## Wednesday, September 30, 2009

### Bits, Nibbles, Bytes, and Words, Lee Bond N7KC

September 30, 2009 Educational Radio Net, PSRG 71st Session

The digital computer is ubiquitous. These computers are everywhere. Kids have their own and, more than likely, so do the grandparents. Most users are very proficient with the keyboard and blaze through the games, documents, spreadsheets, and whatnot without even a thought of what is going on inside that mysterious box called a computer. I am willing to bet that not one in ten users can define a "bit" and I will further wager that not one in 100 users has a good grasp of computer arithmetic or how the rational coding of bits makes their computer tick.

My task tonight is to lead you down that magic path and explore the idea of the "bit" and how it can be used to represent numbers or events. We will then extend the bit notion into nibbles, bytes, and words. Some of the net participants tonight are likely to be expert in this bit world so, if you quality, think of ways to help me clarify the notion when I break for questions or comments.

Let’s start with our very familiar decimal number system and dissect it in a way that I bet few of you have done. We will concern ourselves only with integers so no fractions are allowed in our discussion. Integers are the numbers 1, 2, 3, etc. that have no fractional part. You may call them whole numbers if you please. Integers can be precisely defined but that would muddy the water a bit so we will dispense with such formalism in favor of simplicity. We will start with the integers 1 through 9 and include the notion of nothingness as in zero.

When we think of the decimal integers we must consider the ordering. For example after nothingness as in zero we think of 1, then 2, then 3, etc. in order through 9. Our standard notion of 9 is that it is ‘bigger’ than 8 and 8 is ‘bigger’ than 7 or 6 or 5, etc. In the following discussion throw ordering out the door. We will not be interested in ordering. In fact we will not even use numbers rather we will choose a collection of identical marbles and call then symbols. So, zero (or nothingness) plus 1 through 9 equals 10 symbols which we represent with 10 marbles.

Unfortunately this forum of ours does not include a chalkboard so we must create a virtual chalkboard in your mind. If it helps then close your eyes and imagine the following: in your mind arrange 6 boxes in a 2 row x 3 column array. Each of the boxes in the top row has 10 identically sized marbles and the row of lower boxes have ‘nothing’ or zero marbles. Just so we do not get marbles in the wrong boxes lets choose black marbles for the rightmost box, brown marbles for the center box, and red marbles for the leftmost box.

Ok, lets do something useful with this scheme. Assume that you are sitting on the back porch looking toward the Cascades and you want to tally lightning strikes. When the first strike occurs you move a black marble from the top right box into the box below. At the next strike you repeat the previous operation and move another black marble from the top right box into the box below. As you notice more and more lightning strikes you move black marbles until you have no more black marbles in the top right box. At this point you have used all of your marbles, so to speak, and you indicate this fact by moving a brown marble into the middle box below then move all of the ‘used’ black marbles back into the top right storage box.

The lightning strikes continue and you move black marbles, one by one, into the lower box until you again run out of marbles. At this point you move a brown marble from the top box into the box below and reload the top box with the black marbles. Eventually you will run out of brown marbles in the top center box so you will move a red marble from the top left box to the box below and reload the top boxes with appropriate black and brown marbles.

The point of this mental exercise is to illustrate that you can tally objects with symbols and that every time you exhaust, or cycle through, your symbol set you just indicate this fact by incrementing the symbol set to the left. You can choose as many symbols as you please. In this example, based on the decimal system with 10 symbols, each box to the left is weighted or "heavier" by 10 with respect to its neighbor to the right. As a result, this 10 symbol scheme goes units, tens, hundreds, thousands, etc. so is very convenient for us mortals. Once you know the weighting you can easily figure out how many lightning strikes you tallied by counting the symbols in each weighted box.

In the interest of time let me assert that you can use a set of 12, or 9 or 8 or 7, etc. symbols and the scheme holds true. Now lets examine the case where we have reduced the set to two symbols. Thinking for a moment you realize that "bi" means two as in binary star or binocular. Could the binary number system be as simple as accounting for just two symbols? The answer is, of course, yes. The beauty of using two symbols in numerical computing is that transistors are very good switches and can be used to represent the two binary states perfectly. A SPST switch normally has two states such that it is either on or off. Open or closed. It is a trivial exercise to arrange a circuit with a mechanical switch such that one switch position results in +5 volts and the other position results in zero volts. The symbols associated with a two state switch are zero and one as in 0 or 1.

Now, in a fashion similar to the decimal box exercise above let’s arrange 8 binary boxes side by side in a row. We know that the state of each of these "binary" boxes can be represented by either a 0 or a 1. Assume that the rightmost box is the, so called, least significant box and that all boxes are in the 0 state. Visually the boxes look like 0000 0000. Now, bump the rightmost box to the 1 state and the visual presentation goes to 0000 0001. Well, the rightmost box has now used all of its symbols so to tally the next event it must return to 0 and the box to the left is bumped from 0 to 1 as in 0000 0010. If you carry this process forward you will eventually achieve 1111 1111 and the next event will change all positions back to 0000 0000. The weighting goes by 2 rather than 10 as in the decimal case so each position to the right is ½ of its left neighbor or each position is twice its neighbor to the right. The sequence is 256, 128, 64, 32, 16, 8, 4, 2, 1. These 8 boxes represent a byte and each of the positions is a binary digit or bit. Two sets of 8 boxes as in 2 bytes or 16 bits represents a binary "word". Half a byte as in 4 bits is a nibble (or nybble to some).

The total number of distinct combinations of 8 bits from 0000 0000 to 1111 1111 is 256 and goes as 2^8. A binary word composed of 16 bits can assume 2^16 or 65,536 distinct combinations. Recall that we are limiting ourselves to positive integers so we are only able to handle numbers up to 65,536. There are schemes to handle signed integers such as 2’s complement arithmetic and to be completely flexible there are floating point schemes which look a lot like scientific notation.

Four binary bits can assume 16 distinct combinations and is the basis of the hexadecimal number system which has symbols 0 through 9 plus A, B, C, D, E, and F. Four binary bits are also used for BCD or binary coded decimal notation by throwing away the six unused states.

How would one go about doing math operations on a computer? Well, microprocessors are very good and quick at adding or subtracting but clumsy when multiplying or dividing. Early microprocessors required many operations or, so called, clock cycles to produce numerical products and it was common to have a math coprocessor standing by to do the heavy lifting. For example, if you want to multiply a number by 10 in the decimal system you just move the decimal point one position to the right. In the binary system you would shift left (x2), shift left again (x4), shift left again (x8), and then add twice to achieve the 10x product. Clearly, there is a lot of time consuming overhead doing math in this fashion. Contemporary microprocessors are constructed with onboard and dedicated numerical processors with slick routines and which unburden the main unit.

If you look at the schematic of any computer one thing will pop out immediately. There are lots of parallel paths connecting the various semiconductor packages. These parallel wire structures are called busses and might be as wide as 16 bits… possibly more… for memory addressing as an example. There is likely a data buss as well. One line generally represents the buss with breakouts at the ends. Each wire handles one bit of data on the data buss.

In summary, the intent of this presentation is to show that symbols can be used to tally events and that recycling symbols will work in any number system. Starting with the familiar decimal number system and showing that symbol manipulation leads handily to the very simple two state, or binary, number system. A bit is a two state binary digit, a nibble is four associated bits, a byte is 2 associated nibbles or 8 bits, and a word is 2 bytes of 16 associated bits. Modern computers are said to have 32 bit words and, in some cases, 64 bit words.

This concludes the set up discussion for bits, nibbles, bytes, and words. Are there any questions or comments with regard to tonight's discussion topic?

This is N7KC for the Wednesday night Educational Radio Net

## Wednesday, September 23, 2009

### Phase Shift Keying, Bob, No. 70

Tonight I am continuing with my series on Spread Spectrum Radio. I am going to discuss Phase Shift Keying (PSK), which is the form of modulation used by Direct Sequence Spread Spectrum (DSSS) Radio.

Let's have a short review of spread spectrum. The two main kinds in use in ham radio are Frequency Hopping and Direct Sequence. Frequency hopping is relatively easy to understand. Your carrier, rather than being on a fixed frequency, jumps from one frequency to another. As long as the equipment receiving makes the same jumps you will be able to transmit the signal and prevent interference on any one frequency from causing a problem. This can be used for analog or digital transmission but is most commonly used for digital. Note that you must have a pattern of frequency changes that is known to both parties in order to allow the receiver to follow the transmission.

In the other method, Direct Sequence, you only transmit digital information. In fact the name "Direct Sequence" comes from the 11 digit sequence of ones and zeros that is used to modulate the carrier. If your data bit is a zero then you send the sequence normally. If your data bit is a one then you send a one where the sequence has a zero and send a zero where the sequence has a one. For example 101 would turn into 010. I mentioned that the carrier is modulated by this sequence but I didn't say how. It is modulated using Phase Shift Keying.

Phase Shift Keying (PSK)
So, what exactly is phase shift keying? It is a way of changing the phase of a carrier to transmit a digital signal. In essence what you are doing with PSK is phase hopping. In the simplest example called Binary Phase Shift Keying (BPSK) you hop 180 degrees out of phase to transmit a certain piece of digital data. So the signal is either in it's "normal" phase or in it's 180 degree out phase which is to say, inverted. Note that the frequency of the carrier isn't changed but when you change the phase and modulate the carrier you create side-band emissions. The faster you modulate the signal, the wider the side-bands. In addition to BPSK there is Quadrature PSK (QPSK) where the phase can be one of four values, 0, 90, 180 or -90. There is also 8PSK using 8 phase angles and so on. The general term is Multiple Phase Shift Keying (MPSK).

Differential Phase Shift Keying (DPSK)
Basic BPSK has the signal at normal 0 phase to represent a digital zero and at 180 degrees out to represent a digital 1. A commonly used variation on this is called Differential Binary Phase Shift Keying (DBPSK) and instead of a 1 being 180 degrees out of phase, a 1 always changes the phase while a 0 always keeps the phase the same. This way you don't need a reference signal to know which is 0 degrees and which is 180 degrees.

PSK-31
If you want a very narrow signal you are limited to a slow digital transfer rate. This is the case with PSK-31. The 31 comes from the 31.25 bits per second data rate which generates 31.25Hz sidebands. This is pretty much the opposite of spread spectrum. The whole point of PSK-31 was to use such a small slice of the band that you could actually fit many PSK-31 channels in the space of a typical SSB voice bandwidth.

With spread spectrum you want a very wide bandwidth; by definition, one that is much wider than necessary to convey the information. By encoding each bit of data with the 11 bit sequence used in DSSS you now modulate the signal 11 times for each bit transferred. So for a 1 Mbit/sec transmission, you modulate at 11 MHz. This achieves the spread spectrum you are looking for. This modulation is encoded using DBPSK and a suppressed carrier.

## Wednesday, September 16, 2009

### THE VACUUM DIODE EXPOSED, Lee Bond N7KC

September 16, 2009 Educational Radio Net, PSRG 69th Session

What a digital summer we experienced at the hands of Curt Black, WR5J! Curt pulled all the stops on a summers survey of free software downloads for your computer which, more or less, turned your radio into an analytical tool. As impressive as this tour was, one must ultimately realize that the most complex task that your computer performs is based on tiny packets of electrons or "charge" being directed here and there by the software commands. Elemental electrons in motion, or moving electric "charge", is the bottom line idea in electrical theory and all electrical devices including radio equipment.

Let's review what we know about "charge". We know that it enjoys the symbol Q in the literature. We also know that it is an assembly of electrons and can be as few a one electron. We know that a collection of electrons numbering 6.14 x 10^18 is known as a Coulomb of charge. We know that charge will move under the influence of an electric field. We know that the original notion, or conventional notion, of charge was based on the erroneous idea that charge carried a positive sign hence moved in the direction of an applied electric field. Modern theory has reversed the original positive charge idea since electrons are negative entities and, in fact, move counter to the direction of any applied electric field. We know that charge can be motionless as in static charge. We know that an electrical current is charge in motion. It would seem that we know a lot about charge.

The study of charge is best represented by the science of Physics with Physical Chemistry running a close second. The study of charge as it applies to the vacuum tube would come under the heading of Classical Newtonian Physics in contrast to the study of charge in semiconductor materials which would come under the heading of Modern Quantum Physics. In my view the vacuum tube represents the most elegant device for demonstrating the behavior of electrical charge influenced by an electric field. Lucky me to be raised in the heyday of the vacuum tube. As late as 1963 the US Navy destroyer to which I was assigned had a single piece of transmitting equipment that had modern solid state diodes in the circuitry. The selenium rectifier preceded germanium devices and modern silicon devices and required no heater but it is a stretch to include it as more than a rudimentary solid state device. The solid state transistor with all its ramifications is a relative newcomer to the field of electronics. Modern radio equipment's are solid state for the most part and only the old timers can relate stories of warming their hands in the glow of those magnificent glass bottles.

No one has actually seen an electron. These entities are very, very tiny and to image them requires wavelengths small in comparison to the size of an electron. The one common instance of, more or less, stationary electrons occurs in the lattice structure of many crystals and x-ray crystallography has demonstrated diffraction images suggesting that these things are real. The fact that we can manipulate these tiny guys to the degree that we can is testimony to the very clever work of early scientists.

We know that conductors have an abundance of so called "free" electrons. This is in contrast to tightly "bound" electrons which are not available to contribute to electric current flow. For example the neutral copper atom has 29 electrons associated with the nucleus in 4, so called, shells with the innermost shell containing 2 electrons followed by the next shell containing 8 electrons followed by the next shell containing 18 electrons followed by the outermost shell with a single, so called, valence electron. The three inner shells are tightly bound to the atomic nucleus but the outer single electron is easily forced out of place and can contribute to the electric current bumping along a copper wire. Most metals are conductors to varying degrees with silver, copper, and gold at the top. A copper atom missing its valence electron is known as a copper ion.

I think we now have enough information to appreciate how a two terminal vacuum diode works so let's move on to some apparatus to demonstrate the effect. Forget the little glass bottle for the moment. We are going to use a laboratory bell jar and good quality vacuum pump as a part of our apparatus. Everyone has seen the "bell" jar on a stand with a mechanical vacuum pump attached. For our purposes the bell jar stand needs some electrical penetrations so that we can supply potentials to the bell jar innards. The first of the two inside devices is the filament or heater which also serves as a cathode. There are several schemes for heaters so let me select the one known as the directly heated filament cathode. This will be a tungsten wire section which will glow a bright red to orange when filament voltage is applied. Adjacent to the filament-cathode wire we will position a "plate" of flat metal such that it does not touch the filament. This metallic plate is, in fact, called the "plate" electrode in vacuum tube terminology and serves as the anode. Note that the plate may be cylindrical and surround the filament in real world devices.

Our demonstration diode is complete. We have a filament (cathode) and plate (anode) plus a means of producing a good vacuum so on goes the bell jar but someone forgot to start the vacuum pump. Not realizing that a good vacuum is missing we switch on the filament voltage and sure enough the filament starts to light. Then there is a bright flash as the oxygen in the bell jar contributes to the destruction of the filament. Oops. Off with the jar and we install another filament wire. Ok, this time we turn on the pump and let it run until it chortles. Now when we flip the switch for the filament the wire glows a cheery orange. The chortling pump indicates that the internal vacuum (or pressure) is in the 1 to 10 micron range and suitable for our demonstration. The low internal pressure means that atoms of oxygen and nitrogen are scarce and will not interfere with the electronic process that we are interested in observing.

Let's think about the filament voltage for a moment. I did not mention it but the source of the filament voltage is a battery. Traditionally the battery used for filament power is known as the "A" battery.

So, here is our situation... we have a nicely glowing filament with an unconnected plate nearby and both are within a reasonably good vacuum. The glowing filament is probably heated to 800 degrees F or thereabouts and the thermal energy of the filament has caused lightly bound electrons to break free and form a cloud in the immediate vicinity of the filament-cathode. The thermal energy for breakaway is known as the work function and metals vary in this regard. Some substances such as barium offer very low work functions and are used in cooler, indirectly heated, cathode structures.

Our cloud of electrons is negatively charged. If we cause the nearby plate element to become positively charged with respect to the filament-cathode then the electrons will move toward the plate anode and, since moving electrons constitute electrical current, we can measure a plate current if we insert some current measuring device in series with the plate. With regard to the plate, the battery traditionally used to supply plate voltage is known as the "B" battery hence follows the term B+ for the plate positive voltage supply.

There is a third battery associated with vacuum tubes which is known as the "C" battery however it is not relevant here since we are using a two terminal device or diode and the C battery is only relevant in triode structures and beyond for grid biasing.

At this point we have demonstrated that electrons will traverse a vacuum if the plate is positive with respect to the source of electrons. If the plate is negative relative to the source then the electrons are repelled and no plate current will flow. Herein lies the secret of the rectifying diode. If the plate is alternately positive then negative with respect to the cathode as would happen if connected to alternating mains then plate current only flows on positive excursions of plate voltage. Bi-directional current from AC mains becomes unidirectional current in the plate circuit. A single diode offers half wave rectification and a dual diode (or two individual diodes) offers full wave rectification.

The high vacuum in the bell jar or little glass bottle performs two functions. First the pumping process removes virtually all oxygen so the filament suffers no oxidation. Secondly, the high vacuum is synonymous with low pressure both which equate to few residual gas particles present to hinder electron flow from cathode to plate anode. In reality small glass bottles with high internal temperatures will out gas damaging particles which will poison the vacuum so special devices known as "getters" are used internally to trap these vacuum destroyers.
Special attention must be given to the metal leads going through the little glass tube envelopes. If the glass and wire conductors do not expand and contract in the same manner with extreme changes in temperature then the seal will be broken and the tube will be rendered useless in short order. Special wire alloys which match the glass thermal characteristics are used to avoid this problem.

In summary, the physics and mathematics associated with the classic vacuum tube is elegant and a fun pursuit for the very curious. The concept of electronic charge flow within a triode vacuum tube is easily grasped and directly applicable to field effect transistors. The natural extension from diode to triode by introducing a control grid between cathode and plate made possible amplification and the rest is history.

This concludes the set up discussion for the Vacuum Diode. Are there any questions or comments with regard to tonight's discussion topic?

This is N7KC for the Wednesday night Educational Radio Net

## Wednesday, September 9, 2009

### Winlink 2000, Boone Barker KC7RK, no. 68

Boone Barker, KC7RK

Introduction
This paper provides a brief introduction to the Winlink 2000 system—what it is and how it works, and how to become a user.

To start, here are a couple of defining quotes from the Winlink web site at www.winlink.org .

“Winlink 2000 (WL2K) is a worldwide system of volunteer resources supporting e-mail by radio, with non-commercial links to internet e-mail.”

“To use the Winlink 2000 system, you must hold an Amateur Radio license or be a member of a supported organization or agency. Use of the system and all software is free of charge for those who qualify.”

So Winlink is global, with access around the world. It is developed and supported entirely by volunteers. It is free. The system provides e-mail services to licensed Radio Amateurs without access to the internet such as mariners at sea or expeditions in remote areas.

In addition, a growing number of government agencies and organizations have included WL2K in their emergency communication plans. Winlink 2000 can provide user -to-user e-mail services in a familiar format from inside a disaster area, using only a radio to connect to the outside world.

At the heart of the WL2K system are five mirror image, redundant Common Message Server (CMS) hubs. They are located in San Diego (USA), Wein (Austria), Perth (Australia), Halifax (Canada), and Washington DC (USA). With this redundancy, the system will remain operational even if large segments of the internet are down.

Connected to the five CMS sites are a multitude of Radio Message Server (RMS) nodes, like spokes on a wheel. Traffic flows between the CMS hubs and the internet e-mail recipient, and between the end users and the RMS gateways.

As an aside, the term “PMBO” (for a participating mailbox) is being phased out, but still shows up in Winlink writings.

The radio network has both RMS HF stations and RMS VHF/UHF Packet stations.

RMS HF stations form a controlled and frequency-coordinated global network of Winlink stations. The HF stations all use Pactor, a digital ARQ mode that transfers text files and graphics quickly and error-free. Pactor 2 and 3, the faster modes, are only available on TNC’s produced by SCS in Germany. The WL2K Development Team is working on WINMOR, a new HF transmission protocol that will be freely distributed. It will complement, not replace Pactor; RMS HF stations will be able to handle both modes.

RMS VHF/UHF Packet stations are also part of the network, providing automated messaging capability using AX.25 packet radio in combination with the WL2K Common Message Servers. Although limited in range, RMS Packet stations are widely available in the United States and a few other countries. RMS Packet can provide regular local access to Winlink, or a temporary emergency portal for radio e-mail users, or for fixed installation at unattended remote locations where it can provide radio e-mail communications to the “last mile.”

The locations of public and emcomm RMS stations are shown in maps on the Winlink web site. Related status tables list station call sign, with frequency and mode and grid locator. Note that information about frequencies used by RMS HF emcomm stations is limited to authorized sysops and users.

WL2K Client Software
Paclink is a Winlink 2000 radio e-mail client that links to common e-mail programs such as Outlook Express and Mozilla Thunderbird. Paclink adds telnet, VHF packet radio, HF Pactor radio and WINMOR HF radio channels for WL2K connectivity to compatible user e-mail client programs. Installation and configuration are relatively easy.

Airmail is the oldest and most widely used e-mail program for sending and receiving messages on the Winlink system. Airmail supports HF Pactor, VHF/UHF Packet, and telnet connections over any TCP/IP medium including the internet and high-speed radio media like D-Star. Airmail also has position reporting capability, and a very nice HF propagation prediction program. It can be linked to common e-mail programs such as Outlook Express. Installation and configuration are somewhat difficult, but a nice guide is available; just Google “INSTALLATION AND SETUP FOR WINLINK AIRMAIL”.

Airmail is a stand-alone e-mail program. The primary drawback of Airmail is that it only works with a short list of hardware modems. But it is well suited for WL2K HF connections using a Pactor modem. And the latest version of Airmail 3.3.081 can be used with AGW Packet Engine and a soundcard to make VHF/UHF packet connections, by installing AMPE software. See http://www.qsl.net/mararc/ampe.htm web site.

On the other hand, Paclink has to be linked with an e-mail program. But it works with a wide variety of hardware TNC’s and modems and with AGW Packet Engine. TNC initialization scripts can be modified by the user. Also, scripts can be used to connect through a digipeater or packet node. CMS Telnet is simple and easy on Paclink, to send and receive WL2K messages on an internet connection.

Other software
AGW Packet Engine (AGWPE) handles traffic between your TNC or soundcard and packet programs that are configured to use AGWPE. It is free to download from the SV2AGW.com web site. A lengthy tutorial for installation and configuration is located at http://www.kc2rlm.info/soundcardpacket .

Winlink 2000 RMS Packet gateways can also be accessed with regular packet software, to compose and send a message from the keyboard. Just connect to the RMS Packet node and read the greeting. Type H for help and then follow the instructions to compose a message. This is perhaps the easiest way to try out Winlink if you already have a packet setup.

Hardware
For telnet connection to a CMS, the only hardware needed is a computer with internet access. WL2K e-mail through RMS Packet stations requires a VHF radio, a compatible TNC or modem or soundcard, and necessary interface cables.

To connect with an RMS HF gateway on Pactor 1 requires that you have any one of the hardware TNC’s on setup lists in Airmail or Paclink. Pactor 2 or 3 requires that you have a Pactor TNC made by SCS . These cost \$1,000 or more.

How do you get started?
First and obviously, you need an amateur radio license. With that, here is a suggested initial sequence.

Another option: if you already have an operational packet setup, use it to connect to a local RMS Packet node. The Winlink web site has a map that shows all of the active RMS Packet stations. Just zoom in on your local area and pick them out. Then go to the Reports tab on the Winlink Web site, scroll down to RMS Packet Status, and look up frequencies of those stations in the table. Sometimes these local nodes are not functioning. So if you don’t get a connection, try another station.

You can use WL2K client software with packet and your TNC if it is listed in the setup for Paclink Packet TNC Channels or in Airmail VHF Packet Client Setup. Check out the TNC using a simple terminal program. Then try connecting to one of the local RMS Packet stations.

If you want to use a soundcard for packet, first download and install AGW Packet Engine. Create a new radioport for your soundcard. Then configure Paclink or Airmail to use AGWPE. Remember that Airmail requires that AMPE be running. There is a link to AGWPE is the Airmail TNC list but it is not functional.

You might also want to download AGWTerninal, and AGWMonitor from the SV2AGW web site. They are free. AGW Monitor lets you see all the traffic to and from your TNC/modem/soundcard, and AGWTerminal is a nice simple terminal program. AGWTracker is a simple APRS program, also nice. All require that AGWPE or Packet Engine Pro be running.

If an online course is to your liking, there is a “Winlink for Dummies” course that takes you through all the steps. It can be accessed through www/winlink.org/GetStarted.

Some recommendations
You should always format WL2K e-mail messages in plain text. HTML format adds unnecessary bytes to the message. Attachments should be made as small as possible.

Learn about the Winlink Whitelist and how to work with it. This is an anti-spam filter. E-mails to your Winlink e-mail address need to have //WL2K in the subject line or they will be rejected—unless they come from an e-mail account on your Whitelist. E-mail addresses on outgoing messages are automatically added to your Whitelist.

When you configure a radioport in AGWPE for SignaLink USB, select an unused printer port (LPT3) as your PTT port. It’s easy to hang up at this point in the setup.

Summary
Winlink 2000 has been used since 1999 by Radio Amateurs at sea and in the jungles to send e-mail messages by radio. As a result of experience in Katrina recovery operations and other disasters, Winlink has been included in operational plans of a growing number of emcomm organizations such as RACES and ARES units, Red Cross, MARS, Baptist Relief, and the Salvation Army.

Another defining quote from the Winlink web site:

“The WL2K mission is to provide, through a volunteer network, effective last resort communications in civil emergencies and personal communications in non-emergency conditions.”

As a Radio Amateur, you may already have experience with packet radio—that’s all you need to access Winlink by radio. Another simple start involves connecting by telnet over the internet. And if you are already a Winlink user, you might consider becoming the sysop for your own RMS Packet station. Software and guidelines are on the web site.

So have fun setting up Winlink and trying it out. It might well be very useful in an emergency, to you and to your community.

September 9, 2009

## Tuesday, September 1, 2009

### Spread Spectrum Communications, Bob, no. 67

Tonight's topic, spread spectrum communications, may seem to be the stuff of spy fiction and ultra-expensive military hardware, but you have almost certainly used spread spectrum whether you know it or not and you probably have the sophisticated spread spectrum radio gear right in your own house. I'm talking about wireless networking gear, also known as WiFi. More on that later.

According to the ARRL Handbook, spread spectrum is defined as using an RF bandwidth much larger than needed to carry the signal, and where the bandwidth of the signal is independent of the modulation by the signal. It is a form of radio transmission that makes use of a wide bandwidth to avoid interference by noise or other signals. You can imagine that if you are transmitting a simple AM signal over 100 frequencies at the same time then someone transmititng on any one of those frequencies will only contribute one percent toward the final recombined signal. This would be fine until you had someone else also transmitting an AM signal on the same 100 frequencies. Then you would be back to a big interference problem. Partly in order to allow hams to use the same frequency range, there are more sophisticated ways to use those multiple frequencies. We will discuss those in a moment.

• Relative Immunity to Interference
As described above, unless someone else is using the very same spread spectrum technique and is synchronized with you, you likely won't notice the interference
• Security
As we will see, there are sophisticated ways to encrypt a signal. This is why it is still valuable to the military.
• Lower Power Density
By spreading the signal over a range of frequencies, the power at any given frequency is so low it can be below the noise floor and unnoticed.
There is only one real trade-off and that is the technical complexity necessary to accomplish spread spectrum.

In a sense you could say that spread spectrum began with the earliest radio transmitters. Spark gap transmitters created CW signals that covered a very broad spectrum. You could charitably say that this signal could get through interfering signals but really, it was more of an interfering signal.

Early experiments with intentional spread spectrum began in the late 20's but it was World War II and the military that really pushed the technology forward. Unfortunately, because spread spectrum is still used by the military, much of the history is still kept secret.

In 1981, a group called the Amateur Radio Research and Development Corporation (AMRAD) began experimentation with spread spectrum. In 1989 an idea was put forth to use the Wireless LAN (WLAN) devices in ham radio, and in 1999 the FCC relaxed their rules about hams using spread spectrum. This relaxation opened the door for hams to use equipment already being made for WLAN.

Analog signals can be carried over spread spectrum transmissions, but nearly all spread spectrum use today is with digital signals and that is what we will discuss.

COMMON TYPES
As the name implies, the transmitting frequency hops around in a pre-arranged pattern. In the 802.11 spec, there are 3 sets of 26 such patterns using 75 frequencies. By some clever hopping algorithms you can have 802.11 devices using different sequences, or channels, on the same 75 frequencies without interfering with each other. Because there are only 78 sequences, a receiving device could discover the channel being used by the various transmitters and sync up with one.