Amateur Radio Nets

Many single repeaters—or systems of repeaters—host recurring nets. As described above, nets are coordinated ways to talk with other hams, pass traffic, or buy and sell radio equipment. They also help coordinate communications in the event of an emergency or in the context of a large volunteer event. Some nets are scheduled—many occur weekly, biweekly, or daily at specific times. Other nets are set up for a predetermined purpose, such as a volunteer event. Nets are a fantastic way to hear how hams communicate with one another and behave on the air.

What makes a net a net is some form of structure. This can range from everything from an individual coordinating some traffic to an entire team of operators directing hundreds of hams. A common denominator of most nets is net control, or some individual/group that’s “in charge.” Net control is responsible for ensuring smooth operation of the net and resolving any problems that might crop up. In the case of most casual nets that you’ll find on a repeater, net control will likely be an individual facilitating discussion, check-ins, and the passing of messages. You can identify nets in your area via an online search of something along the lines of “amateur radio nets around _____.”

If you have the time of a particular net, and you know the frequency of the repeater that’s hosting it, you should be good to go! Set up your radio, and tune in at the given time. The net control operator will likely start by announcing the name of the net and then ask for check-ins. At this point, amateur radio operators listening to the net will go around and announce their callsign and name. Some also indicate the hardware used for transmission, such as “mobile” for a car-based system or “handheld” for an HT-based contact.

The net control operator would then proceed down the list of individuals checked in, indicating when a given ham can talk. This ham can share information, talk about equipment that he or she has for sale, or simply talk about how his or her day is going!

Since repeaters utilize higher transmit powers than your HT, it can certainly be possible that you can hear a repeater, but the repeater cannot hear you. If you know that there’s going to be a net on a repeater somewhere close to you, it can still be interesting to listen to the net, even if you can’t reach the repeater with your signal.

Volunteering

A common practical use of amateur radio nets is for large events, such as the Head of the Charles Regatta, as shown in Figure 10-1. You’ll find that at the heart of many large events lie amateur radio operators. Event coordinators will arrange for sometimes hundreds of hams to be present to help relay messages and coordinate traffic.

This event, which is the largest regatta in the world by number of boats, utilizes amateur radio operators to assist with communications. Some hams operate on boats on the Charles River (near Boston) to help with rescues and forward information. Other amateur radio volunteers are paired with medical teams or operate by themselves.

Communicating with cell phones during such events would be impractical, as it would take an extremely long time to disseminate messages across an event, and bottlenecks can occur where many people try and call one person. Commercial radio systems, which are essentially advanced networks of walkie-talkies, are utilized for many events. However, these systems are only effective for smaller groups of people and at limited ranges.

This leaves amateur radio as an efficient and convenient way to pass messages and keep event officials informed. Common events where you’ll find amateur radio operators include marathons, large fairs/festivals, and other large public sporting events. Volunteering at such events is a fun way to watch the event and hone your amateur radio skills at the same time. As a communications volunteer, you’ll get inside access to many areas of an event that regular volunteers cannot access, such as the finish line of large races.

Hams at such events fill many different roles. Experienced amateur radio operators with years of volunteer experience can operate net control. These individuals tightly control the various nets utilized by hams and ensure that messages get where they need to go. Net control is responsible for ensuring that hams do what they’re supposed to.

All other hams are out in the field performing various duties. Some act as monitors, roaming around an event searching for abnormalities. If they detect a concern, these individuals immediately report it to net control and await further instruction. Hams can also serve as the communications link between volunteer teams. Teams of medical staff will oftentimes have an amateur radio operator with them to relay messages. Many important event officials also have a dedicated amateur radio operator with them—these officials are very busy, so passing on the burden of communications frees them to complete other tasks.

Volunteer events are also a great way to practice amateur radio skills in a disaster-like scenario. Some events can be chaotic, and many events result in injuries that need communicating. The Boston Marathon uses amateur radio operators to help coordinate the activities of hundreds of medical personnel. In a typical race, there can be over a thousand injuries that require treatment. Operating a radio in such an environment can be both challenging and fun.

NOAA Weather Radio

NOAA, the same government agency that operates the satellites that we received images from in a previous chapter, operates a network of radio transmitters across the United States. These transmitters send out general weather information and warnings—the coverage of the transmitters is excellent, with all but the least densely populated areas of the country able to receive at least one station. A regional coverage map is shown in Figure 10-2. White areas have reliable coverage, while brown areas do not. Areas without coverage on the borders of the map are actually covered by transmitters not shown on the map (e.g., so there is coverage in Eastern Massachusetts).

Stations generally transmit around 162 MHz, with adjacent stations selecting frequencies that do not interfere with each other. You can identify stations around you by going to www.nws.noaa.gov/nwr/Maps/ . Select your state, and then select the transmitter closest to you. Areas in white are areas with coverage. Selecting the station closest to you will give you the strongest signal, but it’s definitely possible to listen to other stations that are farther away. If you don’t live in the United States, your government likely offers a similar service. There’s also a lot of spillover of NOAA stations into Canada and Mexico, as the stations have transmit powers in the hundreds of watts.

Police/Fire/EMS Radio

Surprisingly enough, many public safety communications are completely unencrypted, and totally available for you to listen to via your HT. Most systems operate utilizing a similar repeater network to amateur radio repeaters. Systems will have a high-powered transmitter at a base station, and mobile units’ communications will be picked up and rebroadcast by this repeater. This also makes it easier for you to monitor communications, as the transmissions are broadcast over wide areas.

You can monitor communications from the police, fire departments, ambulances, and just about any other public safety service that utilizes radios. These frequencies typically lie in the range of 450-460 MHz. If you look at that section of the US frequency allocation chart shown in Figure 10-3, you can see that these frequencies are designated as “land mobile,” which makes sense considering these are generally car-mounted radios. Many other countries operate on similar frequencies. RadioReference.com has a great database of the frequencies used in various municipalities—you can find it here: www.radioreference.com/apps/db/ .

Scan Mode

There are likely all manner of communications filtering through the air waves around you that aren’t listed anywhere online. Luckily, your radio has the ability to scan across various frequencies and stop if it finds anything interesting. BaoFeng HTs are by no means the fastest radio scanners available, but they get the job done. Select a frequency to start on, and then press and hold the “∗” key until your radio starts scanning. If it finds an active frequency, it will briefly pause on that frequency, determine if it’s still active, and then continue scanning if nothing is found.

It’s also possible to scan using your RTL-SDR. In fact, this SDR is a much more capable scanning device than your HT considering the greater processing capability available to it. There are a number of applications available online for this purpose.

Radar

While not explicitly radio communications, radar is still a fascinating subject that deals heavily in principles of electromagnetic radiation. At its core, radar is all about detecting things with radio waves. Radar is short for “radio detection and ranging.” That is, radar systems are primarily designed to detect objects and pinpoint their position. Some radar systems also use the Doppler effect to measure the speed of an object.

The core of radar revolves around the principle of reflection. By sending out a pulse of energy and measuring the amount of time it takes for a reflection to come back to the radar system, one can determine how far away an object is. Radio waves travel at the speed of light, so the distance to an object is half the time it takes for the energy to go out and return to the radar multiplied by the speed of light.

In general, the larger an object is, the more energy it will reflect back. There exists a minimum amount of energy that an object must reflect back in order for it to be detected. If this quantity of reflected energy is too small, the energy from the object will be drowned out by noise and other reflections. The probability of detecting a given object is a function of the object’s size, the distance to the object, and the amount of power sent out by the radar system, among other things.

Those wishing to avoid radar detection can only alter their appearance to a radar system, so much work is done by militaries to minimize the amount of energy that military planes and ships reflect. In fact, the odd shape of a modern military aircraft sometimes has as much to do with minimizing the object’s appearance to radar as it does to improving aerodynamics. The F-35 fighter jet, shown in Figure 10-4, is a great example of this.

A primary selling point of this aircraft is its ability to hide from enemy radar. The surfaces and paint of the jet are designed to scatter electromagnetic radiation or direct it away instead of directing it back to a radar array.

A radar array is comprised of many small antennas in a particular configuration. Lots of small antennas allow the operators of the radar system to create an extremely narrow and electronically steerable beam. A narrow beam means that the radar can tell operators where an object is to a greater degree of accuracy than with a wider beam. A good analogy of this is attempting to paint very fine details with a paintbrush as opposed to a can of spray paint. The spray paint can cover a larger amount of area, but it’s not very precise.

The design of radar systems varies greatly depending upon their use. A common usage of radar is in military applications. Most missiles use radar to find their targets. All modern fighter jets have radar arrays in their noses. Practically every country monitors its airspace with radar installations. The list of radar in military applications is extensive. An interesting example is airliner-mounted radar arrays for surveillance purposes—see Figure 10-5.

There are as many civilian uses for radar as there are military ones. If your car has adaptive cruise control (where your car automatically adjusts its speed to avoid hitting a vehicle in front of it), then it also has an integrated radar system. A car with adaptive cruise control utilizes radar to measure its distance to cars around it. This radar array allows for other interesting applications, such as front crash prevention.

5G

5G promises to revolutionize not only cellular communications, but also the modern technological world. An important way in which 5G improves upon existing 4G infrastructure is through enhanced mobile broadband (eMBB). Mobile broadband is simply cellular data services—every time you access the Internet through a cell tower instead of Wi-Fi, you’re utilizing a mobile broadband service.

4G LTE supports real-world maximum data transfer rates of around 100 Mbps (million bits per second) in most places. As you can likely guess, eMBB greatly increases this figure. Real-world data transfer rates of over 1 Gbps (billion bits per second) are easily achievable with 5G hardware. This increase in speed comes mainly from the allocation of new parts of the electromagnetic spectrum for cellular data services.

4G generally utilizes frequencies between 500 MHz and 1 GHz. These frequencies still support decent data transfer rates, but also can propagate effectively across a large urban or rural environment. 5G will extend cellular communications into the range of tens of gigahertz. These are known as “millimeter-wave” bands, as the wavelength of such radio waves is on the order of millimeters or tens of millimeters. Higher-frequency electromagnetic waves support higher-frequency communications, which is primarily how 5G will achieve its remarkable increase in speed.

This increase in frequency comes at a cost, however. As mentioned throughout the text, radio waves with a frequency upward of a few gigahertz do not diffract well around obstacles. This means that an eMBB user will need to practically have a direct line of sight to an antenna. Cellular carriers need to install millimeter-wave antennas all over cities, and rural customers may never see millimeter-wave service.

5G will bring other benefits in addition to speed. The 5G standard can more effectively utilize existing spectrum, providing a speed increase for rural customers or customers without a millimeter-wave antenna near them. 5G also lowers communications latency and increases communications reliability, two items which are key to unlocking 5G’s applicability in industrial and commercial applications.

LoRa

You’ve already got some exposure to using radio communications with microcontrollers in Chapter 6. The 2.4 GHz radios used in those exercises are great when you need a low-cost and short-range data link. However, this scenario only applies to a limited number of use cases. Many more situations require wireless communications across a longer range, for which different hardware and software is better suited. LoRa, short for long range, is such a system.

LoRa is a hardware specification that details a method for long-range and low-power data links for microcontroller-based devices.

Various radio modules utilize LoRa to connect devices across tens of kilometers, which is a much greater range than is achievable with the NRF24L01 module. Such a LoRa module is pictured in Figure 10-6.

LoRa differs from other radio communications protocols in its usage of “chirps” for data transmission. A chirp is a special implementation of frequency modulation that yields interesting signal processing benefits. A single chirp is a signal that starts at one frequency and ends at another, increasing or decreasing linearly or nonlinearly with time. An “up-chirp” goes from a lower frequency to a higher frequency, whereas a down-chirp goes from a higher frequency to a lower frequency. LoRa utilizes a combination of different up and down chirps to encode information.

Chirps were originally used in radar applications to increase the probability of detecting a target. A chirp return from a target is much easier to identify and resolve from noise (after some special signal processing) than a return of a single frequency. Engineers determined that this same principle applies in the context of communications, and LoRa was born.

LoRa modules are relatively inexpensive (around $20 or $30) and are very easy to interface with various microcontrollers. They are a great next step if the section on radio communications with microcontrollers interested you. You can utilize LoRa for many remote sensing and control applications, including those with strict power requirements (such as battery-powered system). It is not suitable for transferring large amounts of data, however.

Satellite Internet and TV

Satellite communications have been a focus of many sections of this text, so it’s important to mention the primary way in which consumers interact with satellites: satellite Internet and TV services. In some cities, satellites are the only reliable way to access the Internet due to a lack of ground-based infrastructure. Satellite Internet is a cost-effective option in such places, so aerial photos of many cities show satellite dishes dotting every roof. In Figure 10-7, you can see a dish adorning the roof of almost every structure.

The telecommunications satellites that serve these customers operate in geostationary orbit. A geostationary orbit is one in which the orbiting object appears to be fixed in the sky. This is only possible at a very particular altitude where the orbital velocity of the satellite exactly matches the rotational velocity of the surface of the Earth. Orbital velocity is a function of the altitude of the orbiting object, so only a very particular altitude is suitable. Simply put, geostationary satellites fall around the Earth at the same speed that the Earth turns. Geostationary satellites also must be directly above the equator, moving exactly parallel to the equator.

If the satellite were to move along any other trajectory, it would not orbit exactly “with” the Earth. Such satellites travel in a geosynchronous orbit, returning to the same place in the sky the same time every day. Geosynchronous and geostationary satellites orbit more than 20,000 miles above the surface of the Earth, or almost 100 times higher than the orbits of amateur radio satellites.

There are several benefits and drawbacks to having a satellite with such a high orbit. Since these satellites are so high up, they can be seen from almost every point on an entire continent. This allows satellite manufacturers to produce only one satellite to serve an entire country or region. The downsides to geostationary satellites’ high altitude come in the form of transmit power and latency. Such satellites must transmit with kilowatts of power in order to have a usable signal down on Earth. Customers on Earth must also employ highly directional dishes to get their signals back up to the satellite. These dishes need to be precisely aligned, and occupy a lot of space.

Latency, or communications delay, is also a huge limitation of geostationary satellite communications networks. Geosynchronous orbit is over 20,000 miles away, a distance that radio waves take over a tenth of a second to traverse. Additionally, packets to and from satellite Internet users must pass through a supporting ground station for the satellite that adds additional latency. Network latency between a server located somewhat close to a client is generally less than 100 milliseconds, and often closer to 10–20 milliseconds. For a satellite Internet customer, this figure is often greater than 500 milliseconds. This high latency can have serious impacts on gaming, streaming, and many other Internet services.

Many space-based Internet companies are racing to deploy networks of satellites into LEO that can offer satellite Internet without high latencies or the need of a dish. These satellites are much closer to Earth, so it’s easier and faster to send data back and forth from space to the ground. The challenging aspect of LEO satellite Internet networks is that many more satellites are needed to serve a region. At least one satellite must be overhead at all times in order for a consumer to access a company’s network. This is only achievable by deploying a fleet of many satellites in carefully choreographed orbits. Network designs by SpaceX and Amazon call for thousands of satellites. These networks are much closer competitors to traditional broadband, as they offer similar latencies and speeds.

Radio Astronomy

Many of the largest radio dishes on Earth are not used for communications purposes. Such dishes are a specialized type of hardware known as radio telescopes. As you can likely guess from the name, radio telescopes are used for astronomical observation. An example of such telescopes is shown in Figure 10-8.

These dishes function in much the same way as optical telescopes—they collect electromagnetic radiation and focus it. Optical telescopes use optical lenses to focus light, whereas radio telescopes use parabolic reflectors to focus radio waves. Radio astronomy is somewhat more challenging than optical astronomy, however. Due to the longer wavelength of radio waves, the resolution of radio telescopes is much less than that of an optical telescope of the same size. Even a large telescope such as the one shown in Figure 10-8 does not have sufficient resolution to image faraway objects. To remedy this, astronomers use a technique called interferometry.

Interferometry is a way to boost the resolution of imaging systems by combining the data from multiple telescopes. The effective size of the combined telescope is that of the greatest distance between telescopes in the array. This offers huge benefits for astronomy, as it allows telescopes spaced all over the globe to be combined into one Earth-size telescope. This technique was utilized to capture the first ever picture of a black hole, as shown in Figure 10-9.