Wednesday, April 29, 2009

Noise, Bob, no. 49

The basic definition of noise is very much like the definition of weeds. A weed is usually defined as any unwanted plant. Similarly, noise can be defined as any unwanted signal. By this definition, even a very clear SSB transmission that is right on top of the one you are trying to hear can be considered noise.

Is it Noise or is it Interference?
By our broad definition above, interference is a special type of noise. Interference is noise that is generated by another electrical system. Usually it is generated by electronics of some kind and often is an intended signal. In the last case I only mean that the signal is intended, not the intereference. A common example of that would be the SSB described above where the person transmitting the unwanted signal is unaware that there is another signal on the frequency. But there are other sources of intereference than electronically generated signals. Usually these are bad electrical connections of one type or another. As the connection wavers between connected and disconnected it can make a spark which will create a bit of broad spectrum noise. If it does it a lot then you get a lot of noise.

As for other noise that is not man-made, the two most common sources are atmospheric noise and circuit noise from within your own electronic system. Atmospheric noise is caused by electrical storms and circuit noise is usually the thermal noise that exists inside of electrical components. Thermal noise is quite low and comes into play in the initial receiving stage where the signal is also very small.

Types of Noise
Let's look a little closer at the different types of noise out there.

  • Atmospheric Noise - This noise is caused by electrical storms (lightning) and is broadband in nature. It is the source of the "background" noise you hear on HF SSB which sounds like a variable hiss. The reason this doesn't sound like individual pops for each lightning strike is due to a combination of the shear number of strikes (over 30 per second, worldwide) and the multiple paths that each strike take to reach your rig.
  • Spark Noise - This is usually caused by electric motors or generators. It is a common problem that must be dealt with in mobile installations and gets the name "alternator whine" based on the sound coming out of the FM transceiver that varies in pitch with the engine RPM's. Especially in mobile installations it is usually picked up on the 12 Volt power wires.
  • HF SSB Interference - This is the extremely common situation of having two or more incoming SSB transmissions on very close frequencies. Because of the chang in pitch when you are tuned off frequency, usually one of the two stations sounds like Donald Duck on helium or like a sort of a growling. Modern radios have very fancy filters that minimize this but they can't eliminate it entirely if the frequencies are close enough. And if the two signals happen to be on the same frequency, there is nothing to be done.
  • VHF/UHF FM Interference - This is entirely different than SSB Interference. Rather than hearing two distinct signals you will likely hear one that is distorted and with a sort of buzzing underneath it. The stronger of the two signals will be the one you hear...stronger at the repeater that is. If the two signals are nearly equal in strength you will likely hear nothing but incomprehensible buzzing.
  • Circuit Noise - This is the thermal noise present in the circuit of your receiver. When you first receive the signal from the antenna, it is very low power. Depending on how noisy your circuit is, it may not be much above the thermal noise. So in each stage of amplification you amplify the noise right along with the signal. I will leave it to Lee in a later session to go into some of the details on how this is dealt with.

Wednesday, April 22, 2009

Morse Code text session #4 042209

QST DE W7VHY = New sunspot 1015 emerged yesterday to break a string of 25 consecutive spotless days. It was a short break. Less than 24 hours after it appeared, the tiny sunspot is already fading away. AR DE W7VHY SK

Regulated Power Supplies, Part 1 by Lee Bond, N7KC

April 22, 2009 Educational Radio Net, PSRG 48th Session

This is the first of a two part series on the subject of regulated power supplies. This initial part will introduce the idea of ‘regulation’ and contrast non regulated and regulated performance then include the linear or series pass regulator. The second part will deal with the switching supply and contrast the linear approach and the switcher approach to regulation.

To begin lets talk about power supplies which are not regulated. The easy example is the battery and a load. All batteries suffer from some finite internal impedance which affects the terminal voltage as a function of load. Some types of batteries exhibit extremely low internal impedance such as the lead acid type where this impedance might be in the tens of milliohms region. Other types, such as the carbon zinc cell, will have significantly higher internal impedance. The performance of this battery/load system is dictated by the relationship of the internal impedance and the external load.

For example, you cannot crank an automobile engine with a AA cell array making 12 volts but the engine will spin handily if a 12 volt lead acid battery is used. In any battery the chemistry producing the terminal voltage and the supply of electrons is in series with the internal impedance. The outside load is then in series with the battery innards and a complete loop, or circuit, is established. Since the internal impedance and the external load impedance are in series the total circuit current is governed by the combination of the two. Therefore, the smaller the internal impedance the less the external load affects the terminal voltage. The automobile engine starter is a low impedance load but much larger in numerical value than the internal impedance of the typical lead acid battery. As a result, the battery chemistry can provide the large cranking current without losses within the battery subtracting from the terminal voltage. The internal impedance of the AA cell is much larger than the lead acid impedance so a heavy load causes the internal impedance to drop a significant portion of the terminal voltage and the battery appears to be ‘dead’. Clearly, matching the type of battery and type of load is very important for good performance.

Earlier sessions of the Educational Radio Net have talked about voltage sources and current sources so lets review the definitions since these terms are commonly used to describe power supply performance. A box described as a voltage source will stubbornly maintain it’s set output voltage under all load conditions and would have zero internal impedance. A box described as a current source will stubbornly maintain it’s set output current regardless of load condition.

Modeling a battery is equivalent to providing a voltage source, i.e. the chemical process to enable work to be done, and including some internal impedance in series with the chemistry. The lower the internal impedance the more closely the battery looks like a voltage source at it’s terminals. The lead acid battery is an excellent facsimile of a voltage source under most load conditions. The AA battery is an excellent facsimile of a voltage source provided that the external load is much larger than the internal impedance. Ohm’s Law reigns supreme here when calculating losses associated with internal impedance.

Lets look at the lead acid battery with various loads to see how the math is done. Assume the internal impedance of the lead acid cell is 100 milliohms or 0.100 ohms and assume that the terminal voltage is 12 volts DC. Measuring the terminal voltage without any load using a voltmeter with normally high input impedance shows 12 volts. Adding a load of 1000 ohms shows a new terminal voltage of 11.998 volts. The difference from 12 volts, 0.002 volt, divided by the original 12 volts shows the regulation to be 0.0167%. That is, the change in terminal voltage when loaded compared to terminal voltage unloaded is 0.0167%. The same example using a load of 100 ohms yields a regulation of 0.0999%. Reducing the load to 1 ohm yields a regulation of 9.1%. These three examples show that the voltage regulation offered by a battery is good provided the load current in conjunction with the internal impedance of the battery does not disturb the terminal voltage significantly. To be a perfect voltage source the lead acid battery would need to have zero internal impedance. No such battery device is perfect.

At this point we know the definition of a voltage source and we know that unregulated power supplies offer a terminal voltage that is dependent on load and whatever internal impedance is present and, additionally, offer poor regulation unless very lightly loaded. Is there some way to compensate for internal impedance and maintain terminal voltage under all load conditions? The answer, of course, is yes and an example will illustrate a method and what is required.

Imagine a transformer with suitable plug to connect with a wall outlet. Following the transformer there is a rectifier assembly followed by a filter followed by a rheostat followed by a voltmeter followed by some sort of load. The transformer could be step up or step down depending on whether you want high voltage DC to run a vacuum tube device or low voltage DC to run your 12 volt DC radio.

Assume that you have the responsibility to ‘regulate’ the output voltage of this power supply manually at 12 volts DC. Assume that the load, such as a transceiver, is initially off and drawing no current. You have your hand on the rheostat knob and your eyes on the meter when someone plugs the cord into the AC outlet. All of a sudden the output voltage starts to increase, according to the meter which you are watching, and starts to shoot past 12 volts so you twist that knob as fast as you can to reduce the output to 12 volts. Ok, things are fine and you can watch the meter and tweak the rheostat as necessary to maintain the output at 12 volts with the very low drain on the system.

Now imagine someone turning on the transceiver load. All of a sudden the supply voltage starts to drop from 12 volts when there is a current demand and you frantically twist the knob to make the correction. In the process you notice that the voltage went too high when you over corrected, then too low when you under corrected, but you were able to eventually hit the 12 volt mark pretty well. In the meantime the transceiver went through hell because it really wanted good, clean, noise free voltage at 12 volts to operate correctly. Finally, someone is operating this transceiver in CW mode and the power supply must supply full current followed by minimum current then full current etc. You are twisting the rheostat control back and forth in vain trying to correct the power supply terminal voltage to the nominal 12 volts DC.

There is a better way and we will describe the necessary elements now. First, the available voltage must be greater than the required terminal voltage. Secondly, the power supply circuitry must be able to provide a maximum current that, at the least, equals and preferably exceeds the requirements of the load. Thirdly, there must be some reference voltage within the power supply for comparison purposes. Finally, there must be some means of comparing the power supply output voltage to the internal reference and adjusting some circuit element to maintain constant a output voltage. In the earlier example your eyes were comparing the metered output voltage with what you knew to be the desired output voltage and then your brain said to turn the rheostat in the correct direction to compensate for any error.

The first regulation method that we want to talk about is the series pass also known as the linear power supply. This methodology constantly samples the output voltage and compares this sample to some internal standard reference voltage. Any comparison difference is called an error voltage and this error voltage is used to adjust some series pass element in such a manner that the error is reduced to zero. The pass element is generally a power transistor or combination thereof located between the rectifier/filter and the power supply output terminal. The higher than output voltage reservoir preceding the pass element enables adding or subtracting from the nominal output voltage to maintain close regulation of the supply.

The down side of series pass regulated power supplies is poor efficiency. The pass element handles the entire supply current and gets hot so it is dissipating energy in addition to the energy used by the load. The closer the high voltage reservoir is to the output voltage the higher the power supply efficiency because losses in the pass element are smaller. Unfortunately the control circuitry requires a finite input/output differential to work correctly so there is a lower limit to how close these two voltages may be. Additionally, linear supplies tend to be transformer operated so large currents require large transformers and they tend to be very heavy in the higher current capacity models.

The up side of series pass regulated power supplies is very low noise generation and excellent regulation of the output voltage. Most of these supplies also offer fold back current sensing circuitry which protects the power supply and/or load from excessive and destructive currents.

The process of reducing an error voltage to zero requires what is called negative feedback circuitry. Additionally, carefully designed loop filters are required to minimize under and over shooting the desired output voltage in response to any dynamic changes. Modern supplies take full advantage of operational amplifiers and advanced filtering to achieve excellent performance.

This concludes the set up discussion for regulated power supplies part one. Are there any questions or comments with regard to tonight's discussion?

This is N7KC for the Wednesday night Educational Radio Net

Wednesday, April 15, 2009

Morse Code text session #3 041509

QST DE W7VHY = Morse code 13/5 wpm follows = For the first time, NASA spacecraft have traced the 3D shape of solar storms known as coronal mass ejections. AR DE W7VHY SK

Squelch and Tone Squelch, Bob, no. 47

Tonight's topic is squelch, including the related topic of tone squelch.

Generally speaking, in the radio world, squelch is a feature that silences unwanted sounds. What we refer to as just squelch is more properly called carrier squelch. Carrier squelch will silence the audio of your receiver when the signal strength is below a given threshold. This threshold is set by the radio operator. Nearly all FM radios have some kind of squelch level adjustment. It used to be standard to have a knob for continuous adjustment of the squelch threshold. Back then it was pretty common that whenever you had a group of hams with their HT's on, at least one would be adjusting the squelch and you would hear the intermittent sound of background noise hiss as the squelch level was adjusted down into the noise then backed off just above it. Some radios still have this knob but it is becoming common now to have the carrier squelch set to one of 3 or so discrete levels from a menu. This is a compromise to allow for smaller and less expensive radios by having one fewer knob.

The reason to have squelch on an FM radio, as opposed to AM and SSB is that an FM radio will "detect" noise at high audio levels even when there is no signal present. With AM and SSB there is a "noise floor" but because of the way this mode is detected the low amplitude noise creates a small sound compared to a stronger signal. In AM and SSB, it is the amplitude of the noise signal that determines the amplitude of the audio signal. In FM this is not the case. The random frequencies detected by an FM radio look like a strong audio signal and create a loud white noise hiss. Squelch is the answer to this problem. By setting the squelch threshold above the noise level, the radio effectively silences it. When a signal is detected above the squelch threshold the squelch circuit is deactivated and the signal is passed through. This is what is known as "breaking squelch".

Squelch circuitry is not magic. It works pretty well to allow the signal through and not the noise but it is not perfect. Some problems you might have heard are someone that has a signal just strong enough to break squelch but not strong enough to be heard above the noise, or someone that breaks squelch and the signal can be heard but it is "scratchy" or has "popcorn" or some other noise component, or someone that has a signal that is so variable and so close to the squelch threshold that he is "in and out", which is to say the signal doesn't consistently break the squelch. Another way to say that last bit is that the person does not "hold" the repeater.

Still, even with these problems, squelch is a necessary function of FM radio and it serves well to allow us to hear the signals we want and only the signals we want.

One case where you can still hear unwanted signals even with carrier squelch is when someone is transmitting on the frequency but not intending to transmit to you. The common reason for this is when you have two repeaters that share the same frequency pair and that are close enough that it is possible for someone intending to break carrier squelch on one repeater will also do so on the other. Sometimes the repeaters don't even have to be all that close. Occasionally atmospheric conditions will carry VHF and UHF signals hundreds of miles further than they would normally go. The solution for this kind of interference is tone squelch. The standard tone squelch that is built into FM radios is called CTCSS which stands for Continuous Tone-Coded Squelch System. You will also hear them called PL tones (Motorola's trademarked name for it), sub-audible tones, or just tones. The way they work is that the entire time you are transmitting you send a single audio frequency tone along with your voice. The repeater is configured to detect the specific frequency you are sending and will only re-transmit the signal if the tone is present. In turn, the repeater transmits the same tone so that your radio will only break squelch if the signal you are receiving is coming from your repeater. The frequency is one of 38 specified frequencies ranging from 67 Hz to 250.3 Hz. The frequency range of these tones was chosen to be low enough as to normally not be heard. Occasionally, especially with the higher frequencies in this range the tone can be heard but it usually doesn't interfere with the voice audio.

Tone squelch detection and transmission can be turned on and off at the repeater just as it can be at your radio. PSRG usually does not require a tone on incoming signals but usually does transmit a tone on outgoing ones. PSRG's output frequency is 146.96 MHz with a CTCSS tone frequency of 103.5 Hz. There is a repeater in Portland that is also on 146.96 MHz. I didn't find a CTCSS tone listed in the online repeater database I checked. On rare occasions we sometimes get complaints about some of our members breaking the carrier squelch on the Portland repeater. One way to prevent this would be to enable CTCSS detection on both repeaters and require all hams to transmit a tone for their own repeater.

There is a catch with tone squelch. You need to remember that even with tone squelch, you are still transmitting and receiving on the same frequencies. So if our Portland repeater were instead located in Bellevue this would not work. Here's why. You are all familiar with the problem of doubling, that is two people transmitting at the same time. It creates a distorted mess most of the time and at most you will understand only one of the two. If we had a repeater on 146.96 MHz in Bellevue with a different CTCSS tone and tones fully enabled for both repeaters here is what would happen. If only one person was transmitting and using the PSRG tone, then only the PSRG repeater would transmit and only those listening to the PSRG repeater would hear the transmission. Same would be true if there was only one person transmitting but with the Bellevue tone. So far this is working just as we want. But what happens when two people transmit at the same time, one sending the PSRG tone and one sending the Bellevue tone? Both repeaters will "break squelch" and transmit because each one has received the tone it was expecting but both repeaters will receive both transmitted signals and they will send out the signal as a double. This is made worse by the fact that someone could be talking on the PSRG repeater and because of the CTCSS the Bellevue repeater won't transmit anything and the folks listening to the Bellevue repeater will not hear anything and will think the frequency is clear. This makes it even more likely to have doubles.

There are situations where you may want to have a group of radios all on a single frequency but with different tones for different sub-groups. You have to make sure it is well coordinated and that everyone listens with the tone squelch off first before transmitting to be sure the frequency is not in use. This is the reason to have that Monitor button on your radio. On my HT and on most radios it disables both the tone squelch and the carrier squelch. This way you can also find out if the frequency is in use even if you are not using tones but the carriers of the parties that are talking are not high enough to break the carrier squelch.

Wednesday, April 8, 2009

Morse Code text session #2 040809

QST DE W7VHY = Morse code 13/5 wpm follows = Readers, if you have a solar telescope, point it at the limb of the sun and enjoy the show. AR DE W7VHY SK

FM Radio Systems by Lee Bond, N7KC

Educational Radio Net, PSRG 46th Session

This week let’s take a look at the typical FM radio system from both the transmit and receive standpoint.

The first step is to understand just what FM or frequency modulation really means. Here is the situation… we have some information that we would like to pass along to others. Assume that this, so called, ‘baseband’ information is voice generated and that the frequency content extends from about 300 hertz to 3000 hertz. In principle we could transmit baseband information directly however the antenna presents problems that are huge given the very long wavelengths associated with audio frequencies and the enormous physical size of any wire array that could do the job. A better scheme is to take advantage of smaller antennas associated with higher frequency ‘carrier’ waves and impress the baseband information onto the carrier in some fashion.

There are three ways to modify a carrier wave using baseband information. We could change the instantaneous amplitude keeping the frequency constant using a process called AM or amplitude modulation, we could change the instantaneous frequency keeping the amplitude constant using a process called FM or frequency modulation, or we could change the instantaneous phase with respect to some reference phase keeping the amplitude constant using a process called PM or phase modulation.

For this discussion lets choose just AM and FM and contrast the differences. Amplitude modulating the carrier as in AM produces sum and difference frequencies on a one to one basis with the baseband information. Assuming a carrier frequency of 10 MHz we would see lower sideband energy extending downward from 9.999700 MHz to 9.997000, the 10 MHz carrier, and upper sideband energy extending upward from 10.000300 MHz to 10.003000 MHz. The required bandwidth is 6000 hertz. The carrier amplitude never changes in AM so, to 100% modulate the carrier, we must add audio power equal to ½ the carrier power and ½ of this audio power ends up in the USB and the other ½ ends up in the LSB. SSB as in single sideband suppressed carrier is a subset of AM and the bandwidth is reduced by slightly more than ½ to 2700 hertz.

The bandwidth required when using FM is somewhat different and is related to a couple of ideas called Carson’s Rule and modulation index. Modulation index is defined as the maximum carrier deviation from nominal resting frequency divided by the highest modulating frequency and Carson’s Rule is a rule of thumb used to estimate the RF bandwidth required to transmit baseband information at a given deviation. In fact frequency modulating a carrier generates an infinite number of sidebands but about 98% of the radiated energy is contained within the limits of Carson’s Rule. As a practical matter, if the modulation index is less than 1 and in the .7 to .9 region then the FM sidebands look very much like what would be observed in an AM signal and the required bandwidth is about twice the highest modulating frequency. On the other hand, if the modulation index is larger than one then Carson’s Rule takes over and the bandwidth is about twice the sum of the deviation and the highest modulating frequency.

Amateur FM transmitters are deviation limited at 5 KHz and if the highest modulating frequency is about 3 KHz then the modulation index is 5/3 or 1.67. Carson’s estimate of the bandwidth required would be 2(5+3) or 16 KHz. This example shows that FM with a modulation index of 1.67 requires almost 2.7 times as much bandwidth as AM transmitting the same baseband information. FM is pretty much relegated to the VHF bands and above where adequate spectral space is available. A more interesting example is given by commercial FM where the highest modulating frequency is 15 KHz and the maximum modulation index is 5. In this case Carson’s Rule shows that the required bandwidth is 2(5*15+15) or 180 KHz.

Wideband FM enjoys an enormous signal to noise ratio which is the primary advantage over AM. FM also allows for transmitting DC baseband information as well, and was widely used in data telemetry before digital techniques came to fore. Old timers will remember the Ampex multi-track FM tape recorders used to collect data down to DC.

The deviation produced by a baseband audio signal is linearly related to the amplitude of this baseband signal. If you want the maximum audio ‘punch’ from your FM transceiver(s) then make sure that you talk loud enough to deviate your equipment to the maximum. All modern transceivers are deviation limited so there is an upper limit to the effects of shouting. If normal use of your equipment yields low audio at the receiving end and the receiver is known to be properly set up then you might want to change the deviation sensitivity setting in your transceiver.

To summarize operation of the FM transmitter… we know that the RF generating oscillator is running at constant amplitude but its instantaneous frequency is linearly related to the amplitude of the modulating signal. For all modulation indices, especially those greater than one, there are an infinite number of sideband energies produced but about 98% of the sideband energy is within the bandwidth defined by Carson’s Rule. Unlike AM the carrier power in FM is redistributed into the sidebands and, in fact, the carrier power can go to zero (called a Bessel zero) for certain deviation ratios. Deviation ratio and modulation index are defined the same except modulation index is based on maximums and deviation ratio is not.

Now let’s look at the typical FM receiver to see how it functions and make some contrast between AM and FM receivers.

Both AM and FM receivers would typically be superheterodyne type instruments. This means that the incoming signal is mixed with a local oscillator to produce a difference frequency which is called the intermediate frequency or IF for short. Unfortunately single conversion receivers suffer from image problems so one solution is to use two mixers and two IF’s and select the 2nd IF frequency such that the input image is out of the 2nd IF passband. These receivers are called dual conversion. The beauty is that these IF amplifier strips run at a single tuned frequency and offer sufficient bandwidth to accommodate the requirements of reconstructing the original baseband signal. Audio leveling in the AM receiver is accomplished by controlling the gain in the IF stages by the use of AGC as in Automatic Gain Control. AGC is a feedback signal developed in the receiver detector and then applied to the earlier IF stages to control overall receiver gain. AGC is very likely associated with your S meter display since the two go hand in hand. Detection in the AM receiver is very straightforward and can be as simple as passing the signal through a rectifying diode. Single Sideband Band suppressed carrier systems, a subset of AM, use a more complex detector since one must restore the missing carrier to make the audio intelligible.

Unlike the AM receiver, the FM receiver has no RF gain control… automatic or otherwise in the IF sections. The FM front end runs wide open with maximum gain at all times. The characteristic and very loud rushing sound produced by FM receivers when the squelch is opened is a result of front end receiver noise being amplified in the absence of any incoming received signal. The typical FM detector (known as a discriminator) is sensitive to both amplitude and frequency changes so, when no ‘on frequency’ RF is present, the detected noise has a strong varying amplitude component. The underlying principle is to amplify any incoming signal to the point of saturation so that amplitude variations are leveled or limited and thus will not produce any output from the FM detector or discriminator. This is the origin of the term ‘quieting’ which is so often mentioned when describing incoming signal amplitude.

Quieting sensitivity was once the defining measure of receiver sensitivity. ‘On frequency’ RF energy was piped into the antenna port and increased in amplitude until the open squelch noise was reduced (quieted) by 20 db. In voltage terms 20 db is a factor of 10 so 20 db quieting is the same as reducing the noise to 1/10 of its original value or a 90% reduction. At this point one noted the microvolts of RF to produce this noise reduction and tagged the receiver with this value which was typically in the sub microvolt region. Note that 20 db quieting is not the same as full quieting. A noise reduction of 40 db or a factor of 100 would be more in the region of "full" quieting. Quieting sensitivity has been replaced by a scheme to determine the number of microvolts required to produce a 12db SINAD relationship between noise and distortion in FM radio receivers.

There are several schemes for recovering baseband audio from frequency modulated RF signals. The Foster-Seely discriminator and ratio detector circuits appeared early on and are very effective in converting frequency changes into analog audio. Slope detection by using a miss-tuned AM detector is possible as well. The phase locked loop or PLL detector is an excellent example of the modern approach to audio recovery.

Post detection circuits in both AM and FM receivers use standard linear audio stages with appropriate filtering and gain controls. State of the art receivers are now using digital filtering, called DSP for Digital Signal Processing, in the audio stages and sometimes in the IF amplifier stages as well. The squelch circuit associated with the FM receiver is activated by receiver noise and the threshold where the squelch switch is activated is controlled by a manually operated pot or potentiometer in most cases.

This concludes the set up discussion for FM radio systems. Are there any questions or comments with regard to tonight's discussion?

This is N7KC for the Wednesday night Educational Radio Net

Wednesday, April 1, 2009

Morse Code text session #1 040109

QST QST QST DE W7VHY = MORSE CODE FOLLOWS = 13/5 WPM = Venus is passing by the sun today and undergoing a transformation from evening star to morning Star. For the next eight months, the brightest of all planets will shine in the pre-dawn sky, changing phases, casting shadows, and occasionally posing with the crescent Moon for a lovely photo op. AR DE W7VHY SK

APRS Part 2, Brian Daly, WB7OML, No. 45

For April Fools Day, we are going to dive deeper into APRS. The last session on APRS was a general high level introductory overview. For tonight’s session, we will go into more detail on the APRS protocol as well as more into settings for APRS.

The APRS Working Group, under TAPR ( developed the APRS Protocol Reference in 2000. The complete specification is available on the TAPR web site ( The working group also had an addendum approved in 2004, and can be found on Bob Bruninga’s web site,, with APRS 1.2 draft work in progress at

APRS was designed to be a real-time tactical communications tool. The power of APRS is from the use of “generic digipeating” – that is, APRS packets are sent to the network without any knowledge of the network configuration. A fundamental APRS concept is the “net cycle time”. This time is the time which a user should be able to hear – at least once – all APRS stations within range. Remember, APRS was designed to be a local communication tool, so the net cycle time gives the APRS user a picture of activity in their local area. The “design philosophy” was for the net cycle time to be 10 minutes. This might be useful for emergency tactical communications, but for everyday use, it adds a lot of traffic on the network. NWAPRS recommends that fixed stations transmit once every 30 minutes, mobiles once every 1-3 minutes.

To understand the APRS protocol, we must start with a basic understanding of data communications. A protocol is a convention or standard that controls or enables the connection, communication, and data transfer between computing endpoints. The Open Systems Interconnection Reference Model (OSI Reference Model or just OSI Model) is an abstract description for layered communications and computer network protocol design. OSI is a seven layer model - from top to bottom these layers are:

  • Layer 7 – Application
  • Layer 6 – Presentation
  • Layer 5 – Session
  • Layer 4 – Transport
  • Layer 3 – Network
  • Layer 2 – Datalink
  • Layer 1 - Physical Layer

Each “layer” in this model is defined by having similar functions that provide services to the layer above it and receives service from the layer below it. We are not going to spend much time on the functions of each layer, but it is important to understand there are 7 layers – we will see where APRS fits in next.
The Physical Layer is our radio – RF transmission and reception as well as modulation. There is nothing special that APRS does at this layer other then to use the services provides to modulate the RF signal and transmit and receive it on 144.39MHz. The modulation tones from the packet protocol are also part of this Physical Layer.

The next layer is the Datalink layer, or just Link layer for short. So is this where APRS resides? Well, not quite. At the Link Layer, APRS uses the standard packet protocol AX.25. AX.25 is an amateur radio version of the standard X.25 protocol. So what is AX.25 and what does it so? AX.25 has one purpose – it is responsible for transferring data (encapsulated in packets) between nodes and detecting errors introduced by the communications channel (in our case, the radio propagation path). AX.25 has two modes of operation – connection-oriented (virtual circuit connected) and connectionless (datagram style). The specification can be found at

The connection-oriented mode can be been used to establish direct, point-to-point links between packet radio stations, without any additional network layers. This is sufficient for keyboard-to-keyboard contacts between stations and for accessing local bulletin board systems and DX clusters. A simple routing mechanism using digipeaters can be used for the connection-oriented mode. Digipeaters act as simplex repeaters, receiving and retransmitting packets from local stations. They allow multi-hop connections to be established between two stations unable to communicate directly.

But recall APRS is not a connection-oriented protocol; that is, we do not send data to anyone in particular. APRS data is sent without expecting any response, and reception at the other end is not guaranteed. APRS uses the AX.25 connectionless mode. APRS uses what is known as an “Unnumbered Information” or “UI” frame in a broadcast fashion.

The UI frame in AX.25 has 9 fields:

  • Flag — The flag field at each end of the frame is the bit sequence 0x7e that separates each frame.
  • Destination Address — This field can contain an APRS destination callsign or APRS data. APRS data is encoded to ensure that the field conforms to the standard AX.25 callsign format (i.e. 6 alphanumeric characters plus SSID). If the SSID is non-zero, it specifies a generic APRS digipeater path.
  • Source Address — This field contains the callsign and SSID of the transmitting station. In some cases, if the SSID is non-zero, the SSID may specify an APRS display Symbol Code.
  • Digipeater Addresses — From zero to 8 digipeater callsigns may be included in this field. Note: These digipeater addresses may be overridden by a generic APRS digipeater path (specified in the Destination Address SSID).
  • Control Field — This field is set to 0x03 (UI-frame).
  • Protocol ID — This field is set to 0xf0 (no layer 3 protocol).
  • Information Field — This field contains more APRS data. The first character of this field is the APRS Data Type Identifier that specifies the nature of the data that follows.
  • Frame Check Sequence — The FCS is a sequence of 16 bits used for checking the integrity of a received frame.

The APRS data itself is contained in the Information Field, and you can look at it as a Network-layer protocol with Application-layer data. The Information Field is broken down into the following:

  • 1 byte APRS Data Type Identifier defines the format of the data.
  • “n” bytes of APRS Data which include one of the 10 types of data:
  • -- Position
  • -- Direction Finding
  • -- Objects and Items
  • -- Weather
  • -- Telemetry
  • -- Messages, Bulletins and Announcements
  • -- Queries
  • -- Responses
  • -- Status
  • -- Other
  • 7-byte APRS Data Extension, defined for the specific data type (for example, Position can have an extension of Course and Speed).
  • “n” byte Comment which is a plain text comment such as a beacon message, or may contain further APRS data such as Altitude, Maidenhead Locator, Bearing, Weather & Storm Data, etc.

The APRS Protocol Specification goes into detail on the format for the various data types.

Let’s look at one application – Messages, Bulletins and Announcements. These are APRS packets that contain free-format strings. A Message goes to a single recipient and an acknowledgement is expected (ok, I did say APRS is “connectionless” – more on this later). Bulletins and announcements are broadcasts to multiple recipients without an acknowledgement. Message text may be up to 67 characters long (47 on the Kenwood D7A), and may have an optional message identifier which is used for the acknowledgement.

This is one case where APRS is used to send a message and to expect a reply, but it still uses a connectionless protocol – that is, there is no guarantee of delivery. The Message Identifier along with the address information is used by the recipient to reply to the message – this is done within APRS and not within AX.25. There is a Message Acknowledgement defined, which is the same as a “Message” with the text being “Ack”. A message reject can also be sent. Note however, that if a station receives multiple copies of the message, which is entirely possible, it will ack each time the message is received. There is no “state” kept.

APRS also defines a National Weather Service Bulletin message format, which can be used to highlight warning areas. There is also a NTS Radiogram format.

There are many other capabilities defined in the APRS protocol which we just do not have time to cover in this session. Perhaps at another session we can focus on the various data types and explore the capabilities to emergency communications further.

Now lets briefly go on to the parameters needed to get started with basic APRS. To transmit APRS data, here is the information that you need (example for the Kenwood D7A):

  • Your call sign and SSID (MY CALL)
  • Optional station icon (mobile, house, etc.)
  • If you do not have a GPS unit connected, you can manually enter your LAT/LON (MY POS)
  • You can send a comment along with your position, such as “In Service” (POS COMMENT)
  • You can send a 20-character status text along with your comment (STATUS TEXT)
  • Packet Path WIDE1-1, WIDE2-1 (PACKET PATH)
  • Select the radio to send out either transmit manually (MANUAL) via PTT or automatically (AUTO) via a beacon (BCON). If you select AUTO you need to set the beacon interval, usually 1-3 minutes for mobiles (TX INTERVAL).

That is the basic programming functions required. You should be able to find yourself after several minutes on one of the APRS mapping programs.

You APRS-enabled radio is also capable of seeing who is around you, sending and receiving messages, and all the other functions we covered here tonight. Next time we will talk more about messaging with APRS.

For tonight, 73s and have fun!