I85PREP
⌘ K
CWT study materials
CWT study resources

CWT practice questions

One question at a time. Build your CWT understanding.

First 50 of 200 questions. Each one lists the choices, the correct answer, and an explanation. The checker at the top of this page saves progress in this browser when it is available.

Continue in the practice tool above to load more questions, 50 at a time.

Question 1RF Fundamentals

A radio signal at 150 MHz is propagating in free space. What is its approximate wavelength?

  1. 0.5 meters
  2. 20 meters
  3. 2 meters Correct answer.
  4. 150 meters

Explanation

Wavelength equals the speed of light divided by frequency, so 300 divided by 150 MHz gives 2 meters. The shortcut lambda (in meters) = 300 / f (in MHz) is standard for free-space propagation.

Question 2RF Fundamentals

Which unit is used to express transmitter power in the RF domain?

  1. Decibel-milliwatt (dBm) Correct answer.
  2. Siemens per meter (S/m)
  3. Henry (H)
  4. Volt-ampere reactive (VAR)

Explanation

dBm is a logarithmic power unit referenced to 1 milliwatt and is the standard way to state RF power levels. The other units describe reactive power, conductivity, and inductance respectively.

Question 3RF Fundamentals

A signal's power is doubled. How many decibels of gain does this represent?

  1. 6 dB
  2. 2 dB
  3. 10 dB
  4. 3 dB Correct answer.

Explanation

A doubling of power equals 10 log10(2), which is approximately 3 dB. This 3 dB rule is a core reference point for reading link budgets and gain stages.

Question 4RF Fundamentals

Which of these is a common coaxial connector used on handheld and mobile VHF/UHF radio equipment?

  1. PL-259/SO-239 Correct answer.
  2. SC/APC
  3. RJ-45
  4. DB-9

Explanation

The PL-259 plug and SO-239 socket are UHF-style coaxial connectors widely used on VHF/UHF amateur and land mobile gear. The other options are data, serial, and fiber-optic connectors.

Question 5RF Fundamentals

What does a standing wave ratio (SWR) of 1:1 indicate on a transmission line?

  1. The line is open at the far end
  2. The transmitter is overdriven
  3. The load is perfectly matched to the line impedance Correct answer.
  4. The antenna is disconnected

Explanation

An SWR of 1:1 means no reflected wave exists, which happens only when the load impedance equals the line's characteristic impedance. Any mismatch produces a ratio greater than 1:1.

Question 6RF Fundamentals

Which layer of the atmosphere supports long-distance HF skywave communication?

  1. The troposphere
  2. The mesosphere
  3. The stratosphere
  4. The ionosphere Correct answer.

Explanation

The ionosphere contains ionized layers that refract HF signals back toward Earth, enabling beyond-line-of-sight communication. The lower atmospheric layers do not refract HF in this way.

Question 7RF Fundamentals

What is the primary purpose of a dummy load when testing a transmitter?

  1. To convert RF energy into a usable DC supply
  2. To absorb RF power while presenting a matched load without radiating Correct answer.
  3. To filter harmonics from the output
  4. To amplify the signal before it reaches the antenna

Explanation

A dummy load is a non-radiating resistive load, typically 50 ohms, that lets you test a transmitter without sending a signal over the air. It also provides a good match so the transmitter sees a low SWR.

Question 8RF Fundamentals

Which frequency band lies between 30 MHz and 300 MHz?

  1. UHF
  2. HF
  3. VHF Correct answer.
  4. SHF

Explanation

The very high frequency band spans 30 to 300 MHz, covering services such as FM broadcast and land mobile radio. HF ends at 30 MHz, while UHF begins at 300 MHz.

Question 9RF Fundamentals

A transmitter delivers 100 watts into a 50-ohm load. What is the approximate RMS voltage across the load?

  1. 71 volts Correct answer.
  2. 100 volts
  3. 50 volts
  4. 141 volts

Explanation

Using P = V squared over R, V equals the square root of (100 times 50), which is about 70.7 volts RMS. Rounding gives 71 volts.

Question 10RF Fundamentals

An amplifier has 10 dB of gain followed by a cable with 3 dB of loss. What is the net gain?

  1. 7 dB Correct answer.
  2. 13 dB
  3. 3.3 dB
  4. 30 dB

Explanation

Decibel gains and losses add algebraically along a signal chain, so 10 dB minus 3 dB yields 7 dB net. This additive property makes dB convenient for link budgets.

Question 11RF Fundamentals

What characteristic impedance is most commonly used for coaxial cable in RF systems?

  1. 75 ohms
  2. 600 ohms
  3. 50 ohms Correct answer.
  4. 300 ohms

Explanation

Fifty ohms is the near-universal standard for RF transmission lines and equipment because it balances power handling and low loss. Seventy-five ohms is used mainly for video and broadcast distribution.

Question 12RF Fundamentals

A half-wave dipole is cut for 14.2 MHz. What is its approximate total length in free space?

  1. About 40 meters
  2. About 10 meters Correct answer.
  3. About 5 meters
  4. About 20 meters

Explanation

A half-wave dipole is roughly 143 divided by frequency in MHz, giving about 10 meters at 14.2 MHz. This accounts for the half-wavelength plus the end-effect shortening factor.

Question 13RF Fundamentals

Which phenomenon causes VHF and UHF signals to travel beyond the radio horizon?

  1. Ionospheric skip
  2. Ground wave propagation
  3. Meteor scatter only
  4. Tropospheric refraction and ducting Correct answer.

Explanation

Temperature and humidity gradients in the troposphere can bend VHF and UHF signals, sometimes trapping them in ducts that extend range well past the horizon. The ionosphere generally does not refract these higher frequencies.

Question 14RF Fundamentals

What does a return loss of 20 dB indicate about a transmission line termination?

  1. The line is shorted
  2. Only about 1 percent of the incident power is reflected Correct answer.
  3. All power is reflected
  4. Half of the incident power is reflected

Explanation

Return loss of 20 dB means reflected power is 20 dB below incident power, which is about 1 percent. This corresponds to a well-matched load with a low SWR.

Question 15RF Fundamentals

Which instrument displays signal amplitude across a range of frequencies?

  1. Frequency counter
  2. Multimeter
  3. Spectrum analyzer Correct answer.
  4. Oscilloscope

Explanation

A spectrum analyzer plots amplitude versus frequency, revealing harmonics, spurious emissions, and occupied bandwidth. An oscilloscope instead shows amplitude versus time.

Question 16RF Fundamentals

What happens to the characteristic impedance of a coaxial cable as the ratio of inner conductor diameter to shield diameter increases?

  1. It becomes purely reactive
  2. It decreases
  3. It stays constant
  4. It increases Correct answer.

Explanation

Characteristic impedance rises with the logarithm of the shield-to-center-conductor diameter ratio. A larger ratio therefore yields higher impedance for a given dielectric.

Question 17RF Fundamentals

A directional coupler in a wattmeter is used to sample forward and reflected power. What does a high reflected-power reading most directly indicate?

  1. A perfectly tuned antenna
  2. A significant impedance mismatch at the load Correct answer.
  3. Excessive transmit frequency drift
  4. A faulty power supply

Explanation

Reflected power arises when the load impedance differs from the line impedance, so a high reading signals mismatch. The forward-to-reflected ratio is what SWR quantifies.

Question 18RF Fundamentals

Which type of antenna radiates and receives equally well in all horizontal directions?

  1. Parabolic dish
  2. Omnidirectional vertical Correct answer.
  3. Corner reflector
  4. Yagi-Uda beam

Explanation

A vertical antenna with a symmetric ground plane has a circular horizontal pattern, giving omnidirectional coverage. The beam, dish, and reflector antennas concentrate energy in one direction.

Question 19RF Fundamentals

A 50-ohm system carries 25 watts. What is the approximate peak envelope voltage for a continuous sine wave?

  1. 25 volts
  2. 100 volts
  3. 35 volts
  4. 50 volts Correct answer.

Explanation

RMS voltage is the square root of (25 times 50), about 35.4 volts, and peak voltage is that times the square root of two, about 50 volts. This distinction matters for component voltage ratings.

Question 20RF Fundamentals

Why is a low-pass filter often placed at the output of a transmitter?

  1. To boost the fundamental signal level
  2. To attenuate harmonic emissions above the operating frequency Correct answer.
  3. To convert the signal to a different band
  4. To increase the antenna's resonant frequency

Explanation

A low-pass filter passes the fundamental while suppressing harmonics that could cause interference. This improves spectral purity and helps meet emission limits.

Question 21RF Fundamentals

What is the main advantage of a folded dipole over a simple half-wave dipole?

  1. It eliminates the need for a feed line
  2. It provides a higher feed-point impedance and broader bandwidth Correct answer.
  3. It radiates a circularly polarized wave
  4. It operates without a ground reference at any frequency

Explanation

The folded dipole has roughly four times the feed-point impedance of a simple dipole, near 300 ohms, and offers wider bandwidth. This makes it useful for matching to certain feed lines.

Question 22RF Fundamentals

Two equal in-phase signals each at 0 dBm are combined in an ideal lossless power combiner. What is the resulting output power?

  1. 1 dBm
  2. 6 dBm
  3. 0 dBm
  4. 3 dBm Correct answer.

Explanation

Doubling power adds 3 dB, so two 0 dBm signals combine to 3 dBm. The 3 dB rule follows directly from 10 log10(2).

Question 23RF Fundamentals

A transmission line has a velocity factor of 0.66. What is the physical length of a quarter-wavelength section at 100 MHz?

  1. About 0.5 meters Correct answer.
  2. About 1.5 meters
  3. About 0.75 meters
  4. About 3 meters

Explanation

Free-space wavelength at 100 MHz is 3 meters; a quarter wavelength is 0.75 meters, and multiplying by the 0.66 velocity factor gives about 0.5 meters. Velocity factor shortens the physical length relative to free space.

Question 24RF Fundamentals

An antenna has 6 dBi gain. Approximately how much more power does it radiate in its main direction compared with an isotropic radiator fed the same power?

  1. About ten times
  2. About twice
  3. About six times
  4. About four times Correct answer.

Explanation

Gain in dBi compares to an isotropic source, and 6 dB equals two doublings, or a factor of about four. Each 3 dB represents a doubling of power density.

Question 25RF Fundamentals

A receiver has a noise figure of 3 dB and a gain stage ahead of it. Why does placing a low-noise amplifier first improve overall system noise figure?

  1. Noise figure is independent of stage order
  2. Friis' formula weights the first stage's noise contribution most heavily Correct answer.
  3. The amplifier removes noise already present in the signal
  4. Later stages always have lower gain

Explanation

Friis' formula shows that the noise contribution of each stage is divided by the cumulative gain of preceding stages, so the first stage dominates. A low-noise, high-gain first stage therefore minimizes added system noise.

Question 26RF Fundamentals

Which ITU frequency band designation covers the range 3 MHz to 30 MHz?

  1. MF
  2. HF Correct answer.
  3. VHF
  4. UHF

Explanation

The high frequency (HF) band spans 3 MHz to 30 MHz, corresponding to wavelengths of 100 m down to 10 m. This range supports both ground-wave and skywave propagation, which is why it is heavily used for long-distance amateur and maritime communication.

Question 27RF Fundamentals

Which ionospheric layer is most responsible for absorbing HF signals during daytime propagation?

  1. E layer
  2. F1 layer
  3. D layer Correct answer.
  4. F2 layer

Explanation

The D layer exists only during daylight and sits at the lowest altitude, so signals passing through it are absorbed rather than refracted. This absorption is strongest at lower HF frequencies, which is why daytime communication favors higher bands.

Question 28RF Fundamentals

What is the free-space wavelength of a 300 MHz signal?

  1. 10 meters
  2. 3 meters
  3. 1 meter Correct answer.
  4. 0.3 meters

Explanation

Wavelength equals the speed of light divided by frequency, so 300,000,000 m/s divided by 300,000,000 Hz gives 1 meter. This relationship makes 300 MHz a convenient reference point for estimating antenna dimensions.

Question 29RF Fundamentals

Which propagation mode relies on signals bending around the curvature of the Earth near the surface?

  1. Ground wave Correct answer.
  2. Skywave
  3. Sporadic-E
  4. Tropospheric scatter

Explanation

Ground waves travel along the Earth's surface and follow its curvature through diffraction, which is effective at MF and lower HF frequencies. Higher frequencies suffer greater ground loss and rely on other mechanisms.

Question 30RF Fundamentals

What does the term 'skip distance' describe in HF propagation?

  1. The spacing between successive ionospheric layers
  2. The minimum distance from the transmitter where the skywave returns to Earth Correct answer.
  3. The distance at which the ground wave fully attenuates
  4. The total path length of a two-hop signal

Explanation

Skip distance is the shortest ground distance at which a skywave refracted by the ionosphere returns to Earth for a given frequency and ionospheric condition. Inside that zone, only the ground wave is receivable, producing a characteristic skip zone.

Question 31RF Fundamentals

Which unit expresses antenna gain relative to an isotropic radiator?

  1. dBc
  2. dBd
  3. dBi Correct answer.
  4. dBm

Explanation

The dBi unit expresses gain referenced to a theoretical isotropic radiator that radiates equally in all directions. Because a dipole has about 2.15 dB of gain over isotropic, dBi values are always numerically higher than the corresponding dBd values.

Question 32RF Fundamentals

What is the primary function of a balun in an antenna feed system?

  1. To filter out harmonic radiation
  2. To transform between balanced and unbalanced transmission lines Correct answer.
  3. To measure standing wave ratio
  4. To convert AC mains power to a DC supply

Explanation

A balun performs the balanced-to-unbalanced transition, allowing a balanced antenna such as a dipole to connect to unbalanced coaxial feedline. This prevents common-mode current from flowing on the outside of the coax shield.

Question 33RF Fundamentals

Which term describes the random electrical noise generated by the thermal motion of electrons in a resistor?

  1. Flicker noise
  2. Shot noise
  3. Intermodulation noise
  4. Johnson noise Correct answer.

Explanation

Johnson noise, also called thermal noise, arises from random electron motion and has power proportional to absolute temperature and bandwidth. It sets the fundamental noise floor for a receiver front end.

Question 34RF Fundamentals

A signal at 100 watts is fed into a 50-ohm line. What is the approximate peak voltage of the sine wave?

  1. 200 V
  2. 100 V Correct answer.
  3. 50 V
  4. 70 V

Explanation

RMS voltage equals the square root of power times resistance, giving about 70.7 V for 100 W into 50 ohms. The peak value is RMS multiplied by the square root of two, which is approximately 100 V.

Question 35RF Fundamentals

What is the standing wave ratio of a 50-ohm line terminated in a pure 100-ohm resistive load?

  1. 1.0:1
  2. 1.5:1
  3. 4.0:1
  4. 2.0:1 Correct answer.

Explanation

For a purely resistive mismatch, SWR equals the ratio of the larger to the smaller resistance, here 100 divided by 50. The result is 2.0:1, indicating that one quarter of the incident power is reflected.

Question 36RF Fundamentals

What is the reflection coefficient magnitude for a 50-ohm line terminated in 150 ohms?

  1. 0.25
  2. 0.67
  3. 0.5 Correct answer.
  4. 0.33

Explanation

The reflection coefficient equals (ZL minus Z0) divided by (ZL plus Z0), which here is 100 divided by 200. That gives a magnitude of 0.5, equivalent to an SWR of 3:1.

Question 37RF Fundamentals

A receiver has a 3 dB noise figure. What does this mean in terms of its noise performance relative to an ideal noiseless receiver?

  1. It doubles the received signal level
  2. It adds as much noise as a perfect receiver's thermal noise at room temperature Correct answer.
  3. It attenuates the signal by 3 dB
  4. It halves the bandwidth

Explanation

A noise figure of 3 dB means the receiver's output noise is twice what an ideal noiseless receiver would produce, equivalent to adding one times the reference thermal noise. This directly degrades the achievable signal-to-noise ratio.

Question 38RF Fundamentals

Two tones at 10 MHz and 11 MHz pass through a nonlinear amplifier. Which third-order intermodulation product falls closest to the original signals?

  1. 12 MHz
  2. 9 MHz Correct answer.
  3. 21 MHz
  4. 1 MHz

Explanation

Third-order products follow the forms 2f1 minus f2 and 2f2 minus f1, yielding 9 MHz and 12 MHz here. The 9 MHz product is closest to the 10 MHz tone, making it the most troublesome to filter.

Question 39RF Fundamentals

What is the primary advantage of a Yagi-Uda antenna over a half-wave dipole?

  1. Perfectly omnidirectional coverage
  2. Wider bandwidth across all HF bands
  3. Higher forward gain in a single direction Correct answer.
  4. Elimination of the need for a feedline

Explanation

Adding a reflector and directors to a driven element focuses energy in one direction, raising forward gain at the expense of bandwidth and simplicity. A plain dipole radiates bidirectionally with much lower gain.

Question 40RF Fundamentals

A coaxial cable has a velocity factor of 0.8. How long is a half-wavelength section at 50 MHz?

  1. 2.4 meters Correct answer.
  2. 3.0 meters
  3. 1.2 meters
  4. 6.0 meters

Explanation

A free-space half wavelength at 50 MHz is 3 meters. Multiplying by the velocity factor of 0.8 gives 2.4 meters of physical cable, which is the length that presents a half-wave resonance at that frequency.

Question 41RF Fundamentals

What does a return loss of 10 dB indicate about the reflected power relative to incident power?

  1. One half of the incident power is reflected
  2. One hundredth of the incident power is reflected
  3. All incident power is reflected
  4. One tenth of the incident power is reflected Correct answer.

Explanation

Return loss in decibels is ten times the base-10 logarithm of the incident-to-reflected power ratio. A 10 dB return loss means the incident power is ten times the reflected power, so one tenth is reflected.

Question 42RF Fundamentals

Why does a low-pass filter placed after a transmitter reduce interference to other services?

  1. It converts the signal to a lower frequency
  2. It improves the antenna's radiation resistance
  3. It attenuates harmonic emissions above the fundamental frequency Correct answer.
  4. It increases the transmitter's output power

Explanation

Harmonics are multiples of the operating frequency and can fall in other bands, causing interference. A low-pass filter passes the fundamental while attenuating those higher-frequency products before they reach the antenna.

Question 43RF Fundamentals

What is the characteristic impedance of free space, approximately?

  1. 300 ohms
  2. 50 ohms
  3. 377 ohms Correct answer.
  4. 73 ohms

Explanation

Free space has an intrinsic impedance of about 377 ohms, derived from the square root of the permeability-to-permittivity ratio. This value underlies the relationship between electric and magnetic field strengths in a propagating wave.

Question 44RF Fundamentals

A 100-watt transmitter feeds a line with 2 dB of loss. What power reaches the antenna?

  1. Approximately 80 watts
  2. Approximately 63 watts Correct answer.
  3. Approximately 50 watts
  4. Approximately 98 watts

Explanation

A 2 dB loss corresponds to a power ratio of about 1.585, so the delivered power is 100 divided by 1.585, roughly 63 watts. The same loss also attenuates received signals, so feedline loss matters in both directions.

Question 45RF Fundamentals

Which effect explains why VHF signals sometimes propagate hundreds of kilometers via the troposphere during temperature inversions?

  1. Sporadic-E ionization
  2. Tropospheric ducting Correct answer.
  3. Meteor scatter
  4. Ground-wave diffraction

Explanation

Temperature inversions create layers where the refractive index changes sharply with height, trapping VHF and UHF signals in a duct and carrying them far beyond the normal horizon. This is a lower-atmosphere phenomenon distinct from ionospheric effects.

Question 46RF Fundamentals

What is the purpose of a dummy load in transmitter testing?

  1. To absorb RF power while presenting the correct load impedance without radiating Correct answer.
  2. To measure the frequency of the transmitter
  3. To increase the antenna's gain during tuning
  4. To convert RF power into a lower frequency

Explanation

A dummy load is a non-radiating resistive load, typically 50 ohms, that lets a transmitter be tested at full power without causing interference. It also protects the final amplifier by maintaining a proper match.

Question 47RF Fundamentals

A transmission line has a reflection coefficient of 0.2. What fraction of the incident power is delivered to the load?

  1. 0.04
  2. 0.80
  3. 0.96 Correct answer.
  4. 0.20

Explanation

Reflected power fraction equals the square of the reflection coefficient magnitude, giving 0.04. The remaining 0.96, or 96 percent, of incident power is delivered to the load.

Question 48RF Fundamentals

An antenna has 10 dBi gain and radiates 50 watts. What is the approximate effective isotropic radiated power?

  1. 100 watts
  2. 250 watts
  3. 50 watts
  4. 500 watts Correct answer.

Explanation

EIRP equals the transmitter power multiplied by the numeric gain over isotropic. A 10 dBi gain is a factor of 10, so 50 watts times 10 gives 500 watts EIRP, which indicates the equivalent power an isotropic radiator would need.

Question 49RF Fundamentals

Which condition produces the highest possible standing wave ratio on a transmission line?

  1. A lossless line terminated in its characteristic impedance
  2. A lossless line terminated in a pure reactance Correct answer.
  3. A lossy line terminated in a matched load
  4. A short line terminated in a resistive load equal to twice the characteristic impedance

Explanation

A pure reactance reflects all incident power because it dissipates none and absorbs none, so the reflection coefficient magnitude is unity. That yields an infinite standing wave ratio regardless of the reactance value.

Question 50RF Fundamentals

Two identical 3 dB attenuators are cascaded. What is the total attenuation, and why?

  1. 1.5 dB, because each halves the previous value
  2. 9 dB, because losses multiply
  3. 3 dB, because attenuators cancel
  4. 6 dB, because decibel values add in cascade Correct answer.

Explanation

Decibels are logarithmic units, so cascaded gains and losses simply add. Each 3 dB pad halves the power, and two of them reduce power to one quarter, which is 6 dB.