DMR For EmComm

What is EmComm?

EmComm, or Emergency Communications is the practice of providing communication services during emergency response, especially in situations where traditional communication modes may not be available.

The primary role of ARES is to support the emergency management community (responders, relief and recovery agencies) with communications during times of emergency and disaster when normal communications are unavailable or overwhelmed.

We are communicators not first responders. If you arrive at the scene of an emergency just as the sirens are quieting, check in, keep your mouth shut ,and get out of the way until you get an assignment. We do not provide first aid, transport victims, provide traffic control or any other function normally provided by public service agencies, nor are we running the show. We DO provide communication when public service communications systems are overloaded. We do not “self deploy.” We deploy when a partner agency requests our services.

Many hams think of ARES as a simple extension of the “talk time” in the hobby. This is not true. ARES are organizations that continually learn, train, and develop the skills needed to be part of the emergency response machine. We always need more trained operators that are willing to learn to communicate rather than just talk. Do you have the time and the drive to do it well?

The goal is to provide trained operators that have learned to communicate accurately, clearly, and concisely in a timely fashion regardless of the obstacles in the event. One tool for communication is DMR or Digital Mobile Radio.

What is DMR?

DMR is a radio protocol that uses digitally encoded signals instead of traditional analog signals. This is much like the difference between playing an old LP record that directly recorded analog sound versus playing a CD where the audio is encoded as bits and bytes – these bits and bytes are what is transmitted via the radio signal.

Why use it?

There are a number of reasons that make DMR a good choice for EmComm:

  1. Unlike analog radio – there is no ‘fade’ as a signal gets weaker. As long as the signal gets through the audio is clear and readable. This cuts down on the need to re-transmit messages when there are high levels of noise.
  2. DMR is much more efficient in terms of bandwidth. Using much narrower slices of the radio spectrum, and time slicing the available frequencies (more on this later) as many as four conversations can be held using the same bandwidth that one analog communication would take.
  3. DMR can be more capable – since the exchanges are digital text and data can be exchanged alongside the voice exchange. This can include position information, short text messages, and more.

But there are also a few downsides – most significant is the fact that a DMR exchange either works or it fails – completely. With analog radio, a trained operator can pull voice traffic out of very noisy backgrounds. This can be important in an emergency. With DMR, you generally either have a clear voice with little or no background, or you have nothing – literally nothing. So lets talk about how DMR does its thing.

DMR – How it works

DMR does its magic by squeezing your conversation into a really small space. It does this by converting the audio to digital data. Then, since the binary data does not require as much bandwidth as a voice, DMR only needs 6.5kHz instead of the 12.5kHz that voice takes. Finally, it is possible to use two separate time slices (TS1 and TS2) to share one frequency between two separate transmissions. So in the same bandwidth as one analog transmission, you can squeeze 4 (A, B, C, D) digital transmissions.

One Continuous Xmission









1 Analog Transmission (12.5 kHz)









6.5 kHz









6.5 kHz

Now to make this work, the radio needs to know some specific things to set up a transmission.

Time Slots

First, it needs to know which of the two time slots your conversation will use – both radios, yours and either another DMR (simplex) or the repeater need to agree. Because if you are on TS1 and they are on TS2, you won’t be able to communicate.

Color Codes

Color Codes are a lot like PL tones, they allow your radio to filter out transmissions you don’t want to hear. But, both ends have to agree on the same color code, or your buddy’s radio will filter out your call.

Talk Groups

Now, when you have all this digital data flying around, its kind of like email – you need to know the email address of the recipient. That’s where talk groups come in. Having set up the correct time slot, and agreed on the same color code gets the radios set up to be in sync. But, they also need to know how to address the data. So, you also agree on a talk group. Then everybody who is on the same talk group can hear and talk to each other.

Okay, with simplex operations, this isn’t that important. And if you are both on the same repeater, you could work around it. But once the data is digital, it opens a world of possibilities in terms of linking repeaters – even ones that are hundreds of miles apart. And to do that, everybody needs to agree on the same talk group.

For emergency communications this is a very powerful tool. Our group, Denver ARES, is part of a public safety team for the major interstate I-70. The East region stretches from Denver to the Kansas line – over 170 miles of road. Thanks to the wonderful people at Rocky Mountain Ham – we can use one talk group (Central) and have a conversation with ARES members at the other end of the state – and do it with HTs at each end.

“Okay, okay,” you might be thinking, “You’ve sold me – how do I get started with DMR?”

What do you need (and where to get it)?

It is pretty easy, actually. You only need 3 things:

  • A DMR ID that is programmed into your radio as your personal ‘address’
  • A DMR radio, and
  • A CodePlug – the configuration file that the radio uses to know what to do.


A Radio ID is a unique number assigned to you (and your callsign) by the team. It is kind of like a telephone number or IP address. Your Radio ID identifies you as a unique radio user on the various DMR networks and repeaters around the world. You need to have a radio ID to use a DMR radio.

To get your Radio ID, you need to register with and upload an official copy of your Amateur Radio Licence. For U.S. radio operators, that can be done by1:

  1. Log into FCC system at
    1. The FRN and password are the same you use when applying for Vanity callsigns and address changes.
    2. If you don’t know your FRN number, go to FCC ULS License Search and search for your callsign. Your FRN will be listed in the column FRN.
  2. On the left side menu, click on ‘Download Electronic Authorizations’.
  3. On the new page, select your callsign and click on the button ‘ADD >’.
  4. On the bottom right of the page, click on the button ‘Download >’.
    1. Make sure your pop-up blocker is disable, otherwise you won’t be able to download the PDF for your license.
      5. You will be asked to save a PDF file. Save it and submit it to RadioID, Simply create your account at RadioID and upload the License.
  5. That’s it. You will receive an email, usually within 24 – 48 hours, with your unique Radio ID, which you can promptly program into your radio(s).
    1. Note: You only need one radio ID even if you have multiple radios.
    1. DMR Radio

      Okay, yeah – you are going to need a DMR radio. There are too many on the market for us to give you a good overview.

      But, keep in mind that in many cases Emergency Communications groups, like ARES, will maintain a cache of radios to be distributed in the case of a deployment. So check with your team to see what is available.


      In many ways setting up a DMR radio is just like setting up an analog radio that is used to connect to repeaters. While Code Plug sounds like some odd programming process, it is really just a configuration file. It will generally have three sections:

      • A Contact list – which are the talk groups you want to use
      • Channel Information – this is just like setting up a repeater, frequencies, and offsets, but you need a few more things for a the DMR like the time slot and the color code and so forth
      • Zone information – this lets you organize the talk groups and channels into groups that can be arranged by purpose (EmComm, Personal, etc.) or geography (North, Mountains, Vacation house, etc.)

      While you can build a code plug starting with a blank sheet, more often you will start with an existing Code Plug, and modify it to fit your needs.

      For EmComm usage, it is important to have a common operating model. Since many EmComm groups have caches of DMR radios to be distributed during a deployment, they will be programmed with a standard code plug that meets the needs of the group and its partner agencies.

      So, get the code plug approved by your group, or that is distributed for a specific emergency response.

      Operating procedures

      With an analog radio pressing the PTT immediately keys the transmitter and you’re ready to go. Not so on a DMR radio. When the PTT is pressed, a signal is sent to the repeater which checks to see if the Time Slot is available. If it is, a data stream is sent back to the radio giving you the All Clear, usually generating a beep tone. This occurs in just under a second.
      It is highly recommend that the BCLO (Busy Channel Lock Out) function is enabled. This prevents a station from transmitting on a Time Slot if it is currently active. Another indicator that the TG is in use is an activity light on the handheld. If the LED is lit, the Time Slot is in use.

      Busy Repeater Channel / Time Slot

      You may see the Channel Busy indicator lit, but not hearing a conversation. This is caused by someone activating or using a repeater Talkgroup other than the one you are monitoring.

      Digital Monitor

      Your DMR radio may have a ‘programmable key’ function labeled Digi Monitor or Promiscuous mode. This open allows you to monitor all activity on one or both time slots regardless of the Talk Group in use. This is a monitoring function only.

      General Rules:

      • 3 second pause before PTT
        • This allows for network latency as well as a courtesy pause for those wanting to enter the conversation.
      • 1 second pause after PTT
        • This is required for your radio to sync with the repeater and network.
        • You will get the Go Ahead signal if the repeater is clear.
      • Time Slot in use
        • This is usually shown by an indicator light or a time slot busy tone on your radio.
      • Talkgroup in use
        • You may not immediately hear an active Talkgroup. When switching to a different TG, your radio may need to sync to a conversation already in progress.
      • Announcing your presence
        • Announce both your Call Sign and Talkgroup. This will allow someone who is scanning to identify your Talkgroup so they can answer your call

1These instruction are adapted from the ones provided by DMR For Dummies:

The long path needed to deliver power to your house from the fuel needed at the power station to the substation in your area, to the transmission lines to your house, and to the socket in your wall.

Power Systems

Power Systems – Introduction

We can think of a functioning amateur radio ‘station’ as a tripod – the three legs are Power, Transceiver, and Antenna. If any of these are missing or out of whack, the tripod falls over, and the ‘station’ is inoperable or very inefficient.

The Power leg of the Ham radio station tripod

Power is one of the three critical legs that support your ham radio station.

In this blog, we will focus on the Power System portion of the tripod, what it does, how does it do it, and how can it go wrong. In some ways the Power System is unique. The Transceiver and Antenna components are under your control – you can build or buy whatever antenna you want, and mount it in what ever manner you choose (okay, subject to local ordinances and covenants); you can buy or build whatever radio system you want (subject to federal regulations, and your bank account). But often your power system will depend on an amazingly complex infrastructure – one that you have little or no control over. With that in mind – let’s get started!

What does a Power System do anyway?

The function of a power system is to deliver usable energy to the transceiver. This energy is then modulated to contain a signal on a specific radio frequency, and delivered to the feed line and the antenna. On the receive side, it provides the power to enable the radio to demodulate a received signal, and present the result as audio to the operator. Since it is the starting point for the radio’s function it is a key component of the Radio Station tripod.

So what can go wrong?

Well, of course the simplest thing that can go wrong is that the power simply isn’t there. Let’s begin by looking at three common ways of delivering power to the station, and examine their strengths and weaknesses. The three we will examine are: ‘shore’ power provided by the electrical grid, generator supplied power, and battery power (and its recharging components).

‘Shore’ Power

Power provided by the grid is both the most convenient and the riskiest source of power for your radio station. It is convenient because all you have to do is plug your power supply into the wall socket – everything else is taken care of by other people. It is the riskiest because everything else is taken care of by other people; and there is a lot of other stuff to take care of.

The long path needed to deliver power to your house from the fuel needed at the power station to the substation in your area, to the transmission lines to your house, and to the socket in your wall.

What a long strange trip it’s been.

Lets take a look at what is needed to deliver the power to the socket in your wall.

  1. The power supply that converts the AC from the wall into the correct DC voltage for your radio needs to be operational – this may be built into your radio, or it may be a separate component;
  2. The house wiring needs to be correct and intact;
  3. The line from your house to the street needs to be functional;
  4. The transmission lines and transformers from the substation need to be intact;
  5. The substation needs to be operational;
  6. The distribution lines from the power plant to the substation need to be intact;
  7. The generators in the power plant need to be operational;
  8. The power plant needs to have fuel (coal, natural gas, nuclear, wind, solar); and
  9. The surrounding power networks need to be ‘in sync’

All of these have to be working in concert to enable you to power up your rig. Since ARES focuses on emergency communications, we need to look at what can go wrong.

You could have a local problem: your power supply goes bad, taking out #1, or a tree is blown down in your yard taking out #2 or #3.

Image of the smoke plume from a failed electrical substation at Harrison St in Denver, CO.

When a substation goes, the power can be out for an extended period

A neighborhood problem: A truck plows into a power pole, a transformer blows during severe weather, or your local sub-station blows up taking out #4 or #5.

A regional problem: An earthquake, hurricane, or ice storm results in damage to the distribution lines (#6) or the power station (#7)

A national / global problem: Disasters, labor unrest or transportation problems block fuel delivery, or a nearby power grid becomes unstable forcing a shutdown in adjacent grids, taking out #8 or #9.

Wow, when you look at it this way – it’s amazing that we have reliable electrical power at all! So, what’s your backup plan?

Generator Power

A generator powering the House

Generator make you more self-sufficient. Kind of.

Probably the most common solution is to have a standby generator. This approach is used by most critical facilities such as hospitals, commercial radio and TV stations, emergency operations centers, etc., and by many home owners. Whole house generators are available as are smaller limited use generators feeding a subset of the home perhaps only providing power to the furnace and the refrigerator, and some lights. Of course for radio operators that limited use will include the ham shack 🙂 So with this solution we cut away a lot of the complexity of the power distribution system:

1) The power supply that converts the AC from the wall into the correct DC voltage for your radio needs to be operational (this may be built into your radio, or it may be a separate component)
2) The house wiring needs to be correct and intact;
7) The generators in the (your) power plant need to be operational;
8) The power plant (generator) needs to have fuel (gasoline, natural gas, propane)

Clearly a lot simpler, so less can go wrong. Assuming you can disconnect your home from the power lines in the back yard, you can eliminate #9, and you can almost ignore any neighborhood, regional, or national problems (for a while). You can get around the tree falling down issue by running dedicated lines (extension cords) directly to the equipment you want to run – if you planned ahead.

But this has costs. First there is the cost of the generator and installation – a whole house generator can run into the tens of thousands of dollars – and even a small standalone generator can run you several hundred bucks, and that’s just the monetary costs. You also need to maintain the generator and test it regularly, if it fails you are out of luck (#7). You are still dependent on fuel (#8). This can become problematic in the case of regional disasters where the gas stations may be unable to pump gas (if it is available) and refilling propane tanks may be impossible.

Battery Powered Field Station, a large Battery on the ground with power leads going to a radio on a portable table

The simplest possible power system, a battery. Taken at our Field Station setup exercise. (credit: lgunderson, AE0QN)

Let’s see if we can reduce this even more. Let’s start with a battery – one that supplies the DC voltage needed by the transceiver. That eliminates #1. Since we aren’t using the house wiring at all, #2 is covered. There is no generator so #7 is gone. That leaves only #8 – since a battery is only a storage system, how are we going to recharge it? What fuel can we use?

We can recharge it with a generator or a car (that brings #7 back into the list) but then we will need to ‘recharge’ the generator with some fuel source. Alternatively, we go green. Solar panels and / or wind generators can easily provide the ‘fuel’ for the battery. Because even a small home wind generator requires some form of tower – which could be damaged by that pesky tree that keeps falling over, let’s settle on a simple, portable solar panel set. Pull it out, set it up, plug it in, and wait for dawn. Hopefully you sized your battery to handle the radio traffic overnight, and sized your solar panels to both recharge the battery and provide power for radio operations during the day. Factor in that not every day is a beautiful sunny experience, so add some extra watt-hours for cloudy days. Not a bad solution for the delivery of power, but there is more that just the quantity of energy to think about.

What else can be a problem?

There are two other aspects that arise when you are your own power company – you become responsible for both the quality of the power and you are responsible for keeping things running.

Power Quality

Your transceiver is a very sensitive piece of equipment, and it can be thrown off by a number of things – including your choice of power system. The problems generally come down to two aspects: line noise and radio frequency interference (RFI).

Starting at the simplest side, a battery provides high quality energy. While it lasts, it delivers pure DC voltage with no ripples, pops, or noise.

A stable DC trace on an oscilloscope - a flat line across the screen.

Batteries and high quality regulated power supplies give your radio smooth stable power.

We need to recharge it and the recharge system can be noisy. Note: if you are using ‘smart batteries’ they add electronics to control the discharge voltage and so they can be noisy even when discharging. Some of the noise can be on the line – as the charge controller adjusts charging rates and voltages. These can impact the transmission side of your radio, potentially causing drop outs and adding noise to your outgoing transmission. These controllers can also cause RFI – pops, sizzles, and hums on the radio frequencies. This won’t impact your transmissions, but it can cause havoc on the receive side of things.

If you add a generator into the mix, the same problems are increased. Generators can be incredibly noisy on the RF side – so keep it as far away from your antenna as is practical. They can also provide ‘dirty’ AC line power.

The power companies, bless their hearts, spend literally hundreds of million of dollars to provide as close to pure sinusoidal AC to your house as possible. And your radio power supply depends on that beautifully smooth wave to provide clean DC to your radio.

Side by Side oscilloscope traces, one with smmoth sine wave, one noisy

The quality of the AC power supplied by the power lines or generator can affect your stations performance.

The backyard generator from the local big box store may not do the same. It often provides clunky square-wave like profiles, with hums, spikes and drop-outs. This doesn’t matter to your refrigerator, but is not good for your radio or your radio power supply. In addition, with all those moving parts (stator rings, whirling magnets, and so forth) they can actually generate more RF noise than your transmitter. So, if you go the generator path – check reviews by radio operators before you sink cash into a noise farm.

Moving up to shore power – things get better and worse. As I said above, the power companies want to deliver clean power to your house, and they generally do. But there are a lot of parts to the delivery chain. Substations and transformers can inject noise into the line – and even more into the RF spectrum. There are lots of stories and techniques for hunting down RFI in the power grid, and, once it is brought to their attention, the companies are pretty good about fixing the problems. But if we are talking about an emergency situation – all bets are off. Not only can damaged grid components generate tons of noise, but during the recovery the focus is on getting any kind of power to the customers, they will worry about clean later. Again, the more complex the system, the more parts are out of your control, but you do get a lot of convenience from just walking into the radio shack and pressing the power button on your rig.

Safety and Maintenance

You are your own safety officer!  When you are delivering power you have to wear a lot of hats.

Generator Safety and Maintenance

If you are delivering 120V AC from a generator – you might be in for a shock – literally.  Shock hazards, whether from a ratty old extension cord of questionable integrity, or due to badly engineered connections are a real risk.  Emergency generator power during storms adds in the risk of water shorting out your system. And remember – generators get hot, and they emit potentially poisonous gasses. So make sure that you place the generator on a level, non-flammable surface that is clear from any nearby fuel sources (gasoline, dried grass, piles of newspaper, you get the idea) and do not operate a generator inside, neither your house or the garage.  For generators, maintenance items include making sure you have fresh fuel (either gas or propane on site), the oil has been changed according to the manufacturer’s schedule, the generator has been tested, you know how long it will run under what conditions, etc. And test it on a regular schedule – the last thing you want is to confidently run out to fire up the genny and have it not start.

Battery Issues

First, remember that a battery is an energy storage device. There is a lot of chemical energy in there. Remember all those stories of batteries exploding? these things also take care and feeding.  Storing them appropriately is important for safety.

They can emit explosive gasses when charging so think through when and where you plan to charge them, the back of the closet in the kid’s room may not be a good choice, neither is next to the water heater. You will need to make sure there is a good charge on the batteries – notice, I did not say keep them full.

Batteries have a lifespan – ranging from a few hundred charge / discharge cycles to a lifespan measured in years. But eventually they die – you don’t want that to happen on the day you need the battery.

Many batteries have a maximum lifespan when they are kept around 80% full, not continually topped off.     Depending on the battery type, they will have an effective lower discharge limit as well. Factor this in when sizing your battery. A 1000 W-Hr battery may only give you an effective 500 W-Hrs for normal use, or 750 if you are willing to damage the battery. Many batteries prefer to be discharged before they are recharged – follow the manufacturer’s guideline’s and build a maintenance schedule for you emergency power. Then remember to follow your schedule.  Plan your work, then work your plan.

Wrapping Up

So that wraps up our tour of the Power System leg of the radio station tripod. We looked at sources of power, we looked at the delivery paths. We looked at what is needed to get AC to your ham shack, and some of the many, many things that can interrupt that flow. We did a quick dive into simplifying the needed infrastructure going from generators to renewable energy sources to pure battery power. Then we did a quick look at the quality of the power, since that also contributes to the operation of your radio station. Next we looked at some of the causes of both line noise and RFI, and made some suggestions about what can be done to reduce that noise. Finally, emergency power requires that you take on the job of the power company in many ways – importantly you are responsible for safety and maintenance of your emergency power system.

Remember to check out the other two legs of your Ham radio station tripod:

For some additional info on Power systems for Amateur Radio – check out this video by Dave Casler:

Antenna Basics

Antenna as part of the radio system

by Jim Gunderson, AD0ZM

We can think of a functioning amateur radio ‘station’ as a tripod – the three legs are Power, Transceiver, and Antenna. If any of these are missing or out of whack, the tripod falls over, and the ‘station’ is inoperable or very inefficient.

A tripod with legs labeld antenna. Power, and transceiver. The antennal leg is bolded.In this blog, we will focus on the antenna portion of the tripod, what it does, how does it do it, and how can it go wrong. Now don’t worry – I’m not going to fill the rest of the blog with equations, curl operations, or any calculus. We will talk about the physics, but more by way of metaphors. Our focus is on Amateur Radio for Emergency Communications (EmComm), and you are not going to be calculating any derivatives in the field. So, let’s get started!

What does an antenna do anyway?

Essentially, an antenna is a transducer – it accepts energy in one form and changes it into energy in another form. Specifically, it takes alternating current electrical energy in and spits electromagnetic waves out – or the other way around, it can take electromagnetic waves in and spit alternating current electricity out (this makes it a bidirectional transducer). That’s it, that is all it does. “But Jim, that sounds like any old piece of wire is an antenna,” you say. Yep – it is! But there is one key thing: an antenna is a passive element (which makes it a passive bidirectional transducer) – it doesn’t add any energy into the system, but it can be really inefficient and waste a lot of energy by turning it into heat. That heat doesn’t get the message out.

Everything about antenna design, construction and use comes down to two things – make the antenna as efficient as possible to maximize the energy transfer, and make the energy go where you want it to go. This latter point is important. A perfect ‘isotropic’ antenna sends the radio waves out in all directions equally, think about the sun or a light bulb. But, you don’t want to talk or listen in all directions. You want to hit that repeater on top of the building downtown a mile due south of you, or send a message to the Emergency Operations Center which is 15 miles to the north-west.

We know that an antenna can’t add any energy to the radio signal – but it sure can change the shape of the radiation pattern, for better (good) or for worse (not good). So the size, position, orientation, and height above ground all affect that radiation pattern, as do nearby objects, buildings, and terrain. These all change how the electrical energy is sent (or received) by the antenna.

In this post we will focus on the resonance of the antenna and a later post will look at the ways to change the radiation pattern.

How does an antenna do this wonderful thing?

We agreed that we would skip the equations, so we are going to explain this in words. By definition, it won’t be a precise, nor will it be as accurate, but it will get the message across, I hope. We will start with the transmission side of the transducer, what happens when you push the Push To Talk button? Through the engineering inside your radio (whether it is a little HT or a huge $10,000 rig) the sound of your voice is changed into alternating electrical current coming out the antenna connection on the radio. That signal arrives at the antenna.

The antenna is just a conductor, and so it conducts the current. Since it is alternating current it flows into the antenna for half a cycle, and flows back out during the other half. You can think of it like a wave sloshing in a bathtub, back and forth, back and forth. You might think it would just keep doing this forever, but we have a funny old universe.

It turns out that when an electrical current flows in a conductor, it causes a magnetic field to form, it transfers some of the electrical energy into the magnetic field energy. Well, that’s not so bad is it? The energy is still all there, just in a different form. But we have alternating current, so after half a cycle the current flow reverses, and the magnetic field begins to collapse. So what happens to the energy, you ask? Well, some really smart physicists figured out that when the magnetic field collapses it turns the energy into an electric field. So when we have alternating current in a conductor, we get a series of electrical and magnetic fields playing catch with all that energy, back and forth between alternating electrical and magnetic fields. And that process generates electromagnetic waves that take all that energy you pump in from the transmitter for a ride – traveling at the speed of light away from the antenna. You just made a radio transmission.

What affects its efficiency?

So, in a perfect world all the energy from your transmitter gets sent out as radio waves, whether it is 5 watts from an HT or 1500 watts from a massive power amp. Of course the world isn’t perfect – you get some losses because the conductor in your feed-line, and in the antenna itself are not perfect conductors. So you lose a little bit of energy every time the wave runs out to the end of the antenna and makes the round trip back. If the antenna is perfectly resonant, the energy gets transduced into EM waves completely on one trip – you have a standing wave with no reflected signal: and standing wave ratio (SWR) of 1.000000. That is a good thing. If the antenna is not resonant however, some of the energy gets reflected back along the antenna, and (like the bathtub wave) when it hits the other end (at the transmitter) it gets reflected once more out to the antenna, back and forth, back and forth.

But every trip causes a little more energy to be wasted by the resistance in the conductor, and turned into heat. So the worse your SWR – the less power you get out the antenna. It also can do bad things (think unpleasant smells, smoke, blown parts, trips to the radio store for new equipment) to the final stages in the transmitter, so keep the SWR low.

That’s not the end of the process, you need those radio waves to arrive at your intended target, so they have a long trip to make. Okay, not really long for electromagnetic waves – in an ideal environment (like free air or outer space) they will keep going forever. Think about looking up at the stars – if you manage to get a glimpse of the Andromeda galaxy those light waves have traveled 2.4 million light years and you can see them. But that’s a galaxy, you’ve got an HT, and that’s in free space, you are on a planet. And practically everything between you and your target will eat your radio transmission.

Trees, buildings, hills, almost anything will attenuate your signal to some degree. Water is bad. But you say, “Jim, I am not going to transmit through water!” Think about that HT clipped to your belt. On the one side the antenna is facing open air – perfect for sending a radio transmission to infinity and beyond. But, on the other side it is right next to you – and you are 70% water. Figure on about a 6 dB loss going through you – less than 1/4 of the power makes it out the other side. So think about which direction it is to the target, or better yet, pull the HT off your belt and raise it up to head level.

How can you make it better?

Well, it is not so much how you can make it better, as much as what can make it worse. Our reference antenna is a perfect 1/2 wave dipole with an SWR of 1.0, everything else compares to this. Of course it may not be possible to use a perfect antenna in some cases – imagine hiking through a festival with a Dipole on your back! You will most likely use a much smaller antenna.

The effect of using a smaller antenna is significant. Yes, the electrical characteristics can be adjusted so that the transceiver ‘sees’ it as a perfect 50 Ohm connection, but there are costs. Let’s look at the ‘rubber ducky’ that came with your HT. It is probably about 4 inches long and covered with a rubber coating.

The antenna is basically a coil of wire (an inductor) that is tuned for the center of the band. So on that frequency it is resonant, and has its lowest SWR. But there is another factor besides resonance. To be an ideal radiator – the length of the antenna comes into play. This is known as the aperture. The closer the aperture is to the wavelength the more efficient the radiation is. While electrically the rubber ducky is resonant, the length is far short of the ideal aperture, so it looses efficiency. That means more of the signal bounces back and forth – so that means a higher SWR, and that means more loss of radiated power.

There is also one other effect – the range of frequencies with good SWR gets smaller and smaller. So, with a 1/2 wave dipole you can get a low SWR across the entire band, the short antenna gives you a very narrow range where the SWR is good, and over the rest of the band it can be really high.

So, to summarize

The antenna is a key part of your radio system – it is the transducer that changes electrical signals into electromagnetic (radio) waves and vice versa.  In a perfect setup 100% of the electrical energy going in comes out as radio waves, but the world isn’t perfect.

SWR is one measure of the inefficiency of the antenna, the higher the SWR the more round trips the electrical energy makes, the greater the losses to heat.

As second aspect that can affect the efficiency is size – while shorter antennas can be made to ‘look like’ a perfect 50 Ohm antenna, the coupling between the antenna and the outside world (the aperture) is less than perfect. As a result the antenna has lower gain and is less efficient.

Choosing the right antenna for the mission is key and will be covered in a following post.

If you want to dig into more about the effects of size – check out this Wikipedia article: Rubber Ducky Antenna

Antennas are one part of the tripod that supports your Amateur Radio station, remember to check out the other two legs: