increases, not decreases.
• C. There is no peak at 1/8 wavelength from the end; the impedance keeps rising all the way to the end.
• D. Feed point location strongly affects impedance, which is why off-center feeding is used to change it.

A. Lower ground losses

A vertical antenna depends on the ground near it for its return current, and lossy soil wastes power, so verticals need extensive radial systems. A horizontally polarized antenna such as a dipole does not depend on ground currents in the same way, so its ground losses are lower.

• B. Lower feed point impedance is not an advantage of horizontal antennas; a quarter-wave vertical over ground actually has a lower impedance than a dipole.
• C. A horizontal dipole needs no radials at all, so "shorter radials" does not apply.
• D. Lower radiation resistance is not an advantage of horizontal polarization; low radiation resistance makes losses relatively larger.

D. 33 feet

The usual formula for a half-wave dipole is length (feet) = 468 / frequency (MHz), which allows for end effects. At 14.250 MHz: 468 / 14.25 ≈ 32.8 feet, about 33 feet.

• A. 8 feet is only about 0.12 wavelength at 14.250 MHz; it would be a half wave near 58.5 MHz.
• B. 16 feet is about a quarter wavelength at 14.250 MHz (234 / 14.25 ≈ 16.4 feet), half the needed length.
• C. 24 feet would be a half wave near 19.5 MHz (468 / 24); it is too short for 14.250 MHz.

C. 132 feet

Using length (feet) = 468 / frequency (MHz) for a half-wave dipole: 468 / 3.55 ≈ 131.8 feet, about 132 feet.

• A. 42 feet would be a half wave near 11.1 MHz (468 / 42), far too short for 3.550 MHz.
• B. 84 feet would be a half wave near 5.6 MHz (468 / 84), still too short for 3.550 MHz.
• D. 263 feet is about twice the half-wave length (2 x 131.8 ≈ 263.7 feet), a full wavelength at 3.550 MHz.

A. 8 feet

A quarter-wave monopole is half the length of a half-wave dipole, so length (feet) = 234 / frequency (MHz). At 28.5 MHz: 234 / 28.5 ≈ 8.2 feet, about 8 feet.

• B. 11 feet would be a quarter wave near 21.3 MHz (234 / 11), on the 15-meter band, not 28.5 MHz.
• C. 16 feet is about a half wavelength at 28.5 MHz (468 / 28.5 ≈ 16.4 feet), twice the quarter-wave length.
• D. 21 feet is longer than even a half wavelength (about 16.4 feet) at 28.5 MHz.

A. Larger-diameter elements

Thicker conductors lower the Q of each element, so its impedance changes more slowly with frequency. A Yagi with larger-diameter elements keeps a good SWR and pattern over a wider range of frequencies.

• B. Closer element spacing tightens coupling and usually narrows the bandwidth, rather than widening it.
• C. Loading coils make shortened elements high-Q, which narrows the bandwidth.
• D. Elements are tapered mainly for mechanical strength and lighter weight; tapering is not a way to increase bandwidth.

B. 1/2 wavelength

The driven element of a Yagi is essentially a half-wave dipole, about 1/2 wavelength long. The parasitic reflector and directors are tuned slightly longer and shorter than it.

• A. A quarter-wave element is a monopole that needs a ground plane, not the driven element of a Yagi.
• C. A Yagi driven element is not 3/4 wavelength; it is a resonant half-wave dipole.
• D. A full-wave element fed at the center would have a high feed point impedance (thousands of ohms); the Yagi driven element is about a half wavelength.

A. The reflector is longer, and the director is shorter

In a Yagi, the reflector is a few percent longer than the driven element and the director is a few percent shorter. These length differences set the phase of the currents in the parasitic elements, which steers radiation toward the director end.

• B. This reverses the arrangement: the reflector is longer and the director is shorter.
• C. If all elements were the same length, the parasitic currents would not have the phase needed to make the antenna directional.
• D. All the element lengths scale with frequency, but the reflector is always longer and the director shorter than the driven element.

B. Gain in dBi is 2.15 dB higher

dBi compares gain with an isotropic radiator and dBd compares it with a half-wave dipole. A dipole has 2.15 dBi of gain, so gain in dBi = gain in dBd + 2.15 dB; a 7 dBd antenna is 9.15 dBi.

• A. The sign is reversed: because the dipole reference itself has gain over isotropic, dBi figures are higher.
• C. The dBd figure is the lower one, but by 2.15 dB, not 1.25 dB.
• D. The dBd figure is lower than the dBi figure, not higher, and the difference is 2.15 dB, not 1.25 dB.

A. Gain increases

A longer boom with more directors concentrates more of the radiated energy in the forward direction. The main effect is higher forward gain, with a narrower beam.

• B. More gain comes with a narrower beam, so beamwidth decreases, not increases.
• C. Front-to-back ratio depends on how the elements are tuned and spaced; adding directors is done to raise gain, not to reduce front-to-back ratio.
• D. The operating frequency is set mainly by element lengths; a longer boom with more directors is used to raise gain, not to lower the frequency.

C. The power radiated in the major lobe compared to that in the opposite direction

Front-to-back ratio compares the power radiated in the main forward lobe with the power radiated in the opposite direction, usually expressed in dB. A high ratio means the beam rejects signals from behind.

• A. Front-to-back ratio is a radiation measurement, not a count of directors and reflectors.
• B. The positions of the elements affect front-to-back ratio, but the term describes the radiated power ratio, not the element layout.
• D. Forward gain compared with a dipole is the gain in dBd, not the front-to-back ratio.

D. The direction of maximum radiated field strength from the antenna

The main lobe is the part of the radiation pattern in the direction where the antenna radiates its strongest field. A directional antenna is pointed so that its main lobe faces the station being worked.

• A. The main lobe is a direction of maximum radiation, not a measure of vertical angle; the elevation angle of the main lobe is a separate property.
• B. The point of maximum current on an element is a current loop, a point on the conductor, not a lobe of the pattern.
• C. The point of maximum voltage on an element is a voltage loop, near the element ends, not a lobe of the pattern.

B. Approximately 3 dB higher

Two identical antennas properly spaced and fed in phase can about double the effective capture area, which is about 10 log(2) ≈ 3 dB more gain than one antenna.

• A. 1.5 dB is about half of the gain that doubling the number of antennas provides.
• C. 6 dB (10 log 4 ≈ 6.0) would require four antennas, not two.
• D. 9 dB (10 log 8 ≈ 9.0) would require eight antennas, not two.

D. All these choices are correct

Yagi design is a set of trade-offs among forward gain, front-to-back ratio, and SWR bandwidth. Designers adjust boom length, the number of elements, and the spacing of each element to balance them.

• A. True: a longer boom allows more elements and wider spacing, which raises gain, but boom length is only one of the adjustable factors.
• B. True: the number of elements affects gain and front-to-back ratio, but it is only one of the adjustable factors.
• C. True: element spacing affects gain, front-to-back ratio, and feed point impedance, but it is only one of the adjustable factors.

A. A shorted transmission line stub placed at the feed point of a Yagi antenna to provide impedance matching

A beta or hairpin match is a short, shorted section of parallel transmission line connected across the Yagi feed point. It acts as an inductance that, together with a slightly shortened driven element, transforms the feed point impedance to match the feed line.

• B. A quarter-wave section of 75-ohm coax used as an impedance transformer is a Q-section, not a hairpin match.
• C. A series capacitor is not a stub; a hairpin is a shorted section of line connected across the feed point.
• D. A folded dipole has a feed point impedance near 300 ohms and is fed with 300-ohm twin lead directly; that is not a hairpin match.

A. It does not require the driven element to be insulated from the boom

A gamma match connects the feed line between the center of the driven element and a point out along it, through a parallel rod and a series capacitor. The center of a half-wave element is at zero RF voltage, so the element can be mounted directly on a metal boom without insulation.

• B. A gamma match uses a series capacitor to cancel the inductive reactance of the gamma rod.
• C. A gamma match is adjusted for one band; it is not suited to multiband antennas.
• D. A gamma match does need a capacitor and is a single-band match, so not all the choices are correct.

A. A horizontal dipole placed between 1/10 and 1/4 wavelength above the ground

NVIS sends signals nearly straight up so they return from the ionosphere over a radius of a few hundred miles. A horizontal dipole only 1/10 to 1/4 wavelength above ground puts its strongest radiation straight up.

• B. A vertical antenna has a null straight up, so it sends little energy at the high angles NVIS needs.
• C. At about 1/2 wavelength high, the ground reflection cancels radiation straight up and the main lobes are near 30 degrees elevation, too low for NVIS.
• D. A vertical dipole has a null straight up, so it is a poor NVIS antenna.

D. Very high

An end-fed half-wave antenna is fed at its end, where the current is near zero and the voltage is at its peak, so its feed point impedance is very high, typically a few thousand ohms. A high-ratio matching transformer at the feed point brings it down to about 50 ohms for coax.

• A. Low feed point impedance occurs where the current is high, such as at the center of a half-wave dipole, not at its end.
• B. About 50 ohms is near what a center-fed half-wave dipole presents; the end of a half-wave wire is far higher and needs a transformer.
• C. About 300 ohms is typical of a folded dipole, not the end of a half-wave wire.

C. Omnidirectional in the plane of the halo

A halo is a half-wave dipole bent into a circle, usually mounted horizontally for horizontally polarized VHF/UHF work, often mobile. It radiates nearly equally in all directions in the plane of the halo.

• A. Broadside to the plane of the halo is straight up and down when it is mounted horizontally; the halo radiates mainly outward in its own plane.
• B. The halo pattern is nearly uniform around the loop; it is not concentrated opposite the feed point.
• D. The halo pattern is nearly uniform around the loop; it is not concentrated on the feed point side.

A. To enable multiband operation

A trap is a parallel LC circuit resonant on a higher band. On that band it acts as a high impedance that isolates the wire beyond it, so the inner section is resonant; on lower bands current passes through and the outer wire adds length. This lets one antenna work on several bands.

• B. Removing spurious signals is the job of filters such as low-pass filters; antenna traps are for multiband operation.
• C. A balanced feed point connection is provided by a balun, not a trap.
• D. Traps do not stop out-of-band transmission; staying within the bands is the operator's responsibility.

D. It narrows the main lobe in elevation

Stacking two horizontally polarized Yagis one above the other narrows the main lobe in the vertical (elevation) plane. The energy is concentrated at lower angles, which adds gain toward the horizon.

• A. Stacked horizontally polarized Yagis remain horizontally polarized; selecting polarization requires antennas with vertical elements as well.
• B. Stacking horizontally polarized Yagis does not add vertical polarization.
• C. Narrowing the main lobe in azimuth requires placing antennas side by side, not one above the other.

A. Wide bandwidth

A log-periodic antenna has elements of gradually changing length, and only the few elements near resonance are active at any frequency. That lets one antenna cover a wide frequency range, such as several HF bands, with fairly constant SWR and pattern.

• B. Because only a few elements are active at any frequency, a log-periodic has less gain per element than a Yagi.
• C. Because it works over a wide frequency range, a log-periodic radiates harmonics that fall within that range rather than suppressing them.
• D. A log-periodic has a single linear polarization set by its orientation; it does not provide polarization diversity.

A. Element length and spacing vary logarithmically along the boom

In a log-periodic antenna, both the element lengths and the spacing between elements change by a constant ratio from one element to the next along the boom, so they vary logarithmically. This structure is what gives it wide bandwidth.

• B. The electrical behavior of a log-periodic repeats with the logarithm of frequency, not with frequency itself, and its impedance changes only slightly across its design range.
• C. Gain stays roughly constant across the design range; it does not vary logarithmically with frequency.
• D. SWR depends on frequency, not on boom length, and stays fairly constant across the design range.

B. By varying the base loading inductance

A screwdriver antenna has a large base loading coil driven by a motor. Moving a contact along the coil changes how many turns are in the circuit, which changes the loading inductance and retunes the antenna to resonance with a matching feed point impedance.

• A. Body capacitance is not used to tune a screwdriver antenna; its motor varies the coil inductance.
• C. The whip itself keeps its length; the motor moves the coil section so that more or fewer turns are in the circuit.
• D. A capacitance hat is a fixed top-loading addition, not the adjustable mechanism of a screwdriver antenna.

A. Directional receiving for MF and low HF bands

A Beverage is a long wire, usually a wavelength or more, run low above the ground and terminated in a resistor at its far end. It is inefficient for transmitting but gives a directional pattern with good signal-to-noise ratio for receiving on MF and the low HF bands, such as 160 and 80 meters.

• B. A Beverage is too inefficient for transmitting; it is a receiving antenna.
• C. A Beverage is a fixed wire often hundreds of feet long, so it is not portable, and it is used on the lower bands.
• D. A Beverage is a fixed wire often hundreds of feet long, so it is not used for portable direction finding.

B. Broadside to the loop

An electrically small loop radiates and receives most strongly in the plane of the loop and has deep nulls broadside to it, along the axis through its center. Receiving loops use these sharp nulls to reject noise or find the direction of a signal.

• A. The plane of the loop is where the radiation is strongest, not where the nulls are.
• C. Only the broadside directions are nulls; the plane of the loop is the direction of maximum response.
• D. A small loop has deep broadside nulls, so it is not omnidirectional.

D. They have poor harmonic rejection

A multiband antenna is designed to radiate well on several frequencies, often harmonically related, such as 40, 20 and 10 meters. If the transmitter produces a harmonic that falls on another resonant band, the antenna radiates it efficiently instead of attenuating it.

• A. Multiband antennas are designed to present a usable impedance, often near 50 ohms, on each band, not a low impedance.
• B. Many multiband antennas, such as trap and fan dipoles, are matched on each design band and need no tuner.
• C. Many multiband antennas are fed with coaxial cable; open wire line is not required.

A. Inverted V

A dipole hung from a single central support, with its two halves sloping down toward the ground, forms an upside-down V and is called an inverted V. It needs only one tall support.

• B. An inverted L is a vertical wire with a horizontal section at the top, fed against ground, not a dipole on a central support.
• C. A sloper is a single wire slanting from a high point down toward the ground at one end.
• D. A lazy H is an array of two pairs of collinear elements stacked one above the other, not a single dipole.