CRAC Bilingual Manual: Antenna Systems and Matching | 天线系统与匹配 (42 questions)

The ideal operating state of an antenna system composed of a half-wave dipole and a transmission line should be:

1. Astanding waves only on the antenna, traveling waves only on the feed line.
2. Btraveling waves only on the antenna, standing waves only on the feed line.
3. Cstanding waves only on both the antenna and the feed line.
4. Dtraveling waves only on both the antenna and the feed line.

The end effect of antenna elements, phase shift of signals, etc., affect the speed of electromagnetic wave propagation along the element. When trimming the antenna, we should consider that the actual element length will therefore be slightly shorter than the calculated value. This is the so-called “shortening factor”. Its empirical value is:

1. A0.95
2. B1.05
3. C0.707
4. D1.414

To make a half-wave dipole antenna operating at frequency f (in MHz). The length of each element arm (in meters) is approximately:

1. A71.3 / f
2. B48.8 / f
3. C142.6 / f
4. D150 / f

The propagation speed of radio waves in a medium is lower than the speed of light; its ratio to the speed of light is called the “velocity factor”, the reciprocal of the square root of the dielectric constant. Polyethylene dielectric coaxial cable is the most commonly used. Its velocity factor is approximately:

1. A0.65
2. B1.54
3. C1.95
4. D1.0006

To build a certain phased antenna array operating at frequency f (in MHz), a coaxial cable of 1/4 wavelength is needed; its approximate length (in meters) should be:

1. A48.8 / f
2. B149.8 / f
3. C75 / f
4. D71.3 / f

If a dipole antenna is to have low input impedance when resonant at the operating frequency, the total length of the elements can be:

1. Aan odd multiple of 1/2 wavelength.
2. Ban integer multiple of 1/2 wavelength.
3. Can even multiple of 1/2 wavelength.
4. Dan odd multiple of 1/4 wavelength.

The necessary and sufficient condition for a dipole antenna to resonate at the desired operating frequency is:

1. Athe total electrical length of both arms is an integer multiple of 1/2 of the operating wavelength.
2. Bthe total electrical length of both arms is an integer multiple of 1/4 of the operating wavelength.
3. Cthe total electrical length of both arms is an odd multiple of 1/2 of the operating wavelength.
4. Dthe total electrical length of both arms is an integer multiple of the operating wavelength.

Which of the following resonant dipole antennas has peak gain in the direction perpendicular to the element?

1. Aelement length is 1/2 operating wavelength
2. Belement length is 1/8 operating wavelength
3. Celement length is 1/4 operating wavelength
4. Delement length is 3/2 operating wavelength

The method to raise a dipole antenna’s resonant frequency is:

1. Ashorten the elements a bit.
2. Bseries-connect a coil somewhere on the element.
3. Clengthen the elements a bit.
4. Dadd an “X”-shaped capacitance hat to the element.

Between the antenna and feed line a component commonly called a “balun” is often inserted. The origin of “balun” is:

1. Aa combination of the initial letters of “balanced” and “unbalanced”.
2. Bthe name of the person who invented the balanced-unbalanced converter.
3. Cthe name of a famous antenna impedance-matching theorist.
4. Dthe name of a major broadband matching-network manufacturer.

Between the antenna and feed line a component commonly called a “balun” is often inserted. Its main function is:

1. Ato transfer RF energy between a balanced circuit and an unbalanced circuit, blocking any parasitic coupling between the two.
2. Bto achieve automatic impedance matching between the antenna and the feed line.
3. Cto broaden the antenna’s operating bandwidth.
4. Dto lower the antenna’s standing-wave ratio.

A dipole whose two arms each have an electrical length of 1/4 wavelength, with the center broken and fed via a balun, measures resonant frequency f and input impedance Z. If the total length is unchanged but the break/feed point is shifted 1/8 wavelength to one side, the most obvious change in the antenna’s characteristics is:

1. Aimpedance Z increases significantly, while the resonant frequency f changes little.
2. Bimpedance Z decreases significantly, while the resonant frequency f changes little.
3. Cthe resonant frequency f rises significantly, while the impedance Z changes little.
4. Dthe resonant frequency f drops significantly, while the impedance Z changes little.

A horizontally polarized dipole oriented north–south, center-fed, connected via 50-ohm cable to a 50-ohm transceiver, with the correspondent to the east–west. The principle for choosing antenna length is:

1. Awhen each arm is a quarter wavelength, the communication effect is best.
2. Bwhen each arm is a half wavelength, the communication effect is best.
3. Cwhen each arm is three-quarters of a wavelength, the communication effect is best.
4. Dthe communication effect is best only when the SWR is close to 1:1, regardless of element length.

The launch (takeoff) angle of short-wave horizontal dipole antennas (including Yagi, etc.) is mainly determined by:

1. Ait is formed by the superposition of the antenna’s radiation and the ground reflection. The ratio of the antenna’s height above ground to the wavelength affects the angle.
2. Bit is determined by the direction the antenna element conductor points.
3. Cit is determined by the direction the Yagi’s boom points.
4. Dit is determined by the length of the antenna element.

When erecting a short-wave antenna, the launch angle should roughly be chosen according to:

1. Afor long-distance communication choose a low launch angle; for short-distance communication choose a high launch angle.
2. Bfor short-distance communication choose a low launch angle; for long-distance communication choose a high launch angle.
3. Cchoose a low launch angle when the surroundings are open, and a high launch angle when there are buildings nearby.
4. Dchoose a low launch angle for lower-frequency communication and a high launch angle for higher-frequency communication.

When erecting a short-wave antenna, the antenna height should roughly be planned according to:

1. Afor long-distance communication choose a greater height; for short-distance communication choose a lower height.
2. Bfor long-distance communication choose a lower height; for short-distance communication choose a greater height.
3. Cchoose a lower height when there are buildings nearby, and a greater height when the surroundings are open.
4. Dchoose a greater height for lower-frequency communication and a lower height for higher-frequency communication.

In DX communication aimed at a specific target, the basic method to calculate the antenna’s optimal launch angle is:

1. Abased on the approximate height of the ionosphere used, the approximate distance to the correspondent, and the number of ionospheric reflections the wave undergoes during propagation, calculate by simple geometric methods.
2. Bbased on the correspondent’s azimuth, Earth’s radius, the other station’s antenna height, the actual operating frequency, and the average sunspot number, calculate by table lookup.
3. Cbased on the correspondent’s azimuth, the elevation angle blocked by obstacles in the communication direction, the ground conductivity around your station, and the actual operating frequency, find a formula to calculate.
4. Dbased on both stations’ transmit power, antenna polarization, the elevation angle blocked by obstacles in the communication direction, and the solar 10.7 cm radio flux, find a formula to calculate.

The effect of ground conductivity around the transmitting/receiving antenna on long-distance HF communication is roughly:

1. AThe higher the ground conductivity, the more ideal the earth’s image (mirror) effect, and the better the antenna’s performance
2. BThe lower the ground conductivity, the weaker the induced current in the earth, and the better the antenna’s performance
3. CLong-distance HF communication relies on skywave reflection and is independent of ground conductivity
4. DGround conductivity that is too high or too low is bad; an intermediate value is best

If the height of a horizontal dipole antenna is lowered to 1/10 wavelength above ground, what change occurs in its feed-point impedance?

1. AIt decreases
2. BIt increases
3. CThe reactive component disappears
4. DThe resistive component disappears

On a good conducting ground, the main factor affecting the radiation elevation angle of a short-wave (HF) antenna is:

1. Athe ratio of the antenna’s height above ground to the wavelength
2. Bthe antenna’s absolute height, independent of wavelength
3. Cthe antenna’s absolute height above sea level
4. Dthe angle between the antenna element (or the Yagi boom) and the ground

When building your own HF dipole, if space limits prevent spreading the element to its natural resonant length at the operating frequency, you can insert a loading coil somewhere in the element, or make the whole element a coil, to build a kind of inductively loaded antenna and tune the inductance to get the desired resonant frequency. Which of the following descriptions of this antenna is correct?

1. ABy iteratively adjusting the coil’s inductance and its position in the element, you can certainly obtain a resonant frequency and feed-point impedance equivalent to those of a full-size antenna. However, the radiation efficiency of this loaded antenna is lower than that of the full-size antenna
2. BWhether in resonant frequency, impedance, or radiation efficiency, this loaded antenna cannot match a full-size antenna
3. CAs long as the design, construction, and adjustment are proper, such a loaded antenna performs as well as a full-size antenna
4. DIf properly designed, constructed, and adjusted, such a loaded antenna can surpass a full-size antenna

The figure shows a 14/21/29 MHz triband dipole commonly used by Chinese amateur radio operators for field-communication activities. The electrical lengths of the antenna segments should respectively be:

1. AA is 1/4 wavelength at 28 MHz, A+B is 1/4 wavelength at 21 MHz, and A+B+C is 1/4 wavelength at 14 MHz
2. BA is 1/2 wavelength at 28 MHz, A+B is 1/2 wavelength at 21 MHz, and A+B+C is 1/2 wavelength at 14 MHz
3. CA is 1/4 wavelength at 28 MHz, B is 1/4 wavelength at 21 MHz, and C is 1/4 wavelength at 14 MHz
4. DA is 1/2 wavelength at 28 MHz, B is 1/2 wavelength at 21 MHz, and C is 1/2 wavelength at 14 MHz [F]LK0941.jpg

The figure shows the 14/21/29 MHz triband dipole commonly used by Chinese amateur radio operators for field communication. The functions of components A, B, C and D are respectively:

1. AA and D are 21 MHz traps, B and C are 28 MHz traps — i.e., parallel-resonant circuits resonating at 21 MHz and 28 MHz respectively
2. BA and D are 21 MHz traps, B and C are 28 MHz traps — i.e., series-resonant circuits resonating at 21 MHz and 28 MHz respectively
3. CA and D are 14 MHz traps, B and C are 21 MHz traps — i.e., parallel-resonant circuits resonating at 14 MHz and 21 MHz respectively
4. DA and D are 14 MHz traps, B and C are 21 MHz traps — i.e., series-resonant circuits resonating at 14 MHz and 21 MHz respectively [F]LK0942.jpg

The element of a resonant dipole must be at least half a wavelength long. If erection space forces you to shorten the element, inserting inductance into the element compensates the lost reactance and resonates the antenna at the desired frequency. To raise transmit efficiency, the best coil position depends on the site. The figure shows three loading schemes. Assuming the (grey) elements are equal in length, the transmit efficiency ranking of schemes A, B and C is:

1. AC = top loading, B = center loading, A = base loading
2. BA = base loading, B = center loading, C = top loading
3. CA = base loading, C = top loading, B = center loading
4. DB = center loading, A = base loading, C = top loading [F]LK0943.jpg

For amateur services that must change bands frequently, operators sometimes parallel the dipoles of the used bands and feed them together, making a multi-band antenna operable with a single cable. The figure shows an example built from 7, 14 and 28 MHz half-wave elements. Which of the following analyses of its working principle is correct?

1. AThe 7 MHz element operates as 3/2 wavelength at 21 MHz, so the antenna works on the 7, 14, 21 and 28 MHz bands. Because the non-resonant elements present high impedance, the total parallel impedance remains about 50 Ω
2. BThe antenna works on 7, 14 and 28 MHz. Because the non-resonant elements present high impedance, the total parallel impedance remains about 50 Ω
3. CThe 7 MHz element operates as 3/2 wavelength at 21 MHz, so the antenna works on the 7, 14, 21 and 28 MHz bands. Because each element’s impedance is about 50 Ω, the total parallel impedance drops to about 17 Ω
4. DThe antenna works on 7, 14 and 28 MHz. Because each element’s impedance is about 50 Ω, the total parallel impedance drops to about 17 Ω [F]LK0945.jpg

What is a Yagi antenna (八木天线)?

1. Aa half-wave dipole antenna that enhances directivity through a reflector and directors
2. Ba general term for any directional antenna that can focus radio waves
3. Can omnidirectional antenna invented by Dr. Hidetsugu Yagi
4. Da center-fed antenna with eight horizontal elements, also called Octopus

A Yagi is a directional antenna made by adding directors and a reflector to a normal dipole (driven element). The visual method to determine the direction of maximum radiation of a full-size Yagi is:

1. AThe element shorter than the driven element is the director, the longer one is the reflector, and the directors point toward the direction of maximum radiation
2. BThe element longer than the driven element is the director, the shorter one is the reflector, and the directors point toward the direction of maximum radiation
3. CThe element shorter than the driven element is the director, the longer one is the reflector, and the reflector points toward the direction of maximum radiation
4. DThe axis of the driven element is the direction of maximum radiation

How is the front-to-back ratio (前后比) of a Yagi antenna defined?

1. Athe ratio of the main-lobe radiated power to the reverse-direction radiated power
2. Bthe ratio of the forward gain to the gain of an ideal point-source antenna
3. Cthe ratio of the number of directors to the number of reflectors
4. Dthe ratio of the number of directors to the number of driven elements

A traveling-wave antenna radiates as RF current travels along a continuously matched long-wire structure. Common amateur traveling-wave antennas are the T2FD and the Beverage (贝伐列奇). Compared with standing-wave antennas, the characteristic of traveling-wave antennas is:

1. Ano natural resonant point; length requirements are not strict but it usually needs to be about or greater than one wavelength
2. Bstrict requirements on construction dimensions; no natural resonant point
3. Ca series of natural resonant points over a wide band; strict length requirement
4. Dcan achieve wideband matching with an antenna tuner; no strict length requirement

Traveling-wave antennas commonly used in amateur radio, such as the T2FD and the Beverage (贝伐列奇), have a non-inductive resistor at the antenna termination. The correct statement about the function of this resistor is:

1. AWhen the electrical length reaches several wavelengths, most of the energy is radiated to space as it travels, and only a small part enters the terminating resistor and is dissipated, avoiding reflection that would cause standing waves
2. BThe resistor acts as a dummy load; although it enables broadband operation, it dissipates most of the energy
3. CRegardless of length, half the input power is dissipated in the resistor, giving only 50% efficiency
4. DSince most traveling-wave antennas are resonant, the series resistor lowers the loaded Q, broadening the response but reducing efficiency

Xiaoming connects a coil at the midpoint of a 20 m wire and lifts the wire horizontally to a hanging position slightly above 20 m. The other end of the coil connects to another 20 m wire that drops vertically toward the ground. He connects the feed-line center conductor to the vertical wire and the feed-line shield to the ground plane. What is the polarization of this T-shaped antenna?

1. Avertical
2. Bhorizontal
3. Chalf vertical and half horizontal
4. Dthe overhead beam is circularly polarized (rotating)

To lower the radiation elevation angle, Xiaoming hangs half of the T2FD dipole vertically. He connects the folded-dipole feed end via a matching transformer to the cable center conductor, and connects the end with the absorbing resistor to the ground plane and the cable shield. The polarization of this antenna is:

1. Avertical
2. Bhorizontal
3. Cright-hand circular
4. Dleft-hand circular

Which of the following statements about a loop antenna is correct?

1. AThe electromagnetic wave emitted by this antenna has its magnetic-field component perpendicular to the plane of the loop
2. BA naturally resonant loop has a radius exactly equal to 1/4 wavelength
3. CThis antenna has a very high radiation angle and is only suitable for short-range low-frequency contacts
4. DInserting a variable capacitor in the loop can significantly cancel the inductive reactance and broaden the operating bandwidth

If the space for erecting an antenna is limited, a magnetically coupled tunable loop antenna may be an option. Which of the following statements about this antenna is correct? (Choose all that apply.)

1. ABy adjusting the sizes of the driven and resonant elements, you can obtain an input SWR close to 1:1
2. BThe variable capacitor in the resonant element should be rated for high voltage, so it can handle somewhat higher input power
3. CThe larger the resonant element, the higher the radiation efficiency, but the highest tunable frequency is also lower
4. DThis antenna is insensitive to the electric field, loses half the electromagnetic energy, and has very low theoretical gain

During field operations or emergency-communication drills, amateurs may find that contacting HF stations 100–200 km away is trickier than contacting those 300–500 km away. Fortunately many techniques help; the simpler ones are: (Choose all that apply.)

1. ATry lower bands, e.g., 40 m by day and 80 m by night
2. BUse a low-mounted horizontal dipole or an end-fed sloper
3. CInstall a reflector below the dipole to direct the main lobe straight up
4. DUse a high-efficiency magnetically coupled resonant loop within 1 m diameter

What is the function of a transceiver-built-in or connected antenna tuner (天线调谐器, colloquially 天调)?

1. ATransform the input impedance of the antenna-feeder system into the load impedance required by the transmitter
2. BBy achieving conjugate matching, improve the overall efficiency of the antenna-feeder system
3. CAllow an antenna to resonate whether used for transmitting or receiving
4. DAutomatically switch to a suitable antenna according to the transmitter’s current band

Regarding the claim that an antenna tuner (天调) improves contacts, which of the following descriptions is correct?

1. AThe tuner can compensate the parameters of a mismatched antenna system so the transmitter delivers its specified RF power
2. BThe tuner achieves overall resonance of the antenna system, greatly raising the element’s radiation efficiency
3. CThe tuner’s compensation eliminates the SWR within the system, lowering transmission loss
4. DThe tuner dissipates the energy reflected by the mismatched antenna as heat; contacts actually get worse

An antenna is connected via a 50 Ω feed line to a 50 Ω-output transceiver. You plan to insert an antenna tuner and a through-line SWR/power meter into the antenna circuit to compensate and monitor the mismatch. The theoretically best connection order is:

1. Aantenna – antenna tuner – SWR power meter – feed line – transceiver
2. Bantenna – feed line – antenna tuner – SWR power meter – transceiver
3. Cantenna – antenna tuner – feed line – SWR power meter – transceiver
4. Dantenna – SWR power meter – antenna tuner – feed line – transceiver

A tower-top antenna is connected via 50 Ω feed line to a 50 Ω-output transceiver. You must insert an antenna tuner (ATU) and a through-line SWR power meter (M) to compensate and monitor the match. Four schemes: 1) ATU and M both at the tower top; 2) ATU and M both at the tower base; 3) ATU at tower base, M in the shack; 4) ATU and M both in the shack. Once tuned, ranked from highest to lowest overall antenna-system efficiency, the schemes are:

1. AScheme 1 best, schemes 2 and 3 next, scheme 4 worst
2. BScheme 4 best, schemes 2 and 3 next, scheme 1 worst
3. CScheme 3 best, scheme 4 next, scheme 2 next, scheme 1 worst
4. DScheme 2 best, scheme 1 next, scheme 4 next, scheme 3 worst [F]LK0938.jpg

Footnote 5.67A of the Radio Frequency Allocation Regulations of the PRC (《中华人民共和国无线电频率划分规定》) says: amateur-service stations using frequencies in 135.7–137.8 kHz shall have a maximum effective isotropically radiated power (EIRP) not exceeding 1 W. This means that amateur stations in this band:

1. AThe actual power fed to the transmitting antenna must not exceed the value obtained by dividing 1 W by the antenna’s absolute gain ratio Gi
2. BThe transmitter’s rated output power must not exceed 1 W
3. CThe power actually fed to the transmitting antenna after deducting feed-line loss must not exceed 1 W
4. DThe power actually radiated by the antenna after deducting total feed-line and antenna loss must not exceed 1 W

An antenna has a gain of 3.85 dBd and 10 W is fed into it. Its equivalent isotropically radiated power (EIRP) is:

1. A40 W
2. B13.85 W
3. C38.5 W
4. D100 W

An antenna has a gain of 5.15 dBi and 10 W is fed into it. Its effective radiated power (ERP) is:

1. A20 W
2. B51.5 W
3. C10 W
4. D15.15 W