Tuesday, December 16, 2008

Amplitude Modulation

December 17, 2008 Educational Radio Net, PSRG 30th session, Lee Bond N7KC

The subject of tonight’s discussion material is amplitude modulation and the fundamentals thereof. This is the first of a three part series dealing with the process of transmitting voice band frequencies via radio. My next session will focus on single sideband processes and the third session will focus on frequency modulation.

If one looks at the bandwidth required to transmit various signals it is immediately apparent that three designations will suffice to describe the bandwidth required to do the job. The first segment is very narrow bandwidth and this includes CW and several of the popular digital modes. The second definable segment would be moderate bandwidth and this includes voice transmissions, facsimile, and slow scan television. The third segment is the very wide bandwidth signals such as fast scan television.

For this session we are interested in moderate bandwidth voice transmissions and, in particular, the amplitude modulation approach to transmitting voice using radio techniques. As a practical matter we are interested in somehow shifting voice range frequencies to a range more suitable to fit our antennas since the antenna is really where the ‘rubber hits the road’. We will assume that our antennas are cut to fit whatever amateur band we choose to use.

Lets define voice range frequencies for radio purposes as those starting at 20 hertz and extending to 2500 hertz. The, so called, high fidelity range extends to 20,000 hertz but most of the important voice energy required for communications is contained in the region under 3000 hertz. The ratio of the high voice frequency to the low frequency is about 125:1. In principle one can transmit audio frequencies in the same manner as ‘radio’ frequencies but the antenna dimensions would be enormous. For example, assuming standard propagation velocity, a half wavelength at 20 hertz is about 4600 miles and a half wavelength at 2500 hertz is about 37 miles. If you were to cut the antenna for midrange then it would be seriously de-tuned at either end frequency. So what to do?

Mathematics to the rescue. Everyone has heard the rule that two frequencies, if mixed, will produce sum and difference frequency spectra and this spectra will include the original two frequencies as well. This ‘mixing’ behavior is predicted using trig product identities and the mathematics is valid for audio frequencies right up through radio frequencies. Let’s play with some numbers to get a feel for how this mixing business works.

First however, we want to appreciate a couple of terms often used to describe the behavior of circuits. Linear and non linear. A linear circuit processes signals in a straight line fashion. For example, if you double the signal feeding a linear amplifier circuit then the output signal will precisely double. There is no perfectly linear active electrical circuit but it is possible to come very close to perfectly linear. A perfectly linear amplifier will process multiple signals without any interaction between signals. One simple measure of linearity is harmonic distortion. If you drive an amplifier with a single perfect sine wave signal then you would expect a perfectly linear amplifier circuit to present only a single output frequency. If a spectrum analyzer shows any energy at multiples of the driving frequency then these added frequencies are a result of harmonic distortion caused by the amplifier and harmonic distortion is an artifact of non linear performance.

On the other hand, there are circuits which have been deliberately designed to be non linear. If a non linear circuit is used as a ‘mixer’ then you can assume that at least two frequencies are being processed by this circuit. Mixing, in actuality, is really multiplication or the product of at least two frequencies as defined by the product identities in trigonometry.

Now, with that aside, let’s get back to playing with our numbers. Assume that we are feeding two audio frequencies into a non linear ‘mixer’. One frequency is 1000 hertz and the other is just twice the first or 2000 hertz. The sum output is 3000 hertz and the difference is 1000 hertz which is the same as one of the driving frequencies.

Now let’s mix another pair, this time 1000 hertz and 3000 hertz. This time the sum is 4000 hertz and the difference is 2000 hertz. A look at the spectra would show four frequencies namely 1 Khz, 2 Khz, 3 Khz, and 4 Khz.

In like manner let’s mix 1000 hertz and 100,000 hertz. The sum is 101,000 hertz and the difference is 99,000 hertz. The spectra shows our original ‘mixing’ frequencies, 1 Khz and 100 Khz, and the product frequencies of 101 Khz and 99 Khz. The maximum difference between upper sideband frequency and lower sideband frequency is just 5000 hertz so the percentage of bandwidth compared to carrier is just 5%.

Finally, let’s mix 1000 hertz and 10,000,000 hertz or 10 Mhz. This time the sum frequency is 10001000 hertz and the difference is 9999000 hertz. The spectra shows our mixing frequencies of 1 Khz and 10 Mhz plus the product frequencies of 10.001 Mhz and 9.999 Mhz. The audio range frequency, 1000 hertz, could be any frequency between 20 hertz and 2500 hertz and would produce mixing products with the ‘carrier’ frequency (10 Mhz) that extend from 9.9975 Mhz to 9.99998 Mhz and from 10.00002 Mhz to 10.0025 Mhz. The, so called, carrier frequency has energy below it called the lower sideband energy and energy above it called the upper sideband energy. The maximum difference between upper sideband frequency and lower sideband frequency is just 5000 hertz so the percentage of bandwidth compared to carrier is just 0.05%. This indicates that both the carrier frequency and sideband frequencies will ‘fit’ our antennas nicely in the band we choose to transmit within. Voice modulating frequencies in the range of 20 to 2500 hertz are so far removed from the carrier energy that they are filtered out of the final product.

Voice modulation is the process of imprinting intelligent baseband information upon a signal suitable for radio transmission. In the case of amplitude modulation the baseband voice information causes the instantaneous carrier amplitude to change and this change can be detected at great distance to reconstruct the original baseband voice information.

Nothing is free and so it is with amplitude modulation. To 100% modulate a 1000 watt carrier using AM it is necessary to provide 500 watts of audio power. The 500 watts ends up being split between the upper sideband and the lower sideband and, spectrally, the carrier amplitude remains constant. Amplitude modulation is inefficient from the power standpoint since the full carrier power is transmitted but this power contributes nothing to the impressed intelligence. Amplitude modulation is also inefficient from the bandwidth standpoint since identical upper and lower sideband information is transmitted requiring a bandwidth twice as large as the modulating signal.

Recovering the impressed information from an AM signal can be as simple as detecting the, so called, envelope of the signal. This amounts to rectifying the signal and filtering out the carrier. What remains is just the analog of the original modulating signal. This is the precise method used by simple ‘crystal’ sets which are still popular with experimenters. One particularly nasty artifact of operating AM is the heterodyning of adjacent carriers. Radio operators put up with this howling until improved techniques made AM obsolete.

In summary, amplitude modulation or AM is a very simple but inefficient means of impressing information on a ‘carrier’ signal. The AM process is very straight forward and easy to understand but lacks the elegance of improved methods of communication.

This concludes the set up for the discussion of AM. Are there any questions or comments?

This is N7KC for the Educational Radio Net

Wednesday, December 10, 2008

EmComm, Brian Daly, WB7OML, week 29

Amateur Radio Emergency Communications, or “Emcomm”
Brian Daly, WB7OML

Let’s start out by defining - what is a communication emergency? According to the definition in the ARRL Level 1 course, a communication emergency exists when a critical communication failure puts the public at risk.

What are some circumstances that can overload or damage critical day-to-day communication systems?
  • Storm knocks down telephone lines or radio towers
  • A massive increase in the use of a communication system that causes it to be come overloaded
  • Failure of a key component in a system
  • Earthquake
  • Volcano

What are some potential Communications Emergencies in Seattle?

Can a communication emergency occur in “normal” circumstances? Yes, definitely, some examples being:
  • Underground cables being dug up
  • Fires in telephone equipment buildings
  • Car crash knocks down a key telephone pole
  • 9-1-1 systems can fail
  • Hospital systems can fail

So what makes a good emcomm volunteer? Amateur emcomm volunteers come from a variety of backgrounds with a range of skills and experience. Emcomm volunteers share one common characteristic – the desire to help others without personal gain, the ability to work as a member of a team, and to take direction from others. An emergency situation will bring a lot of stress and pressure, thus an emcomm volunteer needs the ability to think and act quickly.

Where do you fit in? We amateurs bring equipment, skills, and frequencies necessary to create emergency communications networks under poor conditions. We have licenses; we have pre-authorization for national and inter-national communication. Many of the skills we bring to emcomm are the same things we do on a day-to-day basis; other skills are specific to emcomm and needs to be learned through courses like the ARRL ARECC Level 1 and through drills and exercises.

Radio equipment, frequencies and basic radio skills are not enough. Without specific emergency communication skills, you can easily become part of the problem.

It is also important to know your limits of responsibility as an emergency communicator. What an emcomm volunteer is not - we need to know where to draw the line, what our limitations are. We are not “first responders”, generally we do not have authority – we don’t make decisions for our served agencies, nor do we place demands on them. But we can make some decisions – the decision on whether to participate or not, and decisions affecting your own life and safety. In general we are not in charge – we are there to fulfill the needs of the served agency.

You cannot “do it all”. If the served agency runs short of specialized help, it is not your job to fill it especially if you are not trained for the job. But you can fill in an urgent need or perform jobs where communication is an integral part, if you are qualified.

And remember, leave your ego at the door!

There are differences between “day-to-day” communication and “emergency communication”. First and foremost, in day-to-day communications there is no real pressure to “get the message through”. No one’s life depends on it. You do things at your leisure. Emcomm can involve both amateurs and non-amateurs, it happens in real-time, there is a lot going on simultaneously perhaps on several nets, there may be little or no warning, you may have to set up and be operational anywhere in a short period of time, and there is no schedule. Public service events may come close to emcomm, as they can be “planned disasters” – about the only know piece is the schedule!

Your job as an emcomm volunteers is simple – communicating is job #1. Notice it is “communicating” and not “amateur radio” – there is a significant difference. Our job is to get the message through regardless of how that happens. We as amateurs have many tools available for this job, and amateur radio is just one of those tools. FAX machine, Internet email, cell phones, landline phones, amateur radio, CB radio, FRS radio, served agency radio – all of these are at our disposal and should be considered. We bring communicating skills to the table, not just amateur radio. There are stories of amateurs that pass long supply lists over the radio, tying up repeaters or frequencies, while sitting next to an operational FAX machine. It is not our job to “show off” our radios – it is our job to “communicate”. Just think about the best and fastest way to send it. Of course, when all else fails we do have the amateur radio.

So what happens during a communication emergency? Some scenarios will not require immediate action, for example during a “watch” or “warning” for a severe storm. This is the period to make sure you go-kit is together, and you are ready to go if called. Other scenarios will happen fast and will require immediate need – for example, an earthquake. Once the need for emcomm is identified, the served agency will put out the call for amateurs to help. Most emcomm groups have defined procedures for activation, such as defining a “rapid response team”. Nets will be established to handle resources and logistics, such as the processing and directing of incoming volunteers. Once these operations begin, things can happen quickly – message traffic grows, confusion exists. Do we have relief operators? Do we have food and water? Where will the volunteers sleep? Do we have batteries, fuel, other logistical needs? Communication assignments need to be made – shelters, gathering damage reports, handling supply requests and other logistical needs of the served agency. Nets will be established, rearranged and disassembled as the needs arise. Volunteers need to remain flexible. Finally, the demands of the emcomm communication effort will decrease, nets can be closed, and volunteers released.

But the emcomm event does not end when the last net is shut down. This starts the after action report period, which will help to improve the response next time around.

There are many additional skills to learn to help you become a successful emcomm volunteer – knowing who your served agency is, their organization, basic communication skills, message handling, net operating, and of course, personal safety, survival and health considerations. We will cover more of these topics on this net in the coming months. Also, the ARRL Amateur Radio Emergency Communication Course Level 1 is another opportunity to learn these skills.

Wednesday, December 3, 2008

BALUNS, Jim K7WA, No. 28

December 3, 2008 – Educational Radio Net
Jim Hadlock K7WA

What does a balun do?
What happens if you don't use one?

Bal-Un is a term formed from the words balanced and unbalanced. It refers to a device used to couple an Unbalanced transmission line to a Balanced load. In the real world, we use a balun to couple a coaxial transmission to a balanced antenna, such as a dipole.

Coaxial transmission lines are commonly used to connect our transceivers to antennas. Coax comes in several sizes and types for different applications. It consists of an inner conductor with an insulated covering (dielectric), which is then covered with a braided wire sheathing (shield). The sheathing is covered with a flexible outer jacket. Coax is weatherproof and may be buried underground, run inside a metal mast or taped to a tower without harmful effects. At the transceiver, the center conductor is connected to the transmitter output (or receiver input), and the shield is connected to the chassis. This arrangements works well with an unbalanced load, such as a vertical monopole antenna fed against a ground plane or radials. However, when coax is used to feed a balanced load, such as a dipole antenna, some provision should be made for converting from the unbalanced transmission line to the balanced load. Otherwise, RF currents will flow on the outer conductor of the coax, compromising the effectiveness of the antenna.

To understand this problem, think of a coaxial transmission line as a wire centered inside a metal pipe. When we connect the coaxial transmission line to our transmitter, the RF current flows on the center wire and on the inside surface of the pipe. This is due to what's called the "skin effect". The "skin effect" describes how RF currents flow in a thin layer on the surface of a conductor, proportional in depth to the wavelength of the signal. If we connect the other end of the coaxial transmission to a balanced antenna, such as a dipole, RF current from the center wire flows to one side of the antenna. The current from the inside surface of the pipe however, is connected to two conductors: the other side of the antenna and the outside surface of the pipe. Current flowing on the outside of the pipe is subtracted from the current that should be flowing on the antenna creating voltage and current nodes on the outside surface of the pipe back down to the transmitter where it is grounded. To go back to our coax fed dipole example, RF current on the outside surface of the coaxial transmission line shield will distort the radiation pattern of the antenna and detract from its effectiveness. It may also contribute to television interference.

A properly connected balun will reduce or eliminate the RF current flow on the outside surface of the coaxial transmission line shield. While the most common use of a balun is at the feedpoint of a balanced antenna, they are also used at the output of an antenna tuner to feed a balanced transmission line (Twin Lead) and even part way down a feedline to convert from balanced transmission line to coaxial transmission line (as in the G5RV antenna).

There are several types of baluns available to radio amateurs and described in the literature. Let's begin with the Current Balun (also called the Choke Balun). Current Baluns have become popular for application in the high frequency range (1.8 mHz to 30 mHz) because they are simple, cheap, and effective. In its simplest form, a Current Balun consists of a number of turns of coaxial cable wound into a close coil at the feedpoint of the antenna. The size of the coil is determined by the operating frequency. For example, the installation directions for the Cushcraft A3S tri-band yagi specify eight turns of RG8/U coaxial cable with a six inch diameter. This coil is a high impedance RF choke at the operating frequency of the antenna and prevents RF current from flowing on the outside of the coaxial transmission line shield. Another approach to the Current Balun was introduced by Walter Maxwell, W2DU. This involves slipping a stack of high-permeability ferrite beads over the coaxial transmission line at the feedpoint of the antenna. The stack of ferrite beads creates a high impedance effectively suppressing any RF current from flowing down the outside surface of the transmission line. Current Baluns and ferrite bead kits are available from many sources.

Another approach is the Voltage Balun as described by Jerry Sevick, W2FMI, and others. This design uses inductors to produce equal, opposite phase voltages into the two resistances, or halves of the antenna. An additional feature of the Voltage Balun is that, by using a combination of inductors as a broad-band RF transformer, it can accommodate impedance conversion in addition to balancing the RF voltages. Typical impedance conversion is 4:1, although Sevick describes transmission line transformers with many other ratios in his classic book: Understanding, Building, and Using Baluns and Ununs.

A third balun technique, most often used at VHF and UHF, is the Coaxial Balun made from a half wavelength loop of coaxial transmission line and presenting a high impedance to any RF current that might otherwise flow on the outer shield of the coaxial transmission line. The half wavelength Coaxial Balun gives a 4:1 impedance step-up.

While I have described how a balun improves the effectiveness of a coax fed balanced antenna, it also has other uses. Consider a vertical antenna with elevated radials. The outer surface of the coaxial transmission line shield will "look" to the antenna like another radial. A Current Balun at the feedpoint of the vertical will prevent RF current from flowing on the feedline. According to author John Devoldere, ON4UN, in Low- Band DXing: "Is it harmful to put a current balun on all the coaxial antenna feed lines for all your antennas? Not at all. If the feed point is symmetric, there will be no current flowing and the beads will do no harm. As a matter of fact they may help reduce unwanted coupling from antennas into feed lines of other nearby antennas."

Baluns are an effective means of preventing unwanted RF current on the outer shield of coaxial feedlines from distorting antenna patterns, as well as reducing TVI (radiation coupling into nearby television sets, house wiring, etc.) and RF in the shack.


ARRL Technical Information Service: An Analysis of the Balun, by Bruce A. Eggers
WA9NEW: www.arrl.org/tis/info/pdf/9409061.pdf

Some Aspects of the Balun Problem, by Walter Maxwell W2DU:

Baluns: What They Do and How They Do It, by Roy W. Lewallen W7EL:

Understanding, Building, and Using Baluns and Ununs, by Jerry Sevick W2FMI, CQ
Communications, Inc.

Low-Band DXing (4th Edition), by John Devoldere ON4UN, The ARRL, Inc.

The ARRL Antenna Book (21st Edition), The ARRL, Inc.

The ARRL Handbook, The ARRL, Inc.

Palomer Engineers (1:1 Current Balun Kit): www.palomer-engineers.com

The Radio Works (Baluns, Coax, Antenna Parts, etc.): www.radioworks.com