T9 Antennas and Feed Lines

C. An antenna that concentrates signals in one direction

A beam antenna, such as a Yagi or a quad, concentrates its signal in one direction. This gives gain toward the front and reduces signals to the sides and back.

• A. The name refers to concentrating signals into a beam, not to the shape of the metal used to build the antenna.
• B. A beam antenna is directional, the opposite of omnidirectional, and the name describes its pattern, not an inventor.
• D. A beam antenna concentrates its signal in one main direction, not into two rays.

A. Electrically lengthening by inserting inductors in radiating elements

Loading makes an antenna that is physically shorter than its resonant length act electrically longer. Inserting inductors (loading coils) in the radiating elements brings a short antenna to resonance at the desired frequency.

• B. A resistor turns power into heat; resonance comes from inductance and capacitance, not resistance.
• C. A spring at the base is a mechanical feature that lets the antenna flex; it is not antenna loading.
• D. Strengthening the elements is mechanical reinforcement; it does not change the antenna's electrical length.

B. By the orientation of the electric field

An antenna's polarization is described by the orientation of the electric field it radiates. A vertical antenna produces a vertical electric field (vertical polarization), and a horizontal dipole produces horizontal polarization.

• A. The orientation of the driven element sets the polarization, but polarization is described by the electric field, not by the element's shape.
• C. The magnetic field is at right angles to the electric field; by convention, polarization is named for the electric field.
• D. The direction of radiation describes the antenna's pattern, not its polarization.

A. It has low efficiency

The short flexible antenna on a handheld (often called a rubber duck) is much shorter than a full quarter wavelength and uses loading to reach resonance, so it radiates less of the transmitter's power than a full-size quarter-wave antenna; it has low efficiency.

• B. A handheld's flexible antenna is linearly polarized (vertical when the radio is held upright), not circularly polarized.
• C. Receiver desensitization comes from strong nearby signals, not from the length of the antenna.
• D. An antenna works the same way for analog and digital signals.

C. Shortening it

A dipole's resonant frequency is inversely proportional to its length: frequency (MHz) is about 468 / length (feet). Shortening the dipole raises the frequency; for example, 468 / 66 ft = 7.09 MHz, while 468 / 64 ft = 7.31 MHz.

• A. Lengthening a dipole lowers its resonant frequency.
• B. Coils in series with the radiating wires (inductive loading) make the antenna electrically longer, which lowers its resonant frequency.
• D. Capacitive loading at the ends of the wires also makes the antenna electrically longer and lowers its resonant frequency.

D. Yagi

A Yagi combines a driven element with parasitic elements (a reflector and one or more directors) that concentrate its signal in one direction. That directivity gives it more gain than the other antennas listed, which spread their signal more widely.

• A. A 5/8-wave vertical has modest gain toward the horizon but radiates equally in all horizontal directions, so it has less gain than a Yagi.
• B. An isotropic antenna is a theoretical reference that radiates equally in all directions; by definition its gain is 0 dBi.
• C. A J-pole is an end-fed half-wave antenna with gain similar to a dipole, far less than a Yagi.

A. Signal strength is reduced due to the shielding effect of the vehicle

A vehicle's metal body acts as a shield, so a handheld used inside it loses much of its signal going out and coming in. An externally mounted antenna avoids this shielding loss.

• B. The signal loss comes from the vehicle's shielding, not from a change in antenna bandwidth.
• C. A lower SWR does not reduce signal strength; the loss comes from the vehicle's shielding.
• D. The handheld does not overheat from reflected power inside the vehicle; the problem is the shielding.

C. It is a resonant quarter-wave

A quarter-wave vertical is about 234 / f (MHz) feet long. At 146 MHz that is 234 / 146 = 1.60 ft, or about 19.2 inches, so a 19-inch whip is a resonant quarter-wave antenna on 2 meters.

• A. A quarter-wave vertical is a simple resonant antenna, not a high-gain one.
• B. A half-wave antenna on 2 meters is about 468 / 146 = 3.2 ft, or about 38 inches, twice as long.
• D. The 19-inch length is chosen for resonance, not for reduced RF exposure.

A. It has more gain

A 5/8-wavelength whip concentrates more of its radiation at low angles toward the horizon, giving more gain than a 1/4-wave whip in the directions where other mobile stations and repeaters are.

• B. A 5/8-wave whip radiates at a lower angle than a 1/4-wave whip, not a higher one.
• C. A 5/8-wave whip needs a matching coil at its base to achieve a good SWR, so lower SWR is not its advantage.
• D. Lower impedance is not the advantage; the 5/8-wave whip's impedance is matched to the feed line with a base coil.

D. Broadside to the antenna

A half-wave dipole radiates most strongly broadside, at right angles to the wire, and very little off its ends. Seen from above, its pattern looks like a figure eight.

• A. Radiating equally in all directions describes a theoretical isotropic antenna, not a dipole.
• B. A dipole radiates the least off its ends.
• C. The feed line does not set the pattern; ideally it runs away from the dipole at a right angle so it does not disturb the pattern.

C. The increase in signal strength in a specified direction compared to a reference antenna

Antenna gain is the increase in signal strength in a specified direction compared to a reference antenna, such as an isotropic antenna (dBi) or a dipole (dBd). Gain comes from concentrating power in some directions at the expense of others; the antenna does not create power.

• A. An antenna cannot add power to the transmitter's output; gain comes from concentrating that power in a chosen direction.
• B. Gain is not extra power needed at higher frequencies; it describes how strongly the signal is concentrated in a direction.
• D. Gain is about signal strength in a direction, not an increase in impedance.

D. All these choices are correct

PL-259, BNC and Type N connectors can all let water in when used outdoors, so all should be sealed and taped for weather protection. Water in a connector corrodes it and can travel into and ruin the attached cable.

• A. PL-259 connectors need weather protection outdoors, but they are only part of the answer.
• B. BNC connectors need weather protection outdoors, but they are only part of the answer.
• C. Type N connectors have a gasket but still need weather protection outdoors; they are only part of the answer.

B. 50 ohms

Most coaxial cable used in amateur radio, such as RG-58, RG-8 and RG-213, has a characteristic impedance of 50 ohms, which matches the output impedance of amateur transceivers and amplifiers.

• A. 8 ohms is a typical loudspeaker impedance, not a coaxial cable impedance.
• C. 600 ohms is the impedance of some audio lines and open-wire feed lines, not common coax.
• D. 12 ohms is not a standard coaxial cable impedance.

A. It is easy to use and requires few special installation considerations

Coax is easy to use: the signal stays inside the shield, so the cable can be run near metal, taped to a tower or buried with few special precautions, unlike open-wire or ladder line.

• B. Coax does not have less loss than every other feed line; open-wire line and hardline have less.
• C. Coax does not handle more power than every other feed line; large hardline and open-wire line can handle more.
• D. Coax is not the least expensive feed line; twin-lead and ladder line often cost less.

A. It matches the antenna system impedance to the transceiver's output impedance

An antenna tuner (antenna coupler) transforms the impedance of the antenna system to match the transceiver's output impedance, usually 50 ohms, so the transmitter sees a good match and can deliver its full power.

• B. An antenna tuner does not tune in stations; the receiver's tuning controls do that.
• C. Switching the antenna between transmit and receive is done by the transceiver's own transmit-receive switching.
• D. Selecting the right antenna for each band is the job of an antenna switch, not a tuner.

D. The loss increases

Loss in coaxial cable increases as frequency increases, because conductor loss (from skin effect) and dielectric loss both rise with frequency. A length of coax that works well on HF can be quite lossy at UHF.

• A. The characteristic impedance is set by the cable's dimensions and dielectric and stays essentially constant with frequency.
• B. Loss increases with frequency; it does not decrease.
• C. The characteristic impedance stays essentially constant; it does not rise with frequency.

B. Type N

Type N connectors are constant-impedance, weather-resistant connectors that keep losses low into the gigahertz range, which makes them suitable for frequencies above 400 MHz.

• A. The PL-259 is not a constant-impedance connector and its performance falls off at UHF; it is mainly used at HF and VHF.
• C. RS-213 is not an RF connector; the name resembles RG-213, which is a type of coaxial cable.
• D. A DB-25 is a 25-pin connector used for computer data ports, not for RF.

C. They are commonly used at HF and VHF frequencies

The PL-259, the plug of the so-called UHF connector series, is widely used on HF and VHF amateur equipment. It is inexpensive and rugged, but it is not constant-impedance, so it is not suited to the higher UHF and microwave frequencies.

• A. PL-259 connectors are not suited to microwave use; connectors such as Type N and SMA are used there.
• B. PL-259 connectors are not watertight and must be sealed for outdoor use.
• D. The PL-259 has a threaded coupling ring; the BNC is the common bayonet-type connector.

D. All these choices are correct

Water intrusion increases cable loss, high SWR increases the loss in the feed line, and each connector adds a small loss, so all of these reduce the power that reaches the antenna.

• A. Water intrusion into coaxial connectors increases loss, but it is only part of the answer.
• B. High SWR increases feed line loss, but it is only part of the answer.
• C. Each connector adds some loss, but multiple connectors are only part of the answer.

B. Loose connection in the antenna or feed line

A loose connection in the antenna or feed line changes the impedance unpredictably, for example as wind moves the antenna or cable, so the SWR reading jumps around.

• A. A thunderstorm brings static and lightning danger, but it does not by itself cause erratic SWR readings.
• C. Over-modulation causes distortion and splatter, not erratic SWR.
• D. Overload from a strong local station is a receiver problem and does not affect SWR.

C. RG-213 cable has less loss at a given frequency

RG-213 is a larger cable than RG-58, with a thicker center conductor and dielectric, so it has less loss at a given frequency and can handle more power.

• A. The two cables differ significantly in loss and power handling.
• B. Both cables normally have a single braided shield; the difference is in size and loss.
• D. RG-58 is the smaller cable and handles less power than RG-213.

C. Air-insulated hardline

Air-insulated hardline has a solid or corrugated outer conductor and uses air, with small spacers, as most of its dielectric, so it has lower loss than flexible coax with solid or foam dielectric.

• A. 50-ohm flexible coax has more loss because of its solid or foam dielectric and braided shield.
• B. Multi-conductor unbalanced cable is not designed as a low-loss RF feed line.
• D. 75-ohm flexible coax has losses similar to 50-ohm flexible coax, more than air-insulated hardline.

A. A measure of how well a load is matched to a transmission line

SWR (standing wave ratio) measures how well a load, such as an antenna, is matched to the transmission line feeding it. A perfect match gives 1:1, and higher numbers mean a greater mismatch.

• B. The ratio of output power to input power is gain, not SWR.
• C. Transmitter efficiency compares RF output power to the power drawn from the supply; it is not SWR.
• D. SWR describes the match between line and load; it says nothing about the quality of the station's ground connection.