There are a great deal of tutorials for radio online which amount to little more than instructions without explanation. There is also a great deal of radio theory online which tends to delve deeply into physics and math without properly explaining the connection to the practical. What follows is my attempt to bridge the gap with a very basic introduction to radio theory in five short parts.
1 What is radio?
Radio telecommunications, or simply radio, is the art and science of using portions of the electromagnetic spectrum outside of the narrow range of visible light to communicate over distance.
Radio signals are a form of non-ionizing radiation. Before the standard fear of radiation kicks in, I should clarify, ionizing radiation is the sort of radiation that messes with chemistry in bodies and causes cancer and death. Non-ionizing radiation includes visible light and the heat you feel projected out of a stove window.
This isn't to say that non-ionizing radiation is completely safe. Like the aforementioned stove, it can burn you in a very literal sense. It can release heat into bodies both on the surface and at depth. Generally, you have nothing to worry about with sources rated 50 watts or less. Even the FCC has exempted these from routine assessments as they do not release enough power to possibly hit the exposure thresholds. For reference, most handheld radios are below 10 watts and most vehicle-mounted radios are around 25 watts.
Radio signals behave most like light. They travel in mostly straight lines, with a little scatter and lensing under certain circumstances. Some materials are transparent to them, some are absorbent, and some are reflective. They can be projected in all directions like a lantern's light, or in a particular direction like a flashlight.
Radio telecommunications, or simply radio, is the art and science of using portions of the electromagnetic spectrum outside of the narrow range of visible light to communicate over distance.
Radio signals are a form of non-ionizing radiation. Before the standard fear of radiation kicks in, I should clarify, ionizing radiation is the sort of radiation that messes with chemistry in bodies and causes cancer and death. Non-ionizing radiation includes visible light and the heat you feel projected out of a stove window.
This isn't to say that non-ionizing radiation is completely safe. Like the aforementioned stove, it can burn you in a very literal sense. It can release heat into bodies both on the surface and at depth. Generally, you have nothing to worry about with sources rated 50 watts or less. Even the FCC has exempted these from routine assessments as they do not release enough power to possibly hit the exposure thresholds. For reference, most handheld radios are below 10 watts and most vehicle-mounted radios are around 25 watts.
Radio signals behave most like light. They travel in mostly straight lines, with a little scatter and lensing under certain circumstances. Some materials are transparent to them, some are absorbent, and some are reflective. They can be projected in all directions like a lantern's light, or in a particular direction like a flashlight.
2 What are radio frequency ranges and bands?
While a particular frequency is important for making contacts, contiguous swathes of frequencies share both propagation characteristics and equipment. The legal authorities that govern radio operation have taken this into account and divided the radio spectrum so that different radio services (i.e. business, amateur, commercial broadcast, police and fire, marine, &c.) have blocks in several different swathes with different characteristics.
Confusingly, both the broad ranges of frequency as well as the divisions given to different radio services within them are both often called ‘bands’. For clarity, I will refer to the large swathes by the alternate term ‘frequency range’ and the allotments within these frequency ranges as ‘bands’. In this section, I will first discuss the legal aspects of bands. In the next, I will cover technical aspects of how the different bands work differently from each other.
The International Telecommunications Union (ITU), a treaty organization, is responsible for managing a lot of definitions, names, and broad frequency allocations to allow for both interoperability and mutual understanding of terms of art in radio around the world. The broad frequency allocations from the ITU allow for both interoperability and minimal interference between stations operating in nearby countries. National agencies, such as the Federal Communications Commission in the United States, further define and restrict frequency allotments within their jurisdictions.
Within the allotments for amateur operators, an organization, such as the Amateur Radio Relay League in the United States, will further adopt a band plan. This ensures that certain methods of communication are operating with compatible or non-interfering methods on the same frequencies. This keeps hams interested in QRP DX-ing (attempting to communicate with distant stations on extremely low power) from having to compete with contesters (people who attempt contact with as many stations on a set list during a set time period, usually with high power transmitters) for the same frequencies. It also allows for machine communication to be separated from human, and various other conveniences.
On a more granular level, there are the frequency coordinators for single metropolitan areas. Repeaters are fixed stations in generally high locations with sensitive and bulky antennas to detect weak signals and repeat them strongly over a broad region to allow reliable two-way communications on even handheld radios. Two repeater stations can interfere with each other in obnoxious ways. Frequency coordinators will coordinate your repeater's frequency, taking in to account its location, power, and coverage area to minimize conflict. It's not strictly required, but the FCC privileges coordinated repeaters to maintain their frequencies over un-coordinated repeaters.
While a particular frequency is important for making contacts, contiguous swathes of frequencies share both propagation characteristics and equipment. The legal authorities that govern radio operation have taken this into account and divided the radio spectrum so that different radio services (i.e. business, amateur, commercial broadcast, police and fire, marine, &c.) have blocks in several different swathes with different characteristics.
Confusingly, both the broad ranges of frequency as well as the divisions given to different radio services within them are both often called ‘bands’. For clarity, I will refer to the large swathes by the alternate term ‘frequency range’ and the allotments within these frequency ranges as ‘bands’. In this section, I will first discuss the legal aspects of bands. In the next, I will cover technical aspects of how the different bands work differently from each other.
The International Telecommunications Union (ITU), a treaty organization, is responsible for managing a lot of definitions, names, and broad frequency allocations to allow for both interoperability and mutual understanding of terms of art in radio around the world. The broad frequency allocations from the ITU allow for both interoperability and minimal interference between stations operating in nearby countries. National agencies, such as the Federal Communications Commission in the United States, further define and restrict frequency allotments within their jurisdictions.
Within the allotments for amateur operators, an organization, such as the Amateur Radio Relay League in the United States, will further adopt a band plan. This ensures that certain methods of communication are operating with compatible or non-interfering methods on the same frequencies. This keeps hams interested in QRP DX-ing (attempting to communicate with distant stations on extremely low power) from having to compete with contesters (people who attempt contact with as many stations on a set list during a set time period, usually with high power transmitters) for the same frequencies. It also allows for machine communication to be separated from human, and various other conveniences.
On a more granular level, there are the frequency coordinators for single metropolitan areas. Repeaters are fixed stations in generally high locations with sensitive and bulky antennas to detect weak signals and repeat them strongly over a broad region to allow reliable two-way communications on even handheld radios. Two repeater stations can interfere with each other in obnoxious ways. Frequency coordinators will coordinate your repeater's frequency, taking in to account its location, power, and coverage area to minimize conflict. It's not strictly required, but the FCC privileges coordinated repeaters to maintain their frequencies over un-coordinated repeaters.
3 How do bands differ from each other in operation?
Different frequencies have different propagation characteristics, with lower frequencies generally traveling further around the Earth than higher frequencies. Bands are generally described by the wavelength of the signal. This length can be calculated accurately enough by dividing 300 by the frequency in megahertz (MHz) to give wavelength in meters (m).
For local use, the most common bands fall into the Ultra High Frequency (UHF) and Very High Frequency (VHF) frequency ranges. VHF is 30-300 MHz and UHF is 300-3000 MHz, or 300 MHz to 3 GHz, if we’re mixing metric prefixes. Everybody from taxis to police, fire fighters to warehouse managers, and ambulances to military satellite communications are set among and in between these bands.
These bands are generally line-of-sight. Atmospheric lensing differs at these frequencies from visible light, with greater differences at lower frequencies. As such, the line of sight extends further past the visual horizon with lower signals in this range. Equipment for VHF and UHF is often quite small, and nearly all handheld radios operate in these bands.
The short wavelength of UHF allows it to penetrate deep into structures by way of radio-transparent portions of walls and passing through the sorts of interior spaces a human could traverse. Signals in this frequency range also suffer from significant attenuation by vegetation, which is to say that the signal is partially absorbed by the plants, reducing range when transmitting through vegetation. For that reason, UHF is heavily used in urban areas, but far less so in rural wooded areas.
VHF, however, is not nearly as subject to attenuation by vegetation, but doesn’t penetrate very far into (or out of) structures, and thus sees a lot more use in rural or wooded areas. It is also a little better at getting around terrain, especially at the lower end.
Both UHF and VHF have interesting properties when it comes to reflecting them off of structures, terrain, the moon, or even, in the case of UHF, clouds. These methods can be used to overcome terrain or other obstacles that block line-of-sight paths. It is possible, with high-gain antennas on both ends, to contact a station anywhere on earth where the moon is visible by bouncing a signal off the moon.
VHF and UHF are also used for communications with spacecraft, such as artificial satellites, as the ionosphere, the outermost layer of earth’s atmosphere, is transparent to pretty much all signals above 30 MHz. Below that frequency, the ionosphere can be transparent, reflective, or absorbent of signals, depending on conditions largely driven by solar activity and heavily impacted by time of day.
The possibility of ionospheric reflectivity dominates the use of High Frequency, or HF radio, which is 3-30 MHz. When conditions are favorable, a signal can be bounced between the ionosphere and the earth through several skips, giving varying degrees of signal strength in rings at tens to hundreds of miles distance. As conditions shift, it might be necessary to move between different HF bands, as it is possible that your signal might be skipping past your intended receiving station, and missing them entirely. A system of automated beacons are used by hams to test whether bands are open. Generally, if you can hear your destination, they can hear you, as the path normally runs equally well both ways.
HF reflected signals work best during the daylight hours, with better performance in the afternoon when the ionosphere has been most energized by the sun. Strong solar weather, such as major sunspot activity, improves communications in HF.
It is difficult to get an HF signal to cross the equator, except by using a phenomenon known as gray-line propagation. The gray-line refers to the boundary between day and night, where signals transmitted around dawn or dusk will often travel great distances along that boundary, but only along that boundary.
Still lower bands, which begin to become more difficult to utilize because of the size requirements for their
Different frequencies have different propagation characteristics, with lower frequencies generally traveling further around the Earth than higher frequencies. Bands are generally described by the wavelength of the signal. This length can be calculated accurately enough by dividing 300 by the frequency in megahertz (MHz) to give wavelength in meters (m).
For local use, the most common bands fall into the Ultra High Frequency (UHF) and Very High Frequency (VHF) frequency ranges. VHF is 30-300 MHz and UHF is 300-3000 MHz, or 300 MHz to 3 GHz, if we’re mixing metric prefixes. Everybody from taxis to police, fire fighters to warehouse managers, and ambulances to military satellite communications are set among and in between these bands.
These bands are generally line-of-sight. Atmospheric lensing differs at these frequencies from visible light, with greater differences at lower frequencies. As such, the line of sight extends further past the visual horizon with lower signals in this range. Equipment for VHF and UHF is often quite small, and nearly all handheld radios operate in these bands.
The short wavelength of UHF allows it to penetrate deep into structures by way of radio-transparent portions of walls and passing through the sorts of interior spaces a human could traverse. Signals in this frequency range also suffer from significant attenuation by vegetation, which is to say that the signal is partially absorbed by the plants, reducing range when transmitting through vegetation. For that reason, UHF is heavily used in urban areas, but far less so in rural wooded areas.
VHF, however, is not nearly as subject to attenuation by vegetation, but doesn’t penetrate very far into (or out of) structures, and thus sees a lot more use in rural or wooded areas. It is also a little better at getting around terrain, especially at the lower end.
Both UHF and VHF have interesting properties when it comes to reflecting them off of structures, terrain, the moon, or even, in the case of UHF, clouds. These methods can be used to overcome terrain or other obstacles that block line-of-sight paths. It is possible, with high-gain antennas on both ends, to contact a station anywhere on earth where the moon is visible by bouncing a signal off the moon.
VHF and UHF are also used for communications with spacecraft, such as artificial satellites, as the ionosphere, the outermost layer of earth’s atmosphere, is transparent to pretty much all signals above 30 MHz. Below that frequency, the ionosphere can be transparent, reflective, or absorbent of signals, depending on conditions largely driven by solar activity and heavily impacted by time of day.
The possibility of ionospheric reflectivity dominates the use of High Frequency, or HF radio, which is 3-30 MHz. When conditions are favorable, a signal can be bounced between the ionosphere and the earth through several skips, giving varying degrees of signal strength in rings at tens to hundreds of miles distance. As conditions shift, it might be necessary to move between different HF bands, as it is possible that your signal might be skipping past your intended receiving station, and missing them entirely. A system of automated beacons are used by hams to test whether bands are open. Generally, if you can hear your destination, they can hear you, as the path normally runs equally well both ways.
HF reflected signals work best during the daylight hours, with better performance in the afternoon when the ionosphere has been most energized by the sun. Strong solar weather, such as major sunspot activity, improves communications in HF.
It is difficult to get an HF signal to cross the equator, except by using a phenomenon known as gray-line propagation. The gray-line refers to the boundary between day and night, where signals transmitted around dawn or dusk will often travel great distances along that boundary, but only along that boundary.
Still lower bands, which begin to become more difficult to utilize because of the size requirements for their
antenna systems, fall into the Medium Frequency (MF) frequency range. The MF band available to amateurs is directly above the commercial AM radio band. Like commercial AM radio, these signals travel far in the daytime and exceptionally far at night, with the best conditions allowing communications to cross continents. Signals at this wavelength propagate best at night, and can be heard furthest when the sun is quiescent.
4 Modulations and Modes
The means by which information is placed into a signal is called modulation. There are a number of schemes by which this may be accomplished. The nature of the information placed into the signal, such as digital information, images, or sound, called ‘phone’ in radio parlance, is called ‘mode’.
The simplest and oldest is, of course, turning the signal on and off. This is called Continuous Wave modulation, or CW, and is the method by which Morse code is generally sent. CW modulated signals are the most efficient, with all signal strength focused into a very narrow range of frequency.
The next logical step is to switch the signal between two nearby frequencies, a method known as Frequency Shift Keying, or FSK. FSK allows for better error detection, as unlike with CW, the total loss of a signal would be detected. The two signals on FSK taking the place of the on-off duality in CW, are ‘mark’ and ‘space’. FSK is heavily used for machine signals such as those used with teletype, or TTY. Radio Teletype, or RTTY, remains a popular method for sending amateur communications.
To modulate by varying the frequency of a signal around a carrier frequency is called Frequency Modulation, or FM. Silence on FM resembles a CW on-signal, a strong signal occupying little bandwidth and centered tightly on the carrier frequency. The individual pulses of higher pressure that make up sound push the FM signal out to either side of its carrier frequency symmetrically, with the frequency increasing and decreasing in alternation very rapidly. The amount of deviation corresponds to the strength of the individual vibration of the sound. The frequencies of most radio signals are such orders of magnitude higher than human hearing range that each pulse of a vibration is shown across many oscillations of the radio signal. A specific bandwidth, or allowable deviation from the nominal carrier signal, is set by the equipment, and dictated by law, as over-modulation can result in intruding on neighboring channels and interfering with those transmissions. FM equipment is very simple and inexpensive, but it is the least efficient modulation. Most phone signals on VHF or UHF are FM.
To modulate by varying the strength of the signal is called Amplitude Modulation, or AM. Like FM, AM encodes sounds to a very high fidelity. AM signals take up less bandwidth and are more efficient than FM, but the equipment is slightly more complicated and expensive.
AM signals have a powerful and persistent central spike of energy directly on their carrier frequency, and two mirrored ‘sidebands’ of varying intensity which contain most of the information. With additional equipment, AM signals can have the carrier and one of the sidebands filtered out before the signal reaches the final amplifier before transmission. This scheme, called Single SideBand, or SSB, is the most efficient modulation for the phone mode. Whether the preserved sideband is the one on higher or lower frequency is denoted by referring to these signals as either Upper SideBand, or USB, or as Lower SideBand, or LSB. By custom, USB is used above 10 MHz and LSB is used below 10 MHz. Most phone signals on HF or MF are SSB. The primary downside to SSB, aside from more complicated and expensive equipment, is that stations imperfectly tuned sound either high or low pitched depending on the direction the signal is off. In the days before reliable digital readouts, two SSB stations trying to tune in on each other could chase each other across a whole band, each trying to get the other station to sound right.
Aside from these modulations there are a whole series of digital modulations and modes, both proprietary and open, in use. Trunked communication and the ability to select different groups of recipients are some of the features offered with these new schemes. Many police radio systems have not only converted to digital, but employ encryption in a way that is not legal for private citizens nor corporate license holders, with the exception of cellular companies.
The means by which information is placed into a signal is called modulation. There are a number of schemes by which this may be accomplished. The nature of the information placed into the signal, such as digital information, images, or sound, called ‘phone’ in radio parlance, is called ‘mode’.
The simplest and oldest is, of course, turning the signal on and off. This is called Continuous Wave modulation, or CW, and is the method by which Morse code is generally sent. CW modulated signals are the most efficient, with all signal strength focused into a very narrow range of frequency.
The next logical step is to switch the signal between two nearby frequencies, a method known as Frequency Shift Keying, or FSK. FSK allows for better error detection, as unlike with CW, the total loss of a signal would be detected. The two signals on FSK taking the place of the on-off duality in CW, are ‘mark’ and ‘space’. FSK is heavily used for machine signals such as those used with teletype, or TTY. Radio Teletype, or RTTY, remains a popular method for sending amateur communications.
To modulate by varying the frequency of a signal around a carrier frequency is called Frequency Modulation, or FM. Silence on FM resembles a CW on-signal, a strong signal occupying little bandwidth and centered tightly on the carrier frequency. The individual pulses of higher pressure that make up sound push the FM signal out to either side of its carrier frequency symmetrically, with the frequency increasing and decreasing in alternation very rapidly. The amount of deviation corresponds to the strength of the individual vibration of the sound. The frequencies of most radio signals are such orders of magnitude higher than human hearing range that each pulse of a vibration is shown across many oscillations of the radio signal. A specific bandwidth, or allowable deviation from the nominal carrier signal, is set by the equipment, and dictated by law, as over-modulation can result in intruding on neighboring channels and interfering with those transmissions. FM equipment is very simple and inexpensive, but it is the least efficient modulation. Most phone signals on VHF or UHF are FM.
To modulate by varying the strength of the signal is called Amplitude Modulation, or AM. Like FM, AM encodes sounds to a very high fidelity. AM signals take up less bandwidth and are more efficient than FM, but the equipment is slightly more complicated and expensive.
AM signals have a powerful and persistent central spike of energy directly on their carrier frequency, and two mirrored ‘sidebands’ of varying intensity which contain most of the information. With additional equipment, AM signals can have the carrier and one of the sidebands filtered out before the signal reaches the final amplifier before transmission. This scheme, called Single SideBand, or SSB, is the most efficient modulation for the phone mode. Whether the preserved sideband is the one on higher or lower frequency is denoted by referring to these signals as either Upper SideBand, or USB, or as Lower SideBand, or LSB. By custom, USB is used above 10 MHz and LSB is used below 10 MHz. Most phone signals on HF or MF are SSB. The primary downside to SSB, aside from more complicated and expensive equipment, is that stations imperfectly tuned sound either high or low pitched depending on the direction the signal is off. In the days before reliable digital readouts, two SSB stations trying to tune in on each other could chase each other across a whole band, each trying to get the other station to sound right.
Aside from these modulations there are a whole series of digital modulations and modes, both proprietary and open, in use. Trunked communication and the ability to select different groups of recipients are some of the features offered with these new schemes. Many police radio systems have not only converted to digital, but employ encryption in a way that is not legal for private citizens nor corporate license holders, with the exception of cellular companies.
5 Antennas
There are thousands of antenna designs out there falling into hundreds of broad types. I will cover in very general terms only the simplest and most common here. Most can be made with ordinary wire and wood or plastic radio-transparent materials available at any home improvement store.
Broadly, antenna size increases with wavelength. There are antennas only suitable for reception and antennas suitable for both transmission and reception.
The simplest antenna is called a dipole. It is two wires, laid end-to-end in line with each other, each cut to a length that is resonant with the intended frequency, fed from the ends adjacent to each other. Resonant lengths are multiples or fractions of the wavelength, such as twice-wave, half-wave, quarter-wave, eighth-wave, &c. There are circumstances where you can use peculiar fractions, like 5/8 wave, but that requires other components added to the antenna system. The closer your antenna is to resonance, the less burden it places on your radio set and the more efficient your transmission is. The dipole is freestanding, and can be set up vertically or horizontally, with that orientation determining the polarity of the transmitted signal.
Polarity is simply whether the radio waves are moving vertically or horizontally. When they are moving in both ways, as well as every angle in between, they are said to be circularly polarized. If you try to pick up vertically polarized waves with a horizontal antenna, or vice versa, there will be a drop in received signal strength compared to what you would have received with the correct polarity. Circularly polarized waves can be picked up by either equally well, and circularly polarized antennas can pick up vertical or horizontal waves equally well.
Physically simpler but conceptually more complicated than the dipole is the monopole. Monopoles are necessarily perpendicular to a ground plane, a flat surface of radio-grounding material, which reflects the signal and, in reflection, creates the appearance of a dipole. On higher frequencies, the hand of the radio operator holding the monopole-equipped handheld radio is enough ground plane. On lower frequencies, you might find yourself running thick copper lines in eight rays out from the base of your antenna, connecting them to rods driven deep into the ground, and terminating one of them into a pond for luck just to have enough ground. (There is a reason so many commercial radio stations have their antenna farms in swamps.)
Both the monopole and dipole antennas are, when vertical, omnidirectional, which is to say that the signal goes out in 360 degrees equally well. When laid horizontally, they are a form of beam antenna, as the signal is focused in a beam perpendicular to the broad sides of the antenna. As a receiver, a horizontal monopole or dipole is also far more sensitive in the same orientation. For transmission, the primary difference between dipoles and monopoles is that, in vertical orientation, monopole signals skew upwards away from the ground plane, while dipoles direct their signals out exactly perpendicular to their orientation. Shorter monopoles direct more and more of their signal upwards and less and less out towards the horizon.
The amount that the signal is concentrated in a particular direction is known as gain, and is measured in decibels, or dB. An antenna with 3 dB of gain will, in the direction it is most powerful, direct twice as much signal energy as a theoretical isotropic, or spherically equally onmidirectional, antenna would. A 6 dB antenna, in its strongest direction, directs four times as much signal energy as an isotropic antenna would.
There are directional antennas that focus most of their energy in a single direction. There will always be some lobes of energy going in the wrong direction, but it is quite possible to construct a many-element Yagi-Uda antenna that will give 12 dB of gain, a remarkable improvement in a particular direction. Such high-gain antennas are generally used to hit specific distant targets, such as the moon.
There are thousands of antenna designs out there falling into hundreds of broad types. I will cover in very general terms only the simplest and most common here. Most can be made with ordinary wire and wood or plastic radio-transparent materials available at any home improvement store.
Broadly, antenna size increases with wavelength. There are antennas only suitable for reception and antennas suitable for both transmission and reception.
The simplest antenna is called a dipole. It is two wires, laid end-to-end in line with each other, each cut to a length that is resonant with the intended frequency, fed from the ends adjacent to each other. Resonant lengths are multiples or fractions of the wavelength, such as twice-wave, half-wave, quarter-wave, eighth-wave, &c. There are circumstances where you can use peculiar fractions, like 5/8 wave, but that requires other components added to the antenna system. The closer your antenna is to resonance, the less burden it places on your radio set and the more efficient your transmission is. The dipole is freestanding, and can be set up vertically or horizontally, with that orientation determining the polarity of the transmitted signal.
Polarity is simply whether the radio waves are moving vertically or horizontally. When they are moving in both ways, as well as every angle in between, they are said to be circularly polarized. If you try to pick up vertically polarized waves with a horizontal antenna, or vice versa, there will be a drop in received signal strength compared to what you would have received with the correct polarity. Circularly polarized waves can be picked up by either equally well, and circularly polarized antennas can pick up vertical or horizontal waves equally well.
Physically simpler but conceptually more complicated than the dipole is the monopole. Monopoles are necessarily perpendicular to a ground plane, a flat surface of radio-grounding material, which reflects the signal and, in reflection, creates the appearance of a dipole. On higher frequencies, the hand of the radio operator holding the monopole-equipped handheld radio is enough ground plane. On lower frequencies, you might find yourself running thick copper lines in eight rays out from the base of your antenna, connecting them to rods driven deep into the ground, and terminating one of them into a pond for luck just to have enough ground. (There is a reason so many commercial radio stations have their antenna farms in swamps.)
Both the monopole and dipole antennas are, when vertical, omnidirectional, which is to say that the signal goes out in 360 degrees equally well. When laid horizontally, they are a form of beam antenna, as the signal is focused in a beam perpendicular to the broad sides of the antenna. As a receiver, a horizontal monopole or dipole is also far more sensitive in the same orientation. For transmission, the primary difference between dipoles and monopoles is that, in vertical orientation, monopole signals skew upwards away from the ground plane, while dipoles direct their signals out exactly perpendicular to their orientation. Shorter monopoles direct more and more of their signal upwards and less and less out towards the horizon.
The amount that the signal is concentrated in a particular direction is known as gain, and is measured in decibels, or dB. An antenna with 3 dB of gain will, in the direction it is most powerful, direct twice as much signal energy as a theoretical isotropic, or spherically equally onmidirectional, antenna would. A 6 dB antenna, in its strongest direction, directs four times as much signal energy as an isotropic antenna would.
There are directional antennas that focus most of their energy in a single direction. There will always be some lobes of energy going in the wrong direction, but it is quite possible to construct a many-element Yagi-Uda antenna that will give 12 dB of gain, a remarkable improvement in a particular direction. Such high-gain antennas are generally used to hit specific distant targets, such as the moon.
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FIRST AID TO THE INJURED.pdf
2.4 MB
What causes injuries and how they are they divided by cause?
Anything that can damage the body can cause an injury: blunt or sharp objects, impact at high speed, falls, animal or insect bites, fire or extreme heat, and exposure to chemicals and toxins. According to the cause the injury can be divided into:
-Mechanical injury: injury to any portion of the body from a blow, crush, cut, or penetrating force (bullet)
– Thermal injury: injury caused by exposure to excess heat and excess cold sufficient to cause damage to the skin, and possibly deeper tissue
- Electrical injury: injuries caused by exposure to natural lightning or electricity in the home or workplace
- Injury produced by ionizing radiation
Anything that can damage the body can cause an injury: blunt or sharp objects, impact at high speed, falls, animal or insect bites, fire or extreme heat, and exposure to chemicals and toxins. According to the cause the injury can be divided into:
-Mechanical injury: injury to any portion of the body from a blow, crush, cut, or penetrating force (bullet)
– Thermal injury: injury caused by exposure to excess heat and excess cold sufficient to cause damage to the skin, and possibly deeper tissue
- Electrical injury: injuries caused by exposure to natural lightning or electricity in the home or workplace
- Injury produced by ionizing radiation
Full-First-Aid-Manual-FM-2111.pdf
2.3 MB
Knowing how to use your IFAK is just as crucial. I can go in to further detail about these products in future posts. First aid is a massive topic, here is an excellent PDF