## Extra Class question of the day: matching antennas to feedlines

For many types of antennas, matching the impedance of the antenna to the impedance of the feedline, normally coax, is essential. Mismatched lines create high SWR and, consequently, feedline losses. An SWR greater than 1:1 is characteristic of a mismatched transmission line. (E9E08)

When a feedline and antenna are mismatched, some of the power you are trying to transmit will be reflected back down the feedline. The ration of the amplitude of the reflected wave to the amplitude of the wave you are trying to send is called the reflection ratio, and it is mathematically related to SWR. Reflection coefficient is the term that best describes the interactions at the load end of a mismatched transmission line. (E9E07)

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. (E9E01)

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. (E9E02) The purpose of the series capacitor in a gamma-type antenna matching network is to cancel the inductive reactance of the matching network. (E9E04)The gamma match is an effective method of connecting a 50-ohm coaxial cable feed line to a grounded tower so it can be used as a vertical antenna. (E9E09)

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. (E9E03) 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. An effective way of matching a feed line to a VHF or UHF antenna when the impedances of both the antenna and feed line are unknown is to use the universal stub matching technique. (E9E11)

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. (E9E10) Note that this will only work on one band as the length of 75-ohm coax you use will only be 1/4 of a wavelength on one band.

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. (E9E05) The equivalent lumped-constant network for a hairpin matching system on a 3-element Yagi is an L network. (E9E06)

Some beam antennas use multiple driven elements in order to make them multi-band antennas. The primary purpose of a phasing line when used with an antenna having multiple driven elements is that it ensures that each driven element operates in concert with the others to create the desired antenna pattern. (E9E12)

I’m not sure that Wilkinson dividers are used much in antenna systems, or why this question is in the section on feedline matching, but here it is. The purpose of a Wilkinson divider is that it divides power equally among multiple loads while preventing changes in one load from disturbing power flow to the others. (E9E13)

## Extra Class question of the day: frequency synthesizers

Most modern amateur radio transceivers use digital frequency synthesizers instead of analog oscillators to generate RF signals. On reason for this is that they are much more stable than analog oscillators. The two main types of digital frequency synthesizers are the direct digital synthesizer and the phase-locked loop synthesizer

A direct digital synthesizer is the type of frequency synthesizer circuit that uses a phase accumulator, lookup table, digital to analog converter and a low-pass anti-alias filter. (E7H09) The phase accumulator is a principal component of a direct digital synthesizer (DDS). (E7H12) The information is contained in the lookup table of a direct digital frequency synthesizer is the amplitude values that represent a sine-wave output. (E7H10)

Both the direct digital synthesizer and the phase-locked loop synthesizer have issues with spectral purity. The major spectral impurity components of direct digital synthesizers are spurious signals at discrete frequencies. (E7H11)

For a more detailed explanation of how direct digital synthesizers work, see the electric druid’s  Synth DIY page.

Another type of frequency synthesizer that’s popular are those that use a phase-locked loop. A phase-locked loop circuit is an electronic servo loop consisting of a phase detector, a low-pass filter, a voltage-controlled oscillator, and a stable reference oscillator. (E7H14)

A phase-locked loop is often used as part of a variable frequency synthesizer for receivers and transmitters because it makes it possible for a VFO to have the same degree of frequency stability as a crystal oscillator. (E7H17) Frequency synthesis, FM demodulation are two functions that can be performed by a phase-locked loop. (E7H15)

An important specification for phase-locked loop circuits is the short-term stability of the reference oscillator. The short-term stability of the reference oscillator is important in the design of a phase locked loop (PLL) frequency synthesizer because any phase variations in the reference oscillator signal will produce phase noise in the synthesizer output. (E7H16) Phase noise is the major spectral impurity components of phase-locked loop synthesizers. (E7H18)

Another important specification is capture range. The capture range of a phase-locked loop circuit is the frequency range over which the circuit can lock. (E7H13)

## Extra Class question of the day: operating HF digital modes; error correction

Perhaps the most popular digital mode these days is PSK31. PSK stands for “phase shift keying.” One of its main advantages is that it had a very narrow bandwidth—only 31 Hz. In fact, PSK31 is the digital communications mode that has the narrowest bandwidth. (E2E10)

One of the ways is achieves this narrow bandwidth is that uses variable length coding. That is to say, characters have different numbers of bits, depending on how frequently they appear in normal text. PSK31 is an HF digital mode that uses variable-length coding for bandwidth efficiency. (E2E09)

Another type of modulation commonly used on the HF bands is frequency-shift keying, or FSK. RTTY, for example uses FSK modulation. FSK is a type of modulation that is common for data emissions below 30 MHz. (E2E01) One type of FSK modulation is MFSK16. The typical bandwidth of a properly modulated MFSK16 signal is 316 Hz. (E2E07)

Amateur transceivers use two different methods to modulate a signal using FSK: direct FSK and audio FSK. The difference between direct FSK and audio FSK is that direct FSK applies the data signal to the transmitter VFO. (E2E11) When using audio FSK, audio, typically from a computer sound card, is used to shift the frequency of the transmitted signal.

To tune an FSK signal, one often uses a crossed-ellipse display. You have properly tuned a signal when one of the ellipses is as vertical as possible, and the other is as horizontal as possible. When one of the ellipses in an FSK crossed-ellipse display suddenly disappears, selective fading has occurred. (E2E04)

PACTOR is one digital mode that uses FSK. It also uses the ARQ protocol to detect errors. Because of this, PACTOR is an HF digital mode that can be used to transfer binary files. (E2E08) How does ARQ accomplish error correction? If errors are detected, a retransmission is requested. (E2E05)

Another way to detect and correct errors in a data transmission is forward error correction. The letters FEC mean Forward Error Correction when talking about digital operation. (E2E02) Forward Error Correction is implemented by transmitting extra data that may be used to detect and correct transmission errors. (E2E03)

No matter what type of modulation you use, data transmission over an HF radio link is very slow. 300 baud is the most common data rate used for HF packet communications. (E2E06) In fact, due to bandwidth limitations, 300 baud is the maximum data rate.

Many of the digital modes were designed to allow keyboard-to-keyboard operation. That is to say, that operators can type messages back and forth to one another, almost as if they were having a conversation using SSB. Winlink, however, does not support keyboard-to-keyboard operation. (E2E12)

## Extra Class question of the day: VHF and UHF digital modes; APRS

One of the most commonly understood concepts in digital communications is the baud. A baud is not equal to a bit per second, except for very simple systems. Rather, the definition of baud is the number of data symbols transmitted per second. (E2D02) A data symbol may represent multiple bits.

The baud rate is a measure of how fast a digital communications system can transmit data. Under clear communications conditions, 300-baud packet is the digital communication mode that has the fastest data throughput. (E2D09)

In the past ten years or so, we’ve had an explosion of digital modes become available. JT65 is one example. JT65 is a digital mode especially useful for EME communications. (E2D03) JT65 improves EME communications because it can decode signals many dB below the noise floor using FEC.(E2D12) FSK441 is a digital mode especially designed for use for meteor scatter signals. (E2D01)

One of the most popular digital modes is the Automatic Packet Reporting System, or APRS. AX.25 is the digital protocol used by APRS. AX.25 is more commonly known as packet radio. (E2D07) Unnumbered Information is the type of packet frame used to transmit APRS beacon data. (E2D08)

APRS stations can be used to help support a public service communications activity. An APRS station with a GPS unit can automatically transmit information to show a mobile station’s position during the event. (E2D10) Latitude and longitude are used by the APRS network to communicate your location. (E2D11) 144.39 MHz is a commonly used 2-meter APRS frequency. (E2D06)

Amateurs that enjoy satellite communications also use digital modes. For example, store-and-forward is a technique normally used by low Earth orbiting digital satellites to relay messages around the world. (E2D05) The purpose of digital store-and-forward functions on an Amateur Radio satellite is to store digital messages in the satellite for later download by other stations. (E2D04)

## Extra Class question of the day: Impedance plots and coordinate systems

Most often when we plot values on a graph, we use the rectangular, or Cartesian, coordinate system. The two numbers that are used to define a point on a graph using rectangular coordinates are the coordinate values along the horizontal and vertical axes. (E5C11) In the graph above, point P is at x,y. Rectangular coordinates are often used to display the resistive, inductive, and/or capacitive reactance components of an impedance. (E5C13)

When thinking about how capacitive reactances, inductive reactances, and resistance combine, it’s useful to think in terms of polar coordinates. Polar coordinates are often used to display the phase angle of a circuit containing resistance, inductive and/or capacitive reactance. (E5C14) In a polar-coordinate system, each point on the graph has two values, a magnitude (shown by r in the figure above) and an angle (shown by θ in the figure above).

When using rectangular coordinates to graph the impedance of a circuit, the vertical axis represents the reactive component. (E5C10) To figure out the impedance of a circuit, you first plot the inductive reactance on the positive y-axis and the capacitive reactance on the negative y-axis. The net reactance, X, will be the sum of the two reactances.

When using rectangular coordinates to graph the impedance of a circuit, the horizontal axis represents the resistive component. (E5C09) After you’ve computed the net reactance, you plot the resistance on the x-axis and compute the magnitude of the impedance, shown by r in the graph above. If you consider that r is the third side of a right triangle made up of the sides r, x, and y, r is equal to the square root of x2 and y2.

Let’s take a look at an example. In polar coordinates, is the impedance of a network consisting of a 100-ohm-reactance inductor in series with a 100-ohm resistor is 141 ohms at an angle of 45 degrees. (E5C01) In this example, x=100 and y=100, so

r = sqrt (X2 + R2) = sqrt (1002 + 1002) = sqrt (20000) = 141 ohms.

The cosine of the phase angle θ is equal to x/r, or 100/141, or .707.  If you look up this value in a table of cosines, you’ll find that the angle is 45 degrees.

Here’s another thing to notice. When the value of the reactance is equal to the value of the resistance, the angle will be either 45 degrees or -45 degrees, depending on whether the net reactance is inductive or capacitive.

Now, let’s look at an example with both inductive and capacitive reactance. In polar coordinates, the impedance of a network consisting of a 100-ohm-reactance inductor, a 100-ohm-reactance capacitor, and a 100-ohm resistor, all connected in series is 100 ohms at an angle of 0 degrees. (E5C02) In this case, the inductive reactance and the capacitive reactance are the same, meaning that there is no net reactance. If you plot the impedance of a circuit using the rectangular coordinate system and find the impedance point falls on the right side of the graph on the horizontal axis, you know that the circuit impedance is equivalent to a pure resistance. (E5C12)

Here’s an example with unequal inductive and capacitive reactances. In polar coordinates, the impedance of a network consisting of a 300-ohm-reactance capacitor, a 600-ohm-reactance inductor, and a 400-ohm resistor, all connected in series is 500 ohms at an angle of 37 degrees. (E5C03) Here’s how we got that result:

X = 600 – 300 = 300 ohms

r = sqrt (X2 + R2) = sqrt (3002 + 4002) = sqrt (250000) = 500 ohms

θ = cos-1(x/r) = cos-1(400/500) = 37 degrees

Here are some more examples. I’ll leave the solutions up to you:

• In polar coordinates, the impedance of a network consisting of a 400-ohm-reactance capacitor in series with a 300-ohm resistor is 500 ohms at an angle of -53.1 degrees. (E5C04)
• In polar coordinates, the impedance of a network consisting of a 400-ohm-reactance inductor in parallel with a 300-ohm resistor is 240 ohms at an angle of 36.9 degrees. (E5C05)
• In polar coordinates, the impedance of a network consisting of a 100-ohm-reactance capacitor in series with a 100-ohm resistor is 141 ohms at an angle of -45 degrees. (E5C06)
• In polar coordinates, the impedance of a network comprised of a 100-ohm-reactance capacitor in parallel with a 100-ohm resistor is 71 ohms at an angle of -45 degrees. (E5C07)
• In polar coordinates, what is the impedance of a network comprised of a 300-ohm-reactance inductor in series with a 400-ohm resistor is 500 ohms at an angle of 53 degrees. (E5C08)
• In polar coordinates, the impedance of a series circuit consisting of a resistance of 4 ohms, an inductive reactance of 4 ohms, and a capacitive reactance of 1 ohm is 5 ohms at an angle of 37 degrees. (E5C18)

## Extra Class question of the day: Station control

An important concept in the rules governing amateur radio is the concept of station control and the control operator. The control operator is the licensed radio amateur who is responsible for the transmissions of a station, and the location of that operator is called the control point. There are three ways that a control operator can control a station: local control, remote control, or automatic control.

Local control means direct manipulation of the transmitter by a control operator. (E1C07) So, when you were sitting in front of your radio, you are using local control.

A remotely controlled station is a station controlled indirectly through a control link. (E1C01) A control operator must be present at the control point is the true statement about remotely controlled amateur stations. (E1C06) This is, of course, true for local control as well. 3 minutes is the maximum permissible duration of a remotely controlled station’s transmissions if its control link malfunctions. (E1C08)

Automatic control of a station means the use of devices and procedures for control so that the control operator does not have to be present at a control point. (E1C02) The control operator responsibilities of a station under automatic control differs from one under local control. Under automatic control the control operator is not required to be present at the control point. (E1C03)

Most repeaters are operated with automatic control. Only auxiliary, repeater or space stations are the types of amateur stations that may automatically retransmit the radio signals of other amateur stations. (E1C10) 29.500 – 29.700 MHz is the frequency band available for an automatically-controlled repeater operating below 30 MHz. (E1C09) No repeaters are allowed on any other HF band.

An automatically controlled station may retransmit third party communications only when transmitting RTTY or data emissions. (E1C04) An automatically controlled station may never originate third party communications. (E1C05)

## Extra Class question of the day: Station restrictions and special operations

Part 97 places many different restrictions on how amateurs can use their stations and specifies technical standards that amateur radio station must meet. For example, some rules set standards for spurious emissions A spurious emission is an emission outside its necessary bandwidth that can be reduced or eliminated without affecting the information transmitted. (E1B01) The rules also state that permitted mean power of any spurious emission relative to the mean power of the fundamental emission from a station transmitter or external RF amplifier must be at least 43 dB below for transmitters or amplifiers installed after January 1, 2003, and transmitting on a frequency below 30 MHZ is. (E1B11)

There are also restrictions on erecting antennas. One factor that might cause the physical location of an amateur station apparatus or antenna structure to be restricted is if the location is of environmental importance or significant in American history, architecture, or culture. (E1B02) If you are installing an amateur station antenna at a site at or near a public use airport, you may have to notify the Federal Aviation Administration and register it with the FCC as required by Part 17 of FCC rules. (E1B06)

The 60m band is one band that has a lot of weird restrictions not found on other ham bands. For example, the maximum bandwidth for a data emission on 60 meters is 2.8 kHz. (E1B05) The carrier frequency of a CW signal must be set at the center frequency of the channel to comply with FCC rules for 60 meter operation. (E1B07)

Because RACES operation is quasi-governmental, there are some rules about RACES operations. Any FCC-licensed amateur station certified by the responsible civil defense organization for the area served may be operated in RACES.  (E1B09) All amateur service frequencies authorized to the control operator are authorized to an amateur station participating in RACES. (E1B10)

Finally, there are some questions about random rules in this section:

• The distance at which an amateur station must protect an FCC monitoring facility from harmful interference is 1 mile. (E1B03)
• An Environmental Assessment must be submitted to the FCC must be done before placing an amateur station within an officially designated wilderness area or wildlife preserve, or an area listed in the National Register of Historical Places. (E1B04)
• The amateur station must avoid transmitting during certain hours on frequencies that cause the interference if its signal causes interference to domestic broadcast reception, assuming that the receiver(s) involved are of good engineering design. (E1B08)
• The highest modulation index permitted at the highest modulation frequency for angle modulation is 1.0. (E1B12)

## Extra Class question of the day: field-effect transistors

A field-effect transistor (FET) is a device that uses an electric field to control current flow through the device. Like the bipolar transistor, a FET normally has three terminals. The names of the three terminals of a field-effect transistor are gate, drain, source. (E6A17)

FETs are normally made with a technology called Complementary Metal-Oxide Semiconductor, or CMOS. The initials CMOS stand for Complementary Metal-Oxide Semiconductor. (E6A13) FETs made with CMOS technology are sometimes call MOSFETs.

In Figure E6-2 (below), schematic symbol 1 is the symbol for a P-channel junction FET. (E6A11) In Figure E6-2 (below), schematic symbol 4 is the symbol for an N-channel dual-gate MOSFET. (E6A10)

One characteristic of the MOSFET is that they have a high input impedance. This makes them more attractive for use in many test equipment applications than bipolar transistors. How does DC input impedance at the gate of a field-effect transistor compare with the DC input impedance of a bipolar transistor? An FET has high input impedance; a bipolar transistor has low input impedance. (E6A14)

One disadvantage of using MOSFETs is that they are very sensitive to electrostatic discharge (ESD). Sometimes, they are damaged by static discharges so low that you never even see the spark or feel the shock. To reduce the chance of the gate insulation being punctured by static discharges or excessive voltages many MOSFET devices have internally connected Zener diodes on the gates. (E6A12)

Most FETs are enhancement-mode devices. When using an enhancement-mode FET, you must apply a voltage to the gate to get current to flow from source to drain. Some FETs are, however, depletion mode devices. A depletion-mode FET is an FET that exhibits a current flow between source and drain when no gate voltage is applied. (E6A09)

## Extra Class question of the day: Aurora propagation, selective fading; radio-path horizon; take-off angle over flat or sloping terrain; effects of ground on propagation; less common propagation modes

One of the most interesting propagation phenomena is Aurora propagation. To make use of this phenomenon, radio amateurs actually bounce their signals off of the Aurora Borealis, also known as the “Northern Lights.” All of these choices are correct when talking about effects Aurora activity has on radio communications (E3C01):

• SSB signals are raspy
• Signals propagating through the Aurora are fluttery
• CW signals appear to be modulated by white noise

The cause of Aurora activity is the interaction of charged particles from the Sun with the Earth’s magnetic field and the ionosphere. (E3C02) Aurora activity occurs in the E-region of the ionosphere. (E3C03) CW is the emission mode that is best for Aurora propagation. (E3C04) From the contiguous 48 states, an antenna should be pointed North to take maximum advantage of aurora propagation. (E3C11)

Normally, we think of the ionosphere as a mirror, reflecting HF signals back to Earth at the same angle at which the signal hits the ionosphere. While this is normally the case, sometimes the ionosphere does not get refracted sufficiently to return directly to Earth, but instead travels for some distance in the F2 layer before finally being returned. The name of the high-angle wave in HF propagation that travels for some distance within the F2 region is called the Pedersen ray. (E3C08)

While we say that VHF/UHF communications is “line of sight,” the distance that a VHF/UHF radio wave will travel is slightly longer than the line-of-sight distance. We call this distance the “radio horizon” or “radio-path horizon.” The VHF/UHF radio-path horizon distance exceeds the geometric horizon by approximately 15% of the distance. (E3C06) The radio-path horizon distance exceeds the geometric horizon because of downward bending due to density variations in the atmosphere. (E3C14)

Another phenomenon that sometimes makes VHF signals beyond the line of sight is tropospheric ducting. Tropospheric ducting is usually responsible for causing VHF signals to propagate for hundreds of miles. (E3C09)

One of the most frustrating propagation phenomena is selective fading. Selective fading is partial cancellation of some frequencies within the received pass band. (E3C05) It is frustrating because it sometimes makes portions of an otherwise perfectly readable signal unreadable.

Amateur radio operators may sometimes use ground-wave propagation to communicate. One important thing to know about this type of propagation is that the maximum distance of ground-wave propagation decreases when the signal frequency is increased. (E3C12) Vertical polarization is the best type of polarization for ground-wave propagation. (E3C13) So, if you really want to make a contact via ground wave, use a vertical antenna on the 160m band.

To take advantage of some of these phenomena, or to just make your antenna work better, you should know how antenna’s performance changes with changes in its design or installation. For example, the radiation pattern of a horizontally polarized 3-element beam antenna varies as the height above ground changes. What happens is the the main lobe takeoff angle decreases with increasing height. (E3C07)

The performance of a horizontally polarized antenna mounted on the side of a hill will be different from the performance of same antenna mounted on flat ground. Specifically, the main lobe takeoff angle decreases in the downhill direction. (E3C10)

## Extra Class question of the day: system noise; electrical appliance noise; line noise; locating noise sources; DSP noise reduction; noise blankers

Noise is often a real problem for radio amateurs. Fortunately, by understanding how noise is generated and how to reduce or eliminate it, noise can be tamed.

Atmospheric noise is naturally-occurring noise. Thunderstorms are a major cause of atmospheric static. (E4E06) There’s not much you can do to eliminate, but you can often use a receiver’s noise blanker to help you copy signals better. Signals which appear across a wide bandwidth (like atmospheric noise) are the types of signals that a receiver noise blanker might be able to remove from desired signals. (E4E03) Ignition noise is one type of receiver noise that can often be reduced by use of a receiver noise blanker. (E4E01)

One undesirable effect that can occur when using an IF noise blanker is that nearby signals may appear to be excessively wide even if they meet emission standards. (E4E09)

Many modern receivers now use digital signal processing (DSP) filters to eliminate noise. All of these choices are correct when talking about types of receiver noise can often be reduced with a DSP noise filter (E4E02):

• Ignition noise
• Power line noise

One disadvantage of using some types of automatic DSP notch-filters when attempting to copy CW signals is that the DSP filter can remove the desired signal at the same time as it removes interfering signals. (E4E12)

While filters can be very effective at reducing noise, it is often better to figure out what is  generating the noise and taking steps to reduce or eliminate the amount of noise generated in the first place. For example, one way you can determine if line noise interference is being generated within your home is by turning off the AC power line main circuit breaker and listening on a battery operated radio. (E4E07) If by doing this you determine that an electric motor is a problem, noise from an electric motor can be suppressed by installing a brute-force AC-line filter in series with the motor leads. (E4E05)

All of these choices are correct when it comes to the cause of a loud roaring or buzzing AC line interference that comes and goes at intervals (E4E13):

• Arcing contacts in a thermostatically controlled device
• A defective doorbell or doorbell transformer inside a nearby residence
• A malfunctioning illuminated advertising display

Sometimes your own equipment may be the cause of received noise. A common-mode signal at the frequency of the radio transmitter is sometimes picked up by electrical wiring near a radio antenna. (E4E08)

The main source of noise in an automobile is the alternator. Conducted and radiated noise caused by an automobile alternator be suppressed by connecting the radio’s power leads directly to the battery and by installing coaxial capacitors in line with the alternator leads. (E4E04)

Personal computer and other digital devices can also generate noise. One type of electrical interference that might be caused by the operation of a nearby personal computer is the appearance of unstable modulated or unmodulated signals at specific frequencies. (E4E14) All of these choices are correct when talking about common characteristics of interference caused by a touch controlled electrical device (with an internal microprocessor) (E4E10):

• The interfering signal sounds like AC hum on an AM receiver or a carrier modulated by 60 Hz hum on a SSB or CW receiver
• The interfering signal may drift slowly across the HF spectrum
• The interfering signal can be several kHz in width and usually repeats at regular intervals across a HF band

Noise can even be generated by the most unlikely things. For example, it is mostly likely that nearby corroded metal joints are mixing and re-radiating the broadcast signals if you are hearing combinations of local AM broadcast signals within one or more of the MF or HF ham bands. (E4E11)