Monday, October 27, 2008

Antennas: The Yagi, Bob, Week 23

Tonight we will cover another of the basic ham antennas, the Yagi. This is the most popular rotatable antenna as it is a good compromise between, cost, durability, manageability and performance.

Let me start by saying I fully expected to get a neat simple explanation of the theory of Yagis from the ARRL Antenna Book but it is not there. This seems to be one of those black magic designs that just work. Don't misunderstand, this can be modeled by the popular computer programs and you can see how it works but I didn't find any simple explanation of why it works. With that said, let's dive in anyway and at least describe it and what it does, along with some of the compromises in design.

It consists of a horizontal boom with two or more horizontal elements that are perpendicular to the boom. I believe most of you have seen several Yagis by now so I won't go too much into the appearance. The two necessary elements are the driven element, which is essentially a horizontal dipole like the kind we have covered previously, and the reflector. The reflector, as you might guess is placed "behind" the driven element, that is to say, the radiation of the antenna is primarily in the direction opposite the reflector. A Yagi has only one reflector. Any more elements after the driven element and the reflector are directors and are on the opposite side of the driven element from the reflector. So, going from back to front the elements are: reflector, driven element, director, director, etc.

Early designs of Yagis had all of the elements equally spaced at around 0.15 wavelengths between each one. Optimal designs now have the reflector, driven element and first director more closely spaced (about 0.1 wavelength or less) and the directors spaced farther apart. So, lets look at what we mean by an optimal design. First, what are the design trade-offs of a Yagi?

Even though we wouldn't all agree on what the perfect Yagi was we can agree on three things we would want:
  • 50 Ohm Impedance at the feedpoint; pure resistive (no reactance)
  • Zero gain at the back and sides
  • Maximum possible gain at the front
We might disagree on how narrow we want the gain pattern to be but lets forget that for now and look at the big three.

If we design for a maximum gain antenna what we get is an antenna that has a very narrow range of frequencies with a usable SWR. The ARRL Antenna Book has a nice set of graphs showing this which I will use to share some numbers. The example I've chosen is for a 10 Meter, 3 element Yagi. For those that have the book it is on page 11-5. The table below shows three Yagi designs: maximum gain antenna, the maximum gain per SWR antenna and the optimal antenna.

ValueMax Gain DesignMax Gain per SWR DesignOptimized Design
SWR at 28.4 MHz222
SWR at 28.0 MHz722
SWR at 28.8 MHz1022.2
Gain at 28.4 MHz8.47.67.2
Gain at 28.0 MHz7.97.57.1
Gain at 28.8 MHz8.27.87.4
F/R at 28.4 MHz132222
F/R at 28.0 MHz201520
F/R at 28.8 MHz61823

From this table you can see that you get a modest improvement in gain for the maximum gain design but at a cost of both SWR and the Front to Back gain ratio. The Gain per SWR gives you good SWR across the band and better gain than the optimized but at a cost of decreased Front to Back gain ratio. The optimized Yagi design sacrifices a bit of overall gain but gives you a good SWR across the band and a consistently good Front to Back ratio as well.

There are design considerations for adding more directors as well but I will leave that for another time. This is more specifically for VHF/UHF antennas and we may have a session just on that.

The ARRL Antenna Book rates two element Yagis well and indicates that the increased gain drops off as you start adding directors.

EZNEC Antenna Software by W7EL
Here is the promised link to get the EZNEC antenna modeling software. I have the free version now which limits you to 20 segments. Each wire should have several segments to allow for accurate modeling so 20 segments won't go very far on a multiple element antenna. I will probably end up buying the full version which is $89 for a web purchase and direct download or $99 to get the CD.

Wednesday, October 22, 2008

Potentiometers 101

October 22, 2008 Educational Radio Net, PSRG 22nd session, Lee Bond N7KC

The impedance series is now history. During the course of 13 weeks we looked at several of the most fundamental ideas in the physics of electrical phenomenon and, hopefully, gained some practical knowledge of how these ideas link together to form a basis for our understanding of all things electrical. Let's exercise some of this earlier impedance series material and see how it can be applied to solve practical problems which are routinely encountered on the bench. My choice for the first study is the potentiometer or "pot" in the vernacular.

There is one wee problem with the word potentiometer which we must clear up before proceeding... there are two devices which share the same name but perform different duties in the electrical world. In early laboratories one could find a very elegant device, with many knobs, generally in a nicely crafted wooden box, and which was used to measure electrical potential differences with great accuracy. Today this function is performed by sophisticated digital voltmeters and one rarely sees the older instrument except in museums. The potentiometer that we will study is the familiar device commonly found on the front panels of our radios, which can be rotated to produce some desired action.

These devices are everywhere. Virtually all "level" controls such as audio volume, AGC, squelch, power supply voltage output, and many more are based on the lowly pot so a good grasp of the underlying operational details is a must for your bag of tricks. We all know what a pot looks like physically. It is a resistive device which has 3 contact points. Basically each end of the resistive "element" is attached to one of the points. The remaining contact point, generally known as the "wiper", connects to a sliding assembly which is controlled by some knob or motor, and which makes a mechanically movable contact which is adjustable from one end of the resistive element to the other.

The resistive element proper was carbon in the early days of this device but modern materials have largely replaced carbon. More common today is the very robust cermet element and the even more robust wire wound element. The carbon element tended to abrade as the wiper slid along its surface and they would become "scratchy" and very annoying. Cermet has much less tendency to abrade and also offers what is called infinite resolution. In contrast is the wire wound pot which may or may not offer infinite resolution depending on construction. If the resistance element is just a length of resistance wire formed in a circle then the resolution would be deemed infinite since the slider can find any point on the wire. If the wire resistance element is a helically wound structure which is then formed in a circle then the wiper can only contact discrete points along the main wire and the pot cannot be set infinitely fine.

The most common pot is structured with a linear taper meaning that doubling the angle of rotation will double the resistance from the wiper with respect to a designated end of the element. Additionally, there log taper pots where the resistance changes logarithmically with rotation angle. The log class includes the audio taper pot which produces a uniform change of loudness to your ear with uniform shaft rotation if used as a volume control. Variations here are log clockwise or counter clockwise.

There is a class of very high precision multiturn wire wound potentiometer devices called Helipots by Beckman. Bourns and others offer similar devices. These offer 5 turn, 10 turn, and 20 turn rotations so the total resistance can be controlled over as much as 7200 degrees of rotation. The linearity of these devices as deviations from a straight line are specified and they all offer extraordinary precision. Generally used with turns counting dials.

One last point concerning the use of wire wound pots is in order. In addition to the desired resistance mechanism there is an added component of inductance present. If the element is helically wound then the inductive component is much larger than that encountered with the simple wire element. Inductance/reactance effects limit the use of these sort of pots in AC circuits hence they are more commonly found in DC circuits.

Ok, let's build a circuit. We will need a power supply of some sort so how about using a 10 volt battery. 10 volts will be convenient for our discussion even though a 10 volt battery would be an oddity for sure. Then we need a pot to work with. Lets choose a simple 1000 ohm carbon unit rated at 2 watts and which is structured as a linear device. That's it for our circuit parts... just a battery and a pot. Lets connect the battery and pot in series in the following manner. Pot contacts are normally labeled 1, 2, and 3. Contact 1 is commonly the low potential reference so it will go to the battery negative terminal. Contact 2 is always the wiper and common convention states that the wiper, contact 2, moves toward the contact 3 end with clockwise rotation of the knob. So, to complete our circuit we connect contact 3 to the battery positive terminal.

Now we need a measuring device so let's choose a VOM as in Volt-Ohm-Milliamp meter. In fact we will need two of these meters so let's use the common Simpson 260 VOM. We want to measure the series current flowing in our circuit so disconnect the pot contact 3 from the battery positive and insert, in series, one of the VOM's with positive lead going to the battery positive and the negative (common) lead going to contact 3 on the pot. From Ohm's Law we expect the series current to be 10 volts divided by 1000 ohms and, sure enough, the series meter shows the current to be 0.01 amperes or 10 milliamperes. (Note: let me assert that we are using a "perfect" meter here... one that does not influence the circuit being measured. In real life no such device exists and all measuring instruments change the circuit to some degree. The effect is commonly described as "loading".)

From earlier discussions we know that 1 ampere is defined as 1 coulomb of charge per second past a given point so the 0.01 ampere represents 0.01 coulombs per second flowing in our circuit. Also from earlier discussions we know that you cannot impress any voltage on a resistor without the resistor becoming warmer than it's surrounding environment. The job of a resistor is to convert the energy of moving electrical charge to heat energy. Remember Joule's Law? The power calculation for our little circuit is voltage squared divided by resistance or 0.1 watt. From earlier discussions we know that 1 watt is one joule per second so we conclude that 100 milli-joules of electrical energy per second is being converted to heat in our pot resistive element and a sensitive thermometer would show some upscale movement.

Now, adjust the pot shaft fully clockwise, and let's add the second meter as a voltmeter and place the meter negative lead on the battery negative and the meter positive lead on contact 2 of the pot. Fully clockwise moves contact 2 to contact 3 and we see 10 volts on the meter as you would expect since both are in contact with the battery positive terminal. Now rotate the shaft to mid rotation, half way between rotational extremes, and notice that the meter indicates 5 volts or 1/2 of the previous initial reading. Moving the shaft again such that the wiper moves toward contact 1 shows that voltage goes toward zero whereas moving from midpoint toward contact 3 shows the voltage going toward maximum.

Now consider the shaft at mid position where we measured 5 volts on the meter. Since the pot is linear the mid position resistance should be 1/2 of the 1000 ohms or 500 ohms. At mid point we would expect 1/2 of the total power to be dissipated above the wiper position and 1/2 dissipated below. At mid point we measure 5 volts and we know that the resistance is 500 ohms. So, 5 squared divided by 500 from Joule's Law gives 0.05 watts which is 1/2 of the total 0.1 watts dissipation.

The idea of "voltage drop" follows directly from the lesser amount of energy dissipated as the wiper approaches the reference terminal or contact 1 on the pot. By extension, one could partition the resistor element into 10 equal sections and then argue that the total must be the sum of the parts so each part would then dissipate 0.01 watts. Each partition would then "drop" 1 volt over 100 ohms which computes to 0.01 watts.

My point here is to show that, yes, one can talk glibly about voltage drops around a resistive circuit, but the underlying principle is directly related to energy conversion to heat. Resistors always throw something away but the wiper on a pot allows you to choose at what level you want to save. Basically a pot is an attenuator. The output signal will never be larger than the input because of the energy conversion into heat phenomenon.

The pot is a simple voltage divider and the output voltage can be easily calculated. The fraction of the tapped off resistance divided by the total resistance times the input will yield the output voltage. For example, using our 1000 ohm pot, if the tapped resistance is 133 ohms and the input voltage were 8.5 volts then the output voltage is 133 ohms divided by 1000 ohms times 8.5 volts or 1.1305 volts. Conversely, if one knows the output voltage and the tap ratio then computing the input voltage is a piece of cake. In like fashion, if you know the input voltage and desired output voltage then calculating the tap point is one more piece of cake.

One last point... sometimes you will see a pot symbol wired with terminals 1 and 2 or 2 and 3 connected together. In this case the pot is wired as a rheostat and is nothing more than a variable resistor. It is not possible to voltage divide with a single rheostat. You must have at least two units to achieve voltage division.

In summary, all resistors dissipate energy and will be measurably warmer than their environment. Voltage drops are a direct consequence of energy dissipation in a resistive element. Kirchoff's voltage law which states that the algebraic sum of the voltages in a closed loop is zero is simply a restatement of conservation of energy where total energy converted to heat equals total input energy. Given that work and energy are identical it follows that work in equals work out hence the net work is zero.

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

Something to ponder: Two atoms are leaving a bar when one says to the other "I left my electrons in the bar". The other says to the first "are you sure?" The first replies "I am positive".

This is N7KC for the Wednesday night Educational Radio Net.

Monday, October 13, 2008

General Test Grab Bag, Bob, Week 21

Tonight we will cover some of the procedural rules that appear in the General Class test.

First is a set of three questions dealing with an unusual situation in the ham bands. That is the situation where amateur radio is secondary to others that also use the band. In other words, the other service or services have priority over the amateur radio service in these bands. The bands are the 30 meter band and the 60 meter band.

The rule is a common sense one and allows the greatest flexibility to amateurs. Quoting from Part 97.303, "A station in a secondary service must not cause harmful interference to, and must accept interference from, stations in a primary service." In practice that means anytime there is interference between you and a primary service, where you are secondary, you must stop immediately, even if you are in the middle of operating and the primary service starts interfering with you. You are free to change to another frequency within the band where you aren't interfering and continue operating.

So on to the questions.

G1A14 (C) [97.303]
Which of the following applies when the FCC rules designate the amateur service as a
secondary user and another service as a primary user on a band?
A. Amateur stations must obtain permission from a primary service station before
operating on a frequency assigned to that station
B. Amateur stations are allowed to use the frequency band only during emergencies
C. Amateur stations are allowed to use the frequency band only if they do not cause
harmful interference to primary users
D. Amateur stations may only operate during specific hours of the day, while primary
users are permitted 24 hour use of the band

G1A15 (D) [97.303]
What must you do if, when operating on either the 30 or 60 meter bands, a station in
the primary service interferes with your contact?
A. Notify the FCC's regional Engineer in Charge of the interference
B. Increase your transmitter's power to overcome the interference
C. Attempt to contact the station and request that it stop the interference
D. Stop transmitting at once and/or move to a clear frequency

G1A16 (A) [97.303(s)]
Which of the following operating restrictions applies to amateur radio stations as a
secondary service in the 60 meter band?
A. They must not cause harmful interference to stations operating in other radio
B. They must transmit no more than 30 minutes during each hour to minimize harmful
interference to other radio services
C. They must use lower sideband, suppressed-carrier, only
D. They must not exceed 2.0 kHz of bandwidth

Here is a question that is Emergency Communication related. Once again, the answer is both common sense and allowing the greatest flexibility to the amateur.

G1B04 (A) [97.113(b)]
Which of the following must be true before an amateur station may provide news
information to the media during a disaster?
A. The information must directly relate to the immediate safety of human life or
protection of property and there is no other means of communication available
B. The exchange of such information must be approved by a local emergency
preparedness official and transmitted on officially designated frequencies
C. The FCC must have declared a state of emergency
D. Both amateur stations must be RACES stations

Music and Encryption...don't do it! (With a couple of very interesting exceptions!) The general idea about using codes and really about all communication in amateur radio is that you are not allowed to operate in a way that intentionally obscures the meaning of what you are communicating. If the codes you are using are generally known and so are understood generally then you are okay.

G1B05 (D) [97.113(a)(4),(e)]
When may music be transmitted by an amateur station?
A. At any time, as long as it produces no spurious emissions
B. When it is unintentionally transmitted from the background at the transmitter
C. When it is transmitted on frequencies above 1215 MHz
D. When it is an incidental part of a space shuttle or ISS retransmission
So unless you happen to be in the Space Shuttle or the International Space Station, you don't get to transmit music.

G1B06 (B) [97.113(a)(4) and 97.207(f)]
When is an amateur station permitted to transmit secret codes?
A. During a declared communications emergency
B. To control a space station
C. Only when the information is of a routine, personal nature
D. Only with Special Temporary Authorization from the FCC
Again, unless you happen to be controlling a space station (and how cool would that be!) you don't get to do it.

Here is another question about using codes.
G1B07 (B) [97.113(a)(4)]
What are the restrictions on the use of abbreviations or procedural signals in
the amateur service?
A. Only "Q" codes are permitted
B. They may be used if they do not obscure the meaning of a message
C. They are not permitted because they obscure the meaning of a message to FCC
monitoring stations
D. Only "10-codes" are permitted

Finally here is a catch-all of prohibited activities.

G1B08 (D) [97.113(a)(4), 97.113(e)]
Which of the following is prohibited by the FCC Rules for amateur radio stations?
A. Transmission of music as the primary program material during a contact
B. The use of obscene or indecent words
C. Transmission of false or deceptive messages or signals
D. All of these answers are correct

Monday, October 6, 2008


October 8, 2008 – Educational Radio Net, Session 20
Jim Hadlock K7WA

One of the things that got me interested in radio was hearing stations from far away places. At first I used my clock-radio in the AM broadcast band. I discovered that at night I could hear stations from Los Angeles, Salt Lake City, and even Mexico! Later I built a Knight Kit shortwave radio and began listening to broadcasts from South America, Russia and Japan. As a ham I've enjoyed the DX (long distance communication) aspect of our hobby for nearly fifty years, but the idea of a small radio signal propagating to and from far away places still intrigues me. Last year when I was in the Caribbean I made a 2-way contact with Paul, NG7Z, in Bothell on 40 meter CW – we were both running 5 watts of power. That, to me, is an example of the miracle of radio propagation – a very small signal covering a great distance.

The subject for tonight is High Frequency Propagation. We will discuss some of the factors that determine how a radio signal travels to far away places and resources for analyzing and predicting propagation conditions. If you have spent much time listening or operating in the high frequency bands between 160 meters and 10 meters you know that propagation is highly variable. How far you can communicate depends on many factors.

Lets begin with frequency. As I discovered with my clock-radio, far away signals on the AM broadcast band come in better at night. This characteristic applies to signals in the 160 meter, 80 meter, and 40 meter amateur bands as well. During daylight these bands may provide local coverage, but at night they can support world-wide communications. The higher amateur bands, 20 meters, 15 meters, and 10 meters are usually open during the daytime and quiet at night. These daily effects are due to the sun's radiation ionizing atoms and molecules in the earth's upper atmosphere, and the different layers of ionized material either absorbing, bending, or passing through radio signals of different frequencies. During daylight ionization in what's called the D-Layer, about 50 miles high, tends to absorb radio signals. This absorption is greater at low frequencies than high frequencies. If the signals pass through the D-Layer they can be refracted, or bent back to earth, by ionization in the F-Layer, 150 to 300 miles high. If the frequency is too high to be bent the radio signal will pass through the F-Layer and continue out into space. A few weeks ago Bob, K9PQ, discussed the terms Maximum Useable Frequency (MUF) and Lowest Useable Frequency (LUF) which are defined by this absorption/refraction process. After the sun sets the D-Layer starts breaking down due to the absence of solar radiation and propagation improves on the lower frequency bands. Although the F-Layer also breaks down in the absence of solar radiation, it often supports some propagation through the night. At sunrise solar radiation begins to build the D-Layer and F-Layer again.

In addition to the daily propagation cycle, seasonal effects vary greatly. Spring and Fall are similar, but Winter and Summer are very different. Due to the tilt of the earth's axis, radiation from the sun is weaker in the winter and stronger in the summer. The long winter nights make for very good low band propagation, while during the short summer nights the higher bands may remain open 24 hours a day.

The solar radiation which ionizes atoms and molecules in the earth's atmosphere is not constant. One of the best indications of strong radio propagation is the presence of sunspots on the surface of the sun. Sunspots are areas on the sun associated with ultraviolet radiation which ionizes the upper atmosphere. Sunspots can appear and disappear quickly or remain for several solar rotations (the sun rotates on its axis every 27.5 earth-days). Sunspots have been observed since Galileo invented the telescope in the early seventeenth century. Sunspots have been counted and recorded as long as they have been observed with records going all the way back to 1610. Currently there are two official sunspot numbers in common use, the daily "Boulder Sunspot Number," computed by the NOAA Space Environment Center, and the "International Sunspot Number" recorded in Europe. Both numbers use a method devised by Rudolph Wolf in 1848, which combines a count of groups of sunspots and a count of individual sunspots:

R=k(10g+s), where "R" is the sunspot number, "g" is the number of sunspot groups, "s" is the total number of individual sunspots in all the groups, and "k" is a variable scaling factor (usually <1) href="">

Understanding Solar Indices –

The Sun, the Earth, the Ionosphere: What the Numbers Mean and Propagation
Predictions –

W1AW Propagation Bulletin –


NCDXF Beacons –

Contributed by Wr5J, Curt Black
Big Bear Solar Observatory -

Wednesday, October 1, 2008


Tonight's edition will cover some important communication skills for Emergency Communication (EmComm) situations. This lesson borrows heavily from The ARRL Emergency Communication Handbook. Although there is a lot of overlap between this book and the three ARRL Emergency Communication course books, I still think it is worth having as a ready reference. I would even recommend bringing it along during a real emergency. While you should avoid trying to use it while you are actually operating, you will likely have some down time when you could refresh your memory and incorporate the good practices you find there. This lesson will use information and tips found in Chapter 5, Basic Communication Skills.

Listening Skills
The need for this is obvious but let's break it down a bit.
  1. Train yourself to understand what is being said under difficult conditions; such as when you have a poor connection with lots of static and some drop outs, or when you are in a noisy environment, or simply in an environment with another conversation. Sometimes a quiet room with just one other conversation going on is more distracting than being in the middle of a crowd. And you can't always ask the people having the conversation to keep it down or move it elsewhere. That other conversation may well be another radio operator performing an equally critical function.
    As with any training, the best way to train for it is to do it in realistic environments. For ACS members, our field exercises like Field Day or the SET are good opportunities. Another idea that I believe I got from Brian, WB7OML, is to have two radios on tuned to different talk stations and try to pick out one.
  2. Use headphones if possible to reduce the level of the noise or conversations around you. The Seattle EOC is fitted out with Fire Engine headsets and they work great. Airplane headsets would be another good choice. I don't have my own yet but they are on my wish list.
  3. Be sure to leave enough of a break between the end of the received transmission and the start of your transmission to allow for breaking stations. This is courteous practice all the time but is critical during emergencies.

Be Boring
This is not the time for creativity in your use of language. Simplicity, clarity and predictability in your communications are very good things when you will potentially be describing things that are well outside the ordinary. A few other tips:
  • Be brief. Keep your transmissions short and to the point. This is a bit of a judgment call since you want to make sure your transmission completely conveys the information you want it to; but try to avoid a lot of unnecessary extra descriptions or irrelevant facts.
  • Use plain language, avoiding Q signals, 10-codes and other jargon. One exception to this is the use of pro-words described below.
  • Spell unusual words, abbreviations or names with phonetics. The ARRL standard is to say the word, say "I spell" and then spell the word phonetically. In keeping with the "boring" theme here, it is best to use the standard phonetics rather than some other one, even if it is in common use on the HF bands. A lot of people that will be on the air will not have spent any time there and may not understand what to you is a common alternate phonetic.
  • Avoid contractions. This is one I hadn't thought of but is a very good tip to avoid confusion. I use contractions all the time and will have to concentrate to avoid them.
  • Avoid thinking on the air. If you need to collect your thoughts and still need to continue your transmission say "Stand By" then un-key, decide what you are going to say then key up again and say it.

There is one exception to the "use plain language" rule and that is the use of pro-words also called pro-signs. These are promoted by the ARRL and, I believe, in EmComm generally. So far I have primarily been discussing voice communications but there are pro-signs for both Voice and for Morse/Digital communications. The Morse/Digital code is in parentheses. They are as follows:
  • Roger (R) - message received completely and correctly
  • Over (KN) - indicates that the specific station that is being communicated with should respond
  • Go Ahead (K) - indicates that any stations may respond

    [Bob's Note: I don't know how common it is to differentiate between Over and Go Ahead. I seem to hear them interchangeably and I wouldn't count on the strict difference in voice communications.]

  • Clear (SK) - completed transmissions and releasing frequency. Usually this indicates that you are still listening on the frequency but it is common and, in my opinion good practice to say "Clear and Listening" to remove any doubt.
  • Out (CL) - completed transmission and leaving the air. Will not be listening.
  • Stand By (AS) - The ARRL manual only calls this a temporary interruption of the contact. In my experience it has a different and much more useful meaning. It is explicitly telling the other station to refrain from transmitting and wait for you to transmit again. Another very useful variation on this is "All Stations Stand By" which would normally only be used by the Net Control Station if there was one.

Tactical and FCC Call Signs
It is easy to get confused about the use of Tactical Call Signs and the use of your (or your station's) FCC Call Sign. Hopefully this will make this clear.

One of the first things you learn when learning about ham radio is that you must use your FCC Call Sign once every 10 minutes and at the end of your last transmission. This requirement still holds during an emergency! The only time you wouldn't use your own FCC call sign is when you using the FCC Call Sign of a club station (e.g. W7ACS) or you are not the control operator and you are using the call sign of the control operator of the station you are on. That second one is not likely to ever occur. The ARRL manual has a very good rule of thumb to accomplish this without going overboard. Since nearly all tactical exchanges are less than 10 minutes in length, give your FCC Call Sign at the end of each full exchange. This doubles as a signal to the other station and anyone else listening that you are finished with that exchange of transmissions.

The use of Tactical Call Signs are purely for better communication and have nothing to do with the FCC requirement to use your FCC Call Sign. Tactical call signs are used to identify the particular station either by location or function, etc. This allows the use of more than one operator at a station or the switching of operators without confusion about which station is sending or is intended to receive the transmission. Here are some examples of Tactical Call Signs:
  • Net Control
  • Seattle EOC
  • South Seattle Community College
  • Green Lake Community Center
  • Seattle City Light
The ARRL book recommends a particular protocol such as, "Net, Seattle EOC" to indicate that Seattle EOC is calling Net Control. They even go so far as to recommend simply, "Seattle EOC" which is to imply that they are calling Net Control. I have mixed feelings about this because I think it could be confusing to people that haven't practiced it and you are likely to have people on the net that have not practiced these things. In my personal opinion a better way for Seattle EOC to call Net Control would be, "Net Control, this is Seattle EOC". It doesn't add a lot more to the transmission but is clearer.

So for an exchange it is good practice to use only the tactical call sign for the entire exchange and finish with both call signs together to signal the end of the transmission, for example, "Seattle EOC, K9PQ". This keeps everything legal but allows for clearer transmissions during the exchange.