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)

IARU supports proposal for .radio domain name

From the ARRL Letter, 8/30/12. I’d register kb6nu.radio in a heartbeat…….Dan

The International Amateur Radio Union (IARU) has expressed public support for a .radio top-level domain name. Under the proposal as put forth by the European Broadcasting Union (EBU), registration will be available via the EBU to all eligible radio representative organizations and broadcasters, Internet radios, radio amateurs, radio professionals and their respective representative organizations, as well as companies providing radio-specific products and services in order to create a worldwide radio community. The proposal must be approved by the Internet Corporation for Assigned Names and Numbers (ICANN); this organization is responsible for the coordination of the global Internet’s systems of unique identifiers and, in particular, ensuring its stable and secure operation. Read more here.

 

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)

ARRL Centennial Convention in Hartford, July 2014

This was released by the ARRL today. Sounds like fun to me…..Dan

August 29, 2012  NEWINGTON, CT – ARRL, the national association for Amateur Radio®, announced the organization will hold its national Centennial Convention in Hartford, Connecticut, July 17-20, 2014. The Convention will mark 100 years of the ARRL’s founding in Hartford. The theme for ARRL’s Centennial year is “Advancing the Art and Science of Radio — since 1914.”

Hiram Percy Maxim (1869-1936), a leading Hartford inventor and industrialist, founded the American Radio Relay League (ARRL) in May 1914, together with Clarence Tuska, secretary of the Radio Club of Hartford. Today, ARRL serves over 158,000 members, mostly licensed radio amateurs, in the US and around the world.  The organization’s headquarters has been maintained in the Hartford area since its founding. ARRL’s present facilities are located on Main Street in Newington, Connecticut, a suburb of Hartford, and are visited by nearly 2,000 groups and individuals each year. The site is also home to The Hiram Percy Maxim Memorial Station since 1938. The radio station, W1AW,  is known as “the flagship station for Amateur Radio” and is known world wide.  ARRL employs around 100 people.

“The 2014 Centennial Convention is a once-in-a-lifetime opportunity,” said ARRL Marketing Manager Bob Inderbitzen explaining that event will have all of the trademark elements of a proper convention and hamfest; presentations and forums, exhibits, vendors, demonstrations, flea market, activities for youth, and banquet. “But, plan on some very special centennial-themed activities,” he added, “including tours of ARRL headquarters and W1AW, guest presenters, some surprises, and lots of celebrating! We want ARRL members to come with all of their experiences from the first one hundred years of Amateur Radio and ARRL, and leave with a shared vision for ARRL’s Second Century.”

The decision to host the Centennial Convention in Hartford was reinforced by the organizers of the New England Division Convention, held every two years in Boxborough, MA. “Boxborough’s organizing sponsor, FEMARA, Inc., graciously agreed to forgo holding a convention there in 2014,” said ARRL Chief Operating Officer Harold Kramer. “Instead, FEMARA has offered to help share its expertise and volunteers as we prepare to bring this national level celebration to Hartford.

The area boasts dozens of attractions and activities, making Connecticut a great destination for members who plan to attend the convention with their family and friends. Nearby attractions include the Connecticut Science Center, Mark Twain House, Harriet Beecher Stowe Center, and Wadsworth Athenaeum. Hartford is served by an international airport (Hartford/Springfield BDL), and is conveniently located between Boston and New York City. Hartford’s centrally located Union Station is serviced by Amtrak and major bus companies.

Interested parties may learn more about ARRL, Amateur Radio and the Centennial Convention at www.arrl.org/expo.

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)

IARU Region 1 Monitoring System newsletter available monthly

While each of the three IARU regions has a monitoring program, the Region 1 monitoring program seems to be the best organized. They have a great website, and their monthly newsletters for 2010-2012 are all available there. In addition, the site has other information including radar systems on shortwave (PDF, 10 MB), frequency allocation table, and How to start an Intruder Watch.

Comparing this to the IARU Region 2 Monitoring System Web page, you can see how good it is. The last bulletins, for example, were published in 2009.

 

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)

Interesting stuff on the Internet – 8/27/12

Here’s some more interesting stuff that I’ve run across on the Internet:

  • HamInfoBar. This toolbar, which is available for Firefox, IE, Safari, and Chrome, give you easy access to a wide variety of net info, including callsign info, APRS station info, eQSL data, and electronic component data.
  • Area51 ham radio forum. Area51 is a set of bulletin boards. Some hams are trying to establish a ham radio forum there, but they need a minimum number of “followers” to do so. Click on the link and follow the forum.
  • Simple Ham Antennas. This ham-radio blog, written by KH6JRM, has some very specific content – all of the posts describe simple antennas that can be home-brewed. For example, the last three are a 40m – 10m loop antenna, a vertical antenna for 40/20/15/10m, and stealth antennas.

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

Rectulangar and Polar Coordinates

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)

TAPR announces Digital Communications Conference details

Where: Atlanta, GA Sheraton Gateway Hotel Atlanta Airport 1900 Sullivan Road Atlanta, GA 30337

When: September 21 – 23, 2012

Website: www.tapr.org/dcc

Technical / Introductory Sessions Schedule http://www.tapr.org/pdf/DCC_2012_Schedule.pdf

Technical Sessions Friday – Saturday: Introductory Sessions

Saturday Night Banquet Speaker & Topic: http://www.tapr.org/dcc#banquet
DCC Saturday Night Banquet Speaker will be Bdale Garbee, KB0G talking about the “Sharing the Joy of Making.

Sunday Morning Seminar Speaker & Topic: http://www.tapr.org/dcc#seminar
DCC Sunday Morning Seminar will be a hands-on tutorial using Gnuradio  to design and implement software defined radios on your laptop presented by Tom Rondeau, KB3UKZ, the leader of the Gnuradio project.