As of technology has developed, equipment that utilizes radio waves such as television, mobile phones and so on has been introduced, and now radio waves are present all around us. Radio waves themselves are not something that humans invented. They have existed since the moment when the universe came into being. Presently in the natural world radio waves are radiated onto the surface of the earth from the sun and the other stars, and electromagnetic waves (radio waves) also arise from movement of the earth's crust and from lightening and so on.
However humans discovered radio waves, and have invented devices for generating them. Radio waves are currently used in all sorts of fields. Familiar items include mobile phones, televisions, radios, amateur radio equipment and the like. There has been an explosive spread of mobile phones, allowing people to contact each other wherever they go. Exchanging email is also a very popular function. Television images are sent from countries on the other side of the earth, and for people who are not really interested in technology and the like, the technology behind these communications is not important. What is important is the content of the communication.
Although we will consider the design of equipment using radio modules, it is not necessary for us to inquire particularly deeply into the physical characteristics of radio waves. Just a little knowledge of the subject is sufficient for designing radio equipment. For detailed information on the physical properties of radio waves, please refer to the relevant literature. First we will cover the basic information.
This refers to radio modules, and communication range is up to several kilometers, while the frequency of the radio waves is in a range of from several hundred MHz to several tens of GHz.
Radio waves are electromagnetic waves. Electromagnetic waves include waves such as X rays, ultraviolet light, visible light, infrared rays and so on, but you probably imagine radio waves to be quite different from these waves. Of the various kinds of electromagnetic waves, radio waves have a longer wave length than infrared rays, and are defined by the Radio Law as 'electromagnetic waves with a frequency of less than 3,000 GHz (3 THz)'.
Ultralow frequency radio waves
*K:kilo 1*103, M:mega 1*106, G:giga 1*109, T:tera 1*1012
|Names of radio waves||Frequency||Wave length||Principal applications|
|VLF(Very Low Frequency)||3kHz ~ 30kHz||100km ~ 10km|
|LF(Low Frequency)||30kHz ~ 300kHz||10km ~ 1km||Vessel / Airplane beacon|
|MF(Medium Frequency)||300kHz ~ 3MHz||1km ~ 100m||AM radio, Marine radio, Amateur radio|
|HF(High Frequency)||3MHz ~ 30MHz||100m ~ 10m||Shortwave broadcasting, Marine / Air radio, Amateur radio|
|VHF(Very High Frequency)||30MHz ~ 300MHz||10m ~ 1m||TV, FM, Fire radio, Police radio, Disaster PA radio network|
|UHF(Ultra High Frequency)||300MHz ~ 3GHz||1m ~ 10cm||Low power radio, Mobile-phone, Taxi radio, Amateur radio, TV, Wireless LAN|
|SHF(Super High Frequency)||3GHz ~ 30GHz||10cm ~ 1cm||Satellite broadcasting, Radar|
|EHF(Extremely High Frequency)||30GHz ~ 300GHz||1cm ~ 1mm||Satellite broadcasting, Radio astronomy, Radar|
|submillimeter waves||300GHz ~ 3THz||1mm ~ 0.1mm||
Radio waves are electromagnetic waves. Electromagnetic waves include waves such
as X rays, ultraviolet light, visible light, infrared rays and so on, but you
probably imagine radio waves to be quite different from these waves. Among the
kinds of electromagnetic waves, radio waves have a longer wave length than
infrared rays, and are defined as electromagnetic waves with a frequency of less
than 3,000 GHz. In free space their propagation velocity is the same as that of
light, at approximately 300,000 km in 1 second. The distance from the earth to
the moon is about 390,000 km, so from the moon, a signal would arrive in about
Recall the scientific experiments you did at school. When a copper wire was
placed on top of a compass and a current was applied from a battery, the needle
of the compass shook. Next, when a bar magnet was moved quickly through the
middle of a coil made of copper wire, the needle of the ammeter connected to the
coil shook. In the former case, a magnetic field was generated by the change in
the current (electric field), and in the latter case, a current (electric field)
was generated by the change in the magnetic field.
When a high frequency current flows in an antenna, the electric field changes acutely and a magnetic field is generated in the vicinity. Then due to this magnetic field, an electric field is generated. This process is repeated and matter flying in space moves perpendicularly (at right angles) to the direction of vibration of the electric field and magnetic field according to the form of the radio wave. The electric field (E) and magnetic field (H) vibrate at right angles to each other. In this way, electric fields and magnetic fields have an integral relationship in radio waves, and neither one exists independently of the other.
The qualities of radio waves are often compared to those of sound waves, but the definitive difference is that radio waves are transmitted even in the absence of matter (a medium). (For sound waves to be transmitted air is required.)
In other words, radio waves can even be transmitted in a vacuum. In fact, radio waves are transmitted from communications satellites.
You may have heard the words 'longitudinal wave' and 'transverse wave'. Waves that, like sound, vibrate in the same direction as their direction of propagation are longitudinal waves, while waves that vibrate at right angles to their direction of propagation are transverse waves. Radio waves are one kind of transverse wave.
The shorter the wave length of radio waves becomes, the more they take on the qualities of light, and the greater their straightness becomes. In other words, their energy is concentrated in one direction, and they are said to have strong directivity. Furthermore, the higher the frequency, the more acute is the attenuation of the wave's energy. In general, radio waves are considered to propagate in a straight line, but what happens if there are various physical obstacles in their path such as mountains, buildings, walls, or people and so on? If we consider an example of an urban area where there are many buildings, there are direct waves that arrive directly, reflected waves that arrive after hitting buildings and the like, diffracted waves that circumvent the shadows of buildings, transmitted waves that arrive by passing through the glass or walls of buildings, and so on. The kinds of wave differs according to type (material) of obstacle. Radio waves can pass through glass and ceramics (permeation), but they are reflected by metal and concrete. Furthermore, waves with frequencies higher than several GHz are scattered and absorbed by rain, snow, fog and the like, and their power tends to attenuate.
In free space the propagation speed of radio waves is the same as that of light, at approximately 300,000 km, so they would arrive there in about 1.3 seconds. The speed falls slightly when passing through a conductor such as an antenna or cable. The wave length λ (lambda) of radio waves is as follows; If the frequency of the radio wave is f, and the speed of the radio wave in a vacuum is C, then
As the speed per second of radio waves in a vacuum is about 300,000 km (about 3×108 [m/sec]), at 433 MHz the wave length λ is
There is a close relationship between the wave length of the radio wave and the characteristics of the antenna.
It should be obvious that to receive another party's voice by mobile phone, there must be radio waves in that location. Although whether or not there are radio waves is to a large extent related to the distance from the transmitting antenna, it is the electric field intensity that indicates the strength of the radio waves there.
Electric field intensity at an arbitrary location
When considering the characteristics of radio waves and antennas theoretically, a method taking a virtual antenna as a reference is used. This theoretical antenna is called an isotropic antenna. Isotropic antennas are point-like antennas, that can emit and receive radio waves in all directions with a certain field strength.
As in the diagram below, the power per unit area at a location D [m] distant from an isotropic antenna with P [W] power, is called power density PD [W/m2], and is the value shown in (2).
In addition, the value for the Poynting vector PV [W/m2] at that location is (1) assuming electric field intensity E [V/m]. Since the power density and Poynting vector are equal, assuming (1) = (2), the electric field intensity of the location at distance D [m] is (3).
* Poynting vector: A theorem identified by John Henry Poynting
Electric field intensity at an arbitrary location with an absolute gain Gi antenna
As the formulae above suggest, the general form for finding the electric field intensity of the location d [m] is with an antenna with absolute gain Gi is (4).Replace Gi with the true value.
It is possible to find the received power of an antenna from the electric field intensity at the location where the antenna is placed. The following is the calculation of received power in free space. Actually, there are a variety of propagation loss factors, and it is not possible simply to calculate it, but it is possible to find approximate values and tendencies.
In addition, since there are many mobile radio devices in use at a height of about 1 m above ground level, we present propagation loss calculations for two simple wave models over flat terrain.
Calculation of received power with the Friis transmission formula
The Friis transmission formula shows the relationship between transmitted power and antenna received power at the location D [m]. It is possible to find the received power from the power density PD (1) of the receiving location and the effective area AR (2) of the receiving antenna. Since PR = PD*AR, received power is found as shown in (3).
Calculation of received power with the Poynting vector and antenna effective area
It is possible to find the received power from the Poynting vector PV (1) of the receiving location and the effective area of the receiving antenna AR (2) as shown in (3). If the field strength of the receiving location and the receiving antenna absolute gain is known, it is possible to find the received power.
Calculation of the electric field intensity and received power in free space.
The approximate value of the electric field intensity and received power in free space can be found with using the following calculation tool.
Calculation of the electric field intensity and received power over flat terrain.
The approximate value of the electric field intensity and received power over flat terrain can be found with using the following calculation tool.
In free space (space in which there is nothing to obstruct the progress of the radio waves), radio waves decay proportionally at the square of the distance, and in inverse proportion to the square of the wave length of the radio waves. If we call the ratio of effective received power Wr and transmitted power Wt free space propagation loss L, and we call frequency f [Hz], distance d [m], wave length λ [m], and the absolute gain of the transmitting and receiving antennas Gt and Gr shown in decibels, we get
If the transmitting and receiving antennas used are isotropic antennas, the free space propagation loss is called free space basic propagation loss LB.
Since the decibel value found is a loss calculation, its code is minus.
If we try a frequency of 400 MHz and distance of 500 m, we get LB [dB] = 78.5 dB.
With a frequency of 2,400 MHz and distance of 500 m, we get LB [dB] = 94.0 dB.
It is easy to see the difference in decay due to frequency.
If assumed there is no loss other than free space basic propagation loss, to calculate in decibels the received power at the receiving antenna, we use the following formula.
Note however, that the transmitted power is power applied directly to the antenna. When thinking about the output terminals or input terminals of radio equipment, consideration must be given to connection loss from cables, connectors and the like.
PR [dBm] = PT [dBm] + (GTA [dBi] + GRA [dBi]) – LB [dB]
Propagation loss not covered by the calculation
This free space propagation loss can be applied with slight correction in the case of transmission from the earth to a communications satellite, or two way, line of sight communication between two high locations. However, in the environments in which radio modules are used, there is an impact from the terrain and buildings and so on, and the weather conditions and the like has an effect. Also, the conditions of use at a ground level of about 1 m are greatly affected by the environment, and the calculations do not necessarily apply.
Data is available for a location in the suburbs where line of sight is possible that shows actual differences in decay of 20 dB with the free space propagation loss at a frequency of 400 MHz due to the installed height and surroundings, so it is necessary to carry out actual tests.
Although there are several formulae for propagation loss over flat terrain, we present the following formula.
With this formula, if the communication distance is sufficiently far with regard to the installed height, the following approximation results.
With the following conditions we will obtain the received power [dBm] at the receiver.
Conditions: hT and hR are the height of the transmitting and receiving antennas, and hT = hR = 1.2 m. d is the communication distance and d = 500 m. Transmitted power PT = 10 mW. Transmitting antenna gain is GTA = 0 dBi. Receiving antenna gain is GRA = 0 dBi. Transmitting frequency is 400 MHz.
If transmitted power is expressed in decibels, PT = 10 mW so PT = 10 dBm
The received power received by the antenna is very low at –95 dBm, or 0.316 pW (picowatts)
You have no doubt heard the words multipath and fading, but what do they actually mean?
The radio waves emitted by the transmitter arrive at the receiver by a variety of paths, and at that time, the received field strength varies due to the effects of the different routes taken and differences in distance. This phenomenon is called fading. There are many kinds of fading depending on the causes, but a representative kind is multipath fading.
Multipath means that the radio waves reach the receiver by various paths, and the aggregate radio wave received by the antenna may experience interference and may fluctuate widely. If the signals are in phase, the field strength is high, but when they are out of phase, it gets weak. The wave length of microwaves is particularly short so the impact of multipath is especially acute.
There are a variety of causes of interference to transmissions in the space in which radio communication occurs.Some slight errors may occur with images and sound due to noise and the like, and the data may not reach the receiver correctly, but this is not a problem. In fact, we are not bothered by a slight ghost when we watch television, and with mobile phones it is not much of a problem if the sound is interrupted or distorted.
However, when we consider controlling machinery and the like using radio waves in the industrial sector, if control of the machinery is incapacitated due to data error caused by noise, a major accident could result. Or if we were transmitting an execution file to a computer, executing the application with that file containing an error of even 1 bit might cause the system to freeze with the loss of important internal data.
The causes of these issues are problems such as noise and interference. We will next look at a summary of the possible causes.
|Noise from the environment||Noise is emitted from the engines of trains and automobiles, from industrial equipment such as power lines and industrial plants, and also from consumer appliances.
Furthermore, microwave ovens and fluorescent lighting in the home create noise, and radio wave noise is emitted simply by operating power switches fixed to the wall.
|Noise from the natural world||There are radio disturbances on earth from atmospheric noise and sun flares, and electromagnetic noise caused by natural phenomena such as cosmic noise, movement of the earth's crust and so on.|
|Interference from other radio equipment||This is interference from radio equipment or appliances using the same frequency or a close frequency.
With narrowband radio equipment, only one piece of equipment can be used in the same area, at the same frequency, and at the same time.
|Noise from the device itself||When you come to design your radio equipment, there is a high possibility of errors occurring due to the proximity to noise emanating from around the power supply, the CPU, and from other components.|
|Causes due to the physical properties of radio waves||In the path of the radio waves between the transmitter and the receiver, there may be several kinds of obstacle such as mountains and buildings, walls, people and so on. If we consider an example involving buildings, there are direct waves that arrive directly, reflected waves that arrive after hitting buildings and the like, diffracted waves that circumvent the shadows of buildings, transmitted waves that arrive by passing through the glass or walls of buildings, and so on. These radio waves arrive after a longer delay than direct waves and reach the receiver with different phases. When these are combined, the received signal level becomes variously stronger or weaker, and this causes errors to occur. This phenomenon is called multipath fading. Many problems can be anticipated particularly when moving around with the radio equipment during telecontrol. But even with systems involving fixed equipment, the same issue may occur if there are moving obstacles such as cars and people in the vicinity, and if it happens to be located in the valley of the radio wave or 'dead spot', communication will be unreliable.|
With the spread of mobile phones, there is a lot of attention being given to the impact of radio waves on the human body and the effects on other equipment.
Naturally there is also concern from this point of view when designing radio equipment using radio modules.
|Direct impact of radio waves on the human body||Radio waves are electromagnetic waves, and the types of electromagnetic waves include ultraviolet light and the radio waves that are used in microwave ovens. The chemical action of ultraviolet light is very strong and it is used for the purpose of sterilization in fields related to foods. Also there are many reports of examples of skin cancer caused by absorbing large quantities of ultraviolet light from the sun. Microwave ovens make the water molecules in food vibrate using the energy of radio waves, to heat the food with this frictional heat. On the same principle, it may be thought that radio waves cause the same heating effect with regard to the human body.
But these cases involve extremely large amounts of energy. However, the power output of mobile phones is quite large (output power: about 1,000 mW), and it is said that using them close to your head is not good for you. Similarly, there are people who have reported various effects from proximity to high powered radio stations.
The radio waves that we are going to use with our modules are quite weak in comparison to the examples above (output power: about less than 10 mW). However, there are people who are concerned that absorbing even weak radio waves over a long period of time is not good for the body.
Unfortunately we cannot provide any clear answers to this problem. While there have been a range of reports on this issue, at the present time there is no definitive study report available. We believe that prudence is required concerning this issue, and that it is necessary for users to be aware of it when using the products.
|Effect on medical equipment and Pacemakers||Some people may be aware of the fact that the PHS (Personal Handy Phone) used by medical personnel inside hospitals is understood to have no impact on medical equipment.
The power output of PHS is 10 mW, and the power output of SRD (Short Range Device) used in the ISM band is also less than 10 mW, so it is thought not to have any impact, although unfortunately, we do not have any actual proof of this. Please carry out sufficient practical testing with regard to medical equipment before deciding on the propriety of implementing these products.
Inside hospitals, use of ordinary PHS and mobile phones is forbidden. (In Japan)
|Effect on other equipment||When entering an aircraft, you are required to turn off the power of mobile phones and electronic equipment. This is because it may affect the radio traffic between the electronic equipment of the aircraft and the control tower.
Although the frequencies used for control and the frequencies of mobile phones are different, radio waves are emitted from the mobile phone for the purpose of positioning even when it is not being used, and this has adverse effects on control. Furthermore, the electronic equipment such as computers and digital videos players built into mobile phones emit a wide range of RF noise and the combination of all of these has adverse effects.
This is not only a problem in aircraft; it is the same on the ground too. Electronic equipment must be used so as not to inconvenience others.
|Effect on equipment in the same waveband||Between radio equipment that uses a certain frequency, if communications from another piece of radio equipment on the same waveband interrupt communications, the original two way communication becomes impossible. For this reason, there is a procedure called carrier sensing that senses whether or not the frequency channel to be used is being used by other equipment before starting transmission. In Japan carrier sensing is laid down in the Wireless Telegraphy Act as an essential function that equipment must have, although there is no similar regulation as such in Europe.|