|
RF
design guide What
are radio waves?
|
|
<< RF design guide Top |
<<BACK
NEXT>> |
|
|
|
Here
we will look at the characteristics of radio waves as far as is required
for the design of radio communication equipment. We will explain the
methods of
processing
radio
waves and
the
thinking
behind it,
and gain an
perspective on the
subject.
|
|
|
|
|
|
People
and radio waves
<Menu>
|
|
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. |
Classification
of radio waves <Menu>
|
|
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
below 3kHz
|
radio waves
3kHz~
3THz |
infrared rays
3THz~
380THz |
visible light
380THz~
790THz |
ultraviolet light
790THz~
105THz |
X rays
105THz~
107THz |
γ
rays
above 107THz |
*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 |
|
|
|
What
are the physical properties of radio waves?
<Menu>
|
|
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 1.3 seconds.
|
The generation of radio waves
<Menu>
|
|
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.
|
|
 |
Longitudinal
waves and transverse waves
<Menu>
|
|
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
qualities of radio waves
<Menu>
|
|
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.
|
The
speed and the wavelength of radio waves
<Menu>
|
|
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
|
|
< Click to move to the calculation
window >
There
is a close relationship between the
wave length of the radio wave and
the
characteristics of the antenna.
|
Electric field intensity at an arbitrary location
<Menu>
|
|
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. |
|
 |
|
|
Electric field intensity and received power
<Menu>
|
|
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.
|
|
|
|
 |
|
< Click to move to the calculation
window > |
|
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.
|
|
|
|
 |
|
< Click to move to the calculation
window >
|
Propagation
loss
<Menu>
|
|
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. |
|
|
|
|
|
 |
Example:
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.
|
|
 |
|
< Click to move to the calculation
window >
|
|
(1) What happens to received power?
|
|
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]
|
|
(2) 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.
Example:
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) |
|
 |
|
< Click to move to the calculation
window >
|
Fading
<Menu>
|
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.
|
|
|
|
Why
do communication errors occur?
<Menu>
|
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. |
|
|
|
|
Is
there any impact on the human body or other equipment?
<Menu>
|
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. |
|
|
|
|
<< RF design guide Top |
<MENU> |
<<BACK
NEXT>> |
|
