2020 Extra Class Study Guide

When setting up your amateur radio station, it’s important to know the characteristics of the feed lines you use in your antenna system. For example, did you know that the physical length of a coaxial cable transmission line is shorter than its electrical length? The reason for this is that electrical signals move more slowly in a coaxial cable than in air.

QUESTION: Why is the physical length of a coaxial cable transmission line shorter than its electrical length? (E9F03)
ANSWER: Electrical signals move more slowly in a coaxial cable than in air

The term we use to quantify the difference in how fast a wave travels in air versus how fast it travels in a feedline is velocity factor. The velocity factor of a transmission line is the velocity of the wave in the transmission line divided by the velocity of light in a vacuum. The dielectric materials used in the transmission line is one of the biggest factors that determine the velocity factor of a transmission line.

QUESTION: What is the velocity factor of a transmission line? (E9F01)
ANSWER: The velocity of the wave in the transmission line divided by the velocity of light in a vacuum

QUESTION: Which of the following has the biggest effect on the velocity factor of a transmission line? (E9F02)
ANSWER: Dielectric materials used in the line

Here are some typical velocity factors:

• Solid polyethylene dielectric coaxial transmission line: 0.66
• Foam polyethylene dielectric coaxial transmission line: 0.8
• Air-insulated, parallel-conductor, or open-wire, feedline: 0.98

QUESTION: What is the approximate physical length of a solid polyethylene dielectric coaxial transmission line that is electrically 1/4 wavelength long at 14.1 MHz? (E9F05)
ANSWER: 3.5 meters

A 1/4-wavelength at 14.1 MHz is approximately 5.3 m, but the velocity factor of a solid polyethylene dielectric coaxial transmission line is about 0.66, so the physical length will be 5.3 m x 0.66, which is 3.5 meters.

QUESTION: What is the approximate physical length of a foam polyethylene dielectric coaxial transmission line that is electrically 1/4 wavelength long at 7.2 MHz? (E9F09)
ANSWER: 8.3 meters

A 1/4-wavelength at 7.2 MHz is approximately 10.4 m, but the velocity factor of a foam polyethylene dielectric transmission line is about 0.8, so the physical length will be 10.4 m x 0.8, which is 8.3 meters.

QUESTION: What is the approximate physical length of an air-insulated, parallel conductor transmission line that is electrically 1/2 wavelength long at 14.10 MHz? (E9F06)
ANSWER: 10.6 meters

A 1/2-wavelength at 14.1 MHz is approximately 10.63 m, but the velocity factor of an air-insulated, parallel conductor transmission line is nearly 1.0, so the physical length will be nearly equal to the electrical length.

In general, coaxial cable transmission lines with a foam dielectric have a higher velocity factor than coaxial cables with a solid dielectric. There are other differences, too. A coaxial cable with a foam dielectric has lower safe operating voltage limits and lower loss per unit of length than a coaxial cable with a solid dielectric.

QUESTION: Which of the following is a significant difference between foam dielectric coaxial cable and solid dielectric cable, assuming all other parameters are the same? (E9F08)
ANSWER: All these choices are correct

• • Foam dielectric has lower safe operating voltage limits
  • Foam dielectric has lower loss per unit of length
  • Foam dielectric has higher velocity factor

Arguably, feed line loss is one of the most important characteristic of a transmission line. Obviously, the lower the feed line loss, the stronger the signal your antenna will radiate. Most amateurs use coaxial cable for antenna feed lines, but you should also consider open-wire feedlines or ladder lines. These feed lines have lower losses than most coaxial cable, and certainly have lower loss than small-diameter coaxial cable such as RG-58 at high frequencies.

QUESTION: How does ladder line compare to small-diameter coaxial cable such as RG-58 at 50 MHz? (E9F07)
ANSWER: Lower loss

Sometimes we use various lengths of coax to match an antenna system or to filter out frequencies. A 1/8-wavelength transmission line presents an inductive reactance to a generator when the line is shorted at the far end, and this property could be used match a capacitive load to a transmitter. A 1/8-wavelength transmission line presents a capacitive reactance to a generator when the line is open at the far end. This property could be used to match an inductive load.

QUESTION: What impedance does a 1/8-wavelength transmission line present to a generator when the line is shorted at the far end? (E9F10)
ANSWER: An inductive reactance

QUESTION: What impedance does a 1/8-wavelength transmission line present to a generator when the line is open at the far end? (E9F11)
ANSWER: A capacitive reactance

A length of transmission line has very different characteristics, depending on whether or not the line is open or shorted at the far end. A 1/4-wavelength transmission line presents a very low impedance to a generator when the line is open at the far end. A 1/4-wavelength transmission line presents a very high impedance to a generator when the line is shorted at the far end. On the other hand, a 1/2-wavelength transmission line that is shorted at the far end presents a very low impedance.

QUESTION: What impedance does a 1/4-wavelength transmission line present to a generator when the line is open at the far end? (E9F12)
ANSWER: Very low impedance

QUESTION: What impedance does a 1/4-wavelength transmission line present to a generator when the line is shorted at the far end? (E9F13)
ANSWER: Very high impedance

QUESTION: What impedance does a 1/2-wavelength transmission line present to a generator when the line is shorted at the far end? (E9F04)
ANSWER: Very low impedance

For many types of antennas, matching the impedance of the antenna to the impedance of the feedline, normally coax, is essential. When a feedline and antenna are mismatched, some of the power you are trying to transmit will be reflected back down the feedline or dissipated in the feedline. The ratio of the amplitude of the reflected wave to the amplitude of the incident wave, or the wave that you’re transmitting, is called the reflection coefficient, and it is mathematically related to SWR.

QUESTION: What parameter describes the interactions at the load end of a mismatched transmission line? (E9E07)
ANSWER: Reflection coefficient

To match the impedance of the feedline to the impedance of the antenna, we use a variety of different techniques. The delta matching system matches a high-impedance transmission line to a lower impedance antenna by connecting the line to the driven element in two places spaced a fraction of a wavelength each side of element center. It’s called a delta match because when connected this way, the feedline and antenna look like the Greek letter delta.

QUESTION: What system matches a higher-impedance transmission line to a lower-impedance antenna by connecting the line to the driven element in two places spaced a fraction of a wavelength each side of element center? (E9E01)
ANSWER: The delta matching system

The gamma match is the name of an antenna matching system that matches an unbalanced feed line to an antenna by feeding the driven element both at the center of the element and at a fraction of a wavelength to one side of center. The purpose of the series capacitor in a gamma-type antenna matching network is to cancel the inductive reactance of the matching network. The gamma match is an effective method of shunt feeding a grounded tower so it can be used as a vertical antenna.

QUESTION: What is the name of an antenna matching system that matches an unbalanced feed line to an antenna by feeding the driven element both at the center of the element and at a fraction of a wavelength to one side of center? (E9E02)
ANSWER: The gamma match

QUESTION: What is the purpose of the series capacitor in a gamma-type antenna matching network? (E9E04)
ANSWER: To cancel the inductive reactance of the matching network

QUESTION: Which of the following is used to shunt-feed a grounded tower at its base? (E9E09)
ANSWER: Gamma match

The stub match is the name of the matching system that uses a section of transmission line connected in parallel with the feed line at or near the feed point. What the stub does is to add reactance at the feed point. By varying the length of the stub, you can change the reactance that the stub provides to whatever value is needed.

QUESTION: What is the name of the matching system that uses a section of transmission line connected in parallel with the feed line at or near the feed point? (E9E03)
ANSWER: The stub match

Many directly-fed Yagi antennas have feedpoint impedances of approximately 20 to 25 ohms. One technique often use to match these antennas to 50-ohm coaxial cable is the hairpin match. To use a hairpin matching system to tune the driven element of a 3-element Yagi, the driven element reactance must be capacitive.

QUESTION: How must an antenna’s driven element be tuned to use a hairpin matching system? (E9E05)
ANSWER: The driven element reactance must be capacitive

Lengths of 75-ohm coax can also be used to match impedances. For example, inserting a 1/4-wavelength piece of 75-ohm coaxial cable transmission line in series between the antenna terminals and the 50-ohm feed cable is an effective way to match an antenna with a 100-ohm feed point impedance to a 50-ohm coaxial cable feed line. Note that this only works on one band as the length of 75-ohm coax you use will only be 1/4 of a wavelength on one band.

QUESTION: Which of these feed line impedances would be suitable for constructing a quarter-wave Q-section for matching a 100-ohm loop to 50-ohm feed line? (E9E06)
ANSWER: 75 ohms

QUESTION: Which of these choices is an effective way to match an antenna with a 100-ohm feed point impedance to a 50-ohm coaxial cable feed line? (E9E10)
ANSWER: Insert a 1/4-wavelength piece of 75-ohm coaxial cable transmission line in series between the antenna terminals and the 50-ohm feed cable

Another use for coaxial cable is as a phasing line for antennas that have multiple driven elements. The theory here is that by feeding the driven elements out of phase with one another, you can create a directional radiation pattern. A common application for phasing lines is a phased, vertical array.

QUESTION: What is the primary purpose of phasing lines when used with an antenna having multiple driven elements? (E9E11)
ANSWER: It ensures that each driven element operates in concert with the others to create the desired antenna pattern

Finally, unless you’re going to be doing microwave work, you probably won’t need to know about Wilkinson dividers, but here’s the information anyway. Wilkinson dividers divide power equally between two 50 ohm loads while maintaining 50 ohm input impedance. They’re used mainly in microwave systems.

QUESTION: What is a use for a Wilkinson divider? (E9E08)
ANSWER: It is used to divide power equally between two 50-ohm loads while maintaining 50-ohm input impedance

Antenna gain is one of the most misunderstood topics in amateur radio. There are several reasons for this, including:

• Antennas don’t really have gain in the same way that an amplifier has gain. When you use a linear amplifier, you get more power out than you put in. Since transmitting antennas are passive devices, there’s no way to get more power out than you put in.
• It’s not easy to measure antenna gain. There is no antenna gain meter that you can simply hook up to an antenna to measure its gain.

So, what is meant by antenna gain? Antenna gain is the ratio of the radiated signal strength of an antenna in the direction of maximum radiation to that of a reference antenna. What this means is that when you talk about antenna gain, you have to know what kind of antenna you’re comparing it to.

When talking about antenna gain, antenna engineers often refer to the “isotropic antenna.” In practice, an isotropic antenna is a theoretical antenna that has no gain in any direction. That is to say it radiates the power input to it equally in all directions.

QUESTION: What is an isotropic antenna? (E9A01)
ANSWER: A theoretical, omnidirectional antenna used as a reference for antenna gain

Let’s take a look at a practical example. The 1/2-wavelength dipole antenna is the most basic amateur radio antenna. The dipole actually has some gain over isotropic antenna. The reason for this is that it is directional. The signal strength transmitted broadside to the antenna will be greater than the signal strength transmitted off the ends of the antenna.

The gain of a 1/2-wavelength dipole in free space compared to an isotropic antenna is 2.15 dB. Sometimes, you’ll see this value as 2.15 dBi, where dBi denotes that an isotropic antenna is being used for this comparison. Since the isotropic antenna is a theoretical antenna, some think it’s better to compare an antenna to a dipole antenna. Let’s look at an example:

QUESTION: How much gain does an antenna have compared to a 1/2-wavelength dipole when it has 6 dB gain over an isotropic antenna? (E9A12)
ANSWER: 3.85 dB

You obtain this value by simply subtracting 2.15 dB from the 6 dB figure:

Gain over a dipole = gain over an isotropic antenna – 2.15 dB = 6 dBi – 2.15 dBi = 3.85 dBd

Sometimes, the gain over a dipole is denoted as dBd.

Effective radiated power

When you use an antenna that has gain, you are increasing the effectiveness of the power input to it in the direction the antenna is pointing. We call this the effective radiated power, but it is not just the transmitter’s output power times the gain of the antenna. You also have to take into account losses in other parts of the antenna system.

This is especially true for VHF and UHF repeater systems, where losses in the feedline, duplexer, and circulator can be significant. The power that reaches the antenna may be substantially lower than the power output of the transmitter.

Let’s look at an example. Say that your repeater station had a transmitter output power of 150 watts, a feed line loss of 2 dB, 2.2 dB duplexer loss, and 7 dBd antenna gain. To calculate the effective radiated power, you have to first subtract the losses from the gain, as expressed in dB to get the total gain of the system:

total system gain = 7 dB – 2 dB – 2.2 dB = 2.8 dB.

Now, recall that 3 dB corresponds to a power ration of 2:1, as shown in the table below. 2.8 dB would then be slight less than that. In fact, 2.8dB corresponds to a power ratio of approximately 1.905, so the effective radiated power is the transmitter output power times the total system gain:

effective radiated power = 150 W x 1.905 = 268 W.

QUESTION: What term describes station output, taking into account all gains and losses? (E9A13)
ANSWER: Effective radiated power

QUESTION: What is the effective radiated power relative to a dipole of a repeater station with 150 watts transmitter power output, 2 dB feed line loss, 2.2 dB duplexer loss, and 7 dBd antenna gain? (E9A02)
ANSWER: 286 watts

Let’s look at another example. In this example, your repeater station has 200 watts transmitter power output, 4 dB feed line loss, 3.2 dB duplexer loss, 0.8 dB circulator loss, and 10 dBd antenna gain. The total gain of the system is, therefore, 10 dB – 4 dB – 3.2 dB – 0.8 dB, or 2.0 dB. 2.0 dB corresponds to a power ratio of approximately 1.585, making the effective radiated power 200 W × 1.585 = 317 W. Note that in this system, the high feedline and duplexer losses almost completely negate the benefit of using a high gain antenna.

QUESTION: What is the effective radiated power relative to a dipole of a repeater station with 200 watts transmitter power output, 4 dB feed line loss, 3.2 dB duplexer loss, 0.8 dB circulator loss, and 10 dBd antenna gain? (E9A06)
ANSWER: 317 watts

Here’s a third example. Notice that in this example we comparing the effective radiated power to an isotropic antenna, not a dipole. In this example, the repeater station has 200 watts transmitter power output, 2 dB feed line loss, 2.8 dB duplexer loss, 1.2 dB circulator loss, and 7 dBi antenna gain. The total gain of the system is 7 dB – 2 dB – 2.8 dB – 1.2 dB, or 1.0 dB. 1.0 dB corresponds to a power ratio of approximately 1.26, and the effective radiated power equals 200 W × 1.26 = 252 W.

QUESTION: What is the effective isotropic radiated power of a repeater station with 200 watts transmitter power output, 2 dB feed line loss, 2.8 dB duplexer loss, 1.2 dB circulator loss, and 7 dBi antenna gain? (E9A07)
ANSWER: 252 watts

Feedpoint impedance

Other antenna parameters are also important, of course. One of the most basic antenna parameters is the feedpoint impedance. The reason that the feedpoint impedance is important is that you want the feedpoint impedance to match the impedance of the feedline that you use and the output impedance of the transmitter. When these are all equal, we say that the system is “matched,” and it will radiate the maximum amount of energy.

Many factors may affect the feed point impedance of an antenna, including antenna height, conductor length/diameter ratio and location of nearby conductive objects. For example, we say that the feedpoint impedance of a half-wavelength, dipole antenna is 72 Ω, but that’s only really true if the antenna is in free space. When it’s closer to the ground than a quarter wavelength, then the impedance will be different. That’s why you have to tune the antenna when you install it.

QUESTION: Which of the following factors affect the feed point impedance of an antenna? (E9A04)
ANSWER: Antenna height

Radiation resistance

Another antenna parameter that’s frequently bandied about is radiation resistance. The radiation resistance of an antenna is the value of a resistance that would dissipate the same amount of power as that radiated from an antenna. In the case of an antenna, however, that power isn’t being turned into heat, but rather turned into radio waves. The total resistance of an antenna system is the sum of the radiation resistance and ohmic resistance. The ohmic resistance is the combination of all the physical resistances in the system, including the resistance of the antenna wire, feed line, and connections.

QUESTION: What is the radiation resistance of an antenna? (E9A03)
ANSWER: The value of a resistance that would dissipate the same amount of power as that radiated from an antenna

QUESTION: What is included in the total resistance of an antenna system? (E9A05)
ANSWER: Radiation resistance plus loss resistance

If you know the radiation resistance and the ohmic resistance of an antenna, you can calculate its efficiency. Antenna efficiency equals the radiation resistance divided by the total resistance.

QUESTION: What is antenna efficiency? (E9A09)
ANSWER: Radiation resistance divided by total resistance

Vertical antennas, bandwidth

Vertical antennas are sometimes criticized as being inefficient antennas. The main reason for this is the lack of a good radial system. Installing a good radial system will improve a quarter-wave vertical’s efficiency. Another for poor vertical performance is poor soil conductivity. If soil conductivity is poor, ohmic resistance will be high, and the antenna’s efficiency will be low.

QUESTION: Which of the following improves the efficiency of a ground-mounted quarter-wave vertical antenna? (E9A10)
ANSWER: Installing a radial system

QUESTION: Which of the following factors determines ground losses for a ground-mounted vertical antenna operating in the 3 MHz to 30 MHz range? (E9A11)
ANSWER: Soil conductivity

Antenna bandwidth is the frequency range over which an antenna satisfies a performance requirement. Normally, the performance requirement is an SWR of 2:1 or less. In fact, you’ll sometimes hear this parameter referred to as the 2:1 SWR bandwidth.

QUESTION: What is antenna bandwidth? (E9A08)
ANSWER: The frequency range over which an antenna satisfies a performance requirement

Some modern radios modulate and demodulate signals entirely in software. This type of radio is called a software-defined radio, or SDR. One type of SDR uses a process called direct digital conversion to convert the analog radio signal into a series of numbers. What this type of SDR does is digitize an incoming RF signal with an analog-to-digital converter without being mixed with a local oscillator signal.

QUESTION: What is meant by direct digital conversion as applied to software defined radios? (E7F01)
ANSWER: Incoming RF is digitized by an analog-to-digital converter without being mixed with a local oscillator signal

Analog-to-digital converter specifications are crucial for a software-defined radio. Sample rate is one such specification. The sample rate determines the maximum receive bandwidth of a direct digital conversion SDR. An analog signal must be sampled at twice the rate of the highest frequency component of the signal by an analog-to-digital converter so that the signal can be accurately reproduced.

QUESTION: What aspect of receiver analog-to-digital conversion determines the maximum receive bandwidth of a Direct Digital Conversion SDR? (E7F10)
ANSWER: Sample rate

QUESTION: How frequently must an analog signal be sampled by an analog-to-digital converter so that the signal can be accurately reproduced? (E7F05)
ANSWER: At least twice the rate of the highest frequency component of the signal

Voltage resolution is also important. In the absence of atmospheric or thermal noise, the reference voltage level and sample width in bits sets the minimum detectable signal level for an SDR. The reference voltage is basically the maximum detectable voltage, while the number of bits determines the resolution of the analog-to-digital converter. For example, if an analog-to-digital converter had a reference voltage of 1 V, the minimum number of bits required to sample a signal at a resolution of 1 millivolt is 10 bits. The reason for this is that 2¹⁰ = 1,024, meaning that each bit represents approximately 1 mV.

QUESTION: What sets the minimum detectable signal level for a direct-sampling SDR receiver in the absence of atmospheric or thermal noise? (E7F11)
ANSWER: Reference voltage level and sample width in bits

QUESTION: What is the minimum number of bits required for an analog-to-digital converter to sample a signal with a range of 1 volt at a resolution of 1 millivolt? (E7F06)
ANSWER: 10 bits

Modern software defined radios convert an incoming signal into two data streams: I and Q. The I and Q data streams are 90 degrees out of phase with one another, and as a result, the two data streams not only show how the amplitude of a signal is changing, but how the phase of a signal is changing. Perhaps the most common operation that is performed on the I and Q signals is a Fast Fourier Transform (FFT). The Fast Fourier Transform converts digital signals from the time domain to the frequency domain.

QUESTION: What function is performed by a Fast Fourier Transform? (E7F07)
ANSWER: Converting digital signals from the time domain to the frequency domain

Once a signal has been digitized, or converted into a series of numbers, it can be digitally filtered. We call this digital signal processing, or DSP For example, to remove unwanted noise from a received SSB signal you would use a DSP audio filter called an adaptive filter. Another type of digital filter used in SDRS is the finite impulse, or FIR, filter. An advantage of a Finite Impulse Response (FIR) filter vs an Infinite Impulse Response (IIR) digital filter is that FIR filters delay all frequency components of the signal by the same amount.

QUESTION: What kind of digital signal processing audio filter is used to remove unwanted noise from a received SSB signal? (E7F02)
ANSWER: An adaptive filter

QUESTION: Which of the following is an advantage of a Finite Impulse Response (FIR) filter vs an Infinite Impulse Response (IIR) digital filter? (E7F15)
ANSWER: FIR filters can delay all frequency components of the signal by the same amount

Taps in a digital signal processing filter provide incremental signal delays for filter algorithms. More taps would allow a digital signal processing filter to create a sharper filter response.

QUESTION: What is the function of taps in a digital signal processing filter? (E7F13)
ANSWER: Provide incremental signal delays for filter algorithms

QUESTION: Which of the following would allow a digital signal processing filter to create a sharper filter response? (E7F14)
ANSWER: More taps

Because so many calculations are required to do digital signal processing, SDRs sometimes use a technique called decimation, which allows them to use less-powerful processors. Decimation reduces the effective sample rate by removing samples when using a digital filter. They can get away with this because the signal of interest will usually have a significantly lower bandwidth than the digitized signal. An anti-aliasing digital filter must be used with a digital decimator because it peaks the response of the decimator, improving bandwidth.

QUESTION: What is the function of decimation? (E7F08)
ANSWER: Reducing the effective sample rate by removing samples

QUESTION: Why is an anti-aliasing digital filter required in a digital decimator? (E7F09)
ANSWER: It removes high-frequency signal components that would otherwise be reproduced as lower frequency components

Signals can also be generated using SDR techniques. For example, a common method of generating an SSB signal using digital signal processing is to combine signals with a quadrature phase relationship. The type of digital signal processing filter used to generate an SSB signal is a Hilbert-transform filter.

QUESTION: What is a common method of generating an SSB signal using digital signal processing? (E7F04)
ANSWER: Signals are combined in quadrature phase relationship

QUESTION: What type of digital signal processing filter is used to generate an SSB signal? (E7F03)
ANSWER: A Hilbert-transform filter

Modulation is the process of adding information, such as voice or digital information, to a carrier signal. The most common types of modulation that we use in amateur radio are amplitude modulation (AM) and frequency modulation (FM). Single-sideband, or SSB, is a form of amplitude modulation.

To frequency modulate a carrier, a transmitter will sometimes use a modulator that varies the phase of the signal. This process is called phase modulation (PM), and the type of modulator used to phase modulate a signal is called a reactance modulator. It uses an electrically variable inductance or capacitance to produce PM or FM signals.

QUESTION: Which of the following can be used to generate FM phone emissions? (E7E01)
ANSWER: A reactance modulator on the oscillator

QUESTION: What is the function of a reactance modulator? (E7E02)
ANSWER: To produce PM or FM signals by using an electrically variable inductance or capacitance

When generating FM signals, a pre-emphasis network is often added to an FM transmitter to boost the higher audio frequencies. Conversely, de-emphasis is commonly used in FM communications receivers to maintain compatibility with transmitters using phase modulation.

QUESTION: What circuit is added to an FM transmitter to boost the higher audio frequencies? (E7E05)
ANSWER: A pre-emphasis network

QUESTION: Why is de-emphasis commonly used in FM communications receivers? (E7E06)
ANSWER: For compatibility with transmitters using phase modulation

 

Amplitude modulation and single-sideband signals are produced using mixer circuits. The carrier frequency and the baseband signals are input to the mixer circuit which produces an amplitude modulated output. In other words, the term baseband refers to the frequency range of a modulating signal prior to it being mixed with a carrier signal. A mixer combines the carrier and baseband signals and produces four different output frequencies: the two input frequencies along and the sum and difference of the two input frequencies. When using a mixer, you must take care not to use too high of a signal level at the mixer inputs, or it may generate spurious products.

QUESTION: What is meant by the term “baseband” in radio communications? (E7E07)
ANSWER: The frequency range occupied by a message signal prior to modulation

QUESTION: What are the principal frequencies that appear at the output of a mixer circuit? (E7E08)
ANSWER: The two input frequencies along with their sum and difference frequencies

QUESTION: What occurs when an excessive amount of signal energy reaches a mixer circuit? (E7E09)
ANSWER: Spurious mixer products are generated

Single sideband is most often used for phone transmission on the HF bands and for weak-signal operation on the VHF and UHF bands. One way a single-sideband phone signal can be generated is by using a balanced modulator followed by a filter. A balanced modulator is a type of mixer, but its output contains only the two sidebands, not the carrier or baseband signal. A filter follows the balanced modulator to eliminate the undesired sideband.

QUESTION: What is one way a single-sideband phone signal can be generated? (E7E04)
ANSWER: By using a balanced modulator followed by a filter

At the receiving station, a modulated signal has to be demodulated. Amplitude modulated signals are often demodulated using a simple diode detector circuit. A diode detector rectifies and filters a modulated signal, thereby producing an audio signal at its output. For demodulating SSB signals, you want something a little more sophisticated, like a product detector. A product detector is actually a frequency mixer, followed by a filter . Its output is the product of the modulated signal and a beat frequency oscillator, hence the name.

QUESTION: How does a diode envelope detector function? (E7E10)
ANSWER: By rectification and filtering of RF signals

QUESTION: Which type of detector circuit is used for demodulating SSB signals? (E7E11)
ANSWER: Product detector

FM receivers use a circuit called a discriminator to detect an FM signal. A discriminator converts a signal that changes in frequency to one that changes in amplitude.

QUESTION: What is a frequency discriminator stage in a FM receiver? (E7E03)
ANSWER: A circuit for detecting FM signals