THE ELECTROMAGNETIC SPECTRUM
To start with, to know what a spectrum is: when white light is shone through a prism it is separated out into all the colours of the rainbow; this is the visible spectrum. So white light is a mixture of all colours. Black is NOT a colour; it is what you get when all the light is taken away.
Some physicists pretend that light consists of tiny particles which they call
photons. They travel at the speed of light (what a surprise). The speed of light is about
300,000,000 meters per second. When they hit something they might bounce off, go right
through or get absorbed. What happens depends a bit on how much energy they have. If
they bounce off something and then go into your eye you will "see" the thing they have
bounced off. Some things like glass and Perspex will let them go through; these materials
are transparent. Black objects absorb the photons so you should not be able to see black
things: you will have to think about this one. These poor old physicists get a little bit
confused when they try to explain why some photons go through a leaf, some are
reflected, and some are absorbed. They say that it is because they have different amounts
Other physicists pretend that light is made of waves. These physicists measure the length of the waves and this helps them to explain what happens when light hits leaves. The light with the longest wavelength (red) is absorbed by the green stuff (chlorophyll) in the leaves. So is the light with the shortest wavelength (blue). In between these two colours there is green light, this is allowed to pass right through or is reflected. (Indigo and violet have shorter wavelengths than blue light.)
Well it is easy to explain some of the properties of light by pretending that it is made of tiny particles called photons and it is easy to explain other properties of light by pretending that it is some kind of wave.
The visible spectrum is just one small part of the electromagnetic spectrum. These electromagnetic waves are made up of two parts. The first part is an electric field. The second part is a magnetic field. So that is why they are called electromagnetic waves. The two fields are at right angles to each other.
THE MICROWAVE REGION
Microwave wavelengths range from approximately one millimeter (the thickness
of a pencil lead) to thirty centimeters (about twelve inches). In a microwave oven, the
radio waves generated are tuned to frequencies that can be absorbed by the food. The
food absorbs the energy and gets warmer. The dish holding the food doesn't absorb a
significant amount of energy and stays much cooler. Microwaves are emitted from the
Earth, from objects such as cars and planes, and from the atmosphere. These microwaves
can be detected to give information, such as the temperature of the object that emitted the
Microwaves have wavelengths that can be measured in centimeters! The longer
microwaves, those closer to a foot in length, are the waves which heat our food in a
microwave oven. Microwaves are good for transmitting information from one place to
another because microwave energy can penetrate haze, light rain and snow, clouds, and
Shorter microwaves are used in remote sensing. These microwaves are used for radar like the Doppler radar used in weather forecasts. Microwaves, used for radar, are just a few inches long. Because microwaves can penetrate haze, light rain and snow, clouds and smoke, these waves are good for viewing the Earth from space
Microwave waves are used in the communication industry and in the kitchen as a way to cook foods. Microwave radiation is still associated with energy levels that are usually considered harmless except for people with pace makers.
Here we are going to use the S band of the Microwave Spectrum.
Microwave frequency bands
Designation Frequency range
L Band 1 to 2 GHz
S Band 2 to 4 GHz
C Band 4 to 8 GHz
X Band 8 to 12 GHz
Ku Band 12 to 18 GHz
K Band 18 to 26 GHz
Ka Band 26 to 40 GHz
Q Band 30 to 50 GHz
U Band 40 to 60 GHz
V Band 46 to 56 GHz
W Band 56 to 100 GHz
The frequency selection is another important aspect in transmission. Here we have selected the license free 2.45 GHz ISM band for our purpose.
The Industrial, Scientific and Medical (ISM) radio bands were originally reserved internationally for non-commercial use of RF electromagnetic fields for industrial, scientific and medical purposes.
The ISM bands are defined by the ITU-T in S5.138 and S5.150 of the Radio Regulations. Individual countries use of the bands designated in these sections may differ due to variations in national radio regulations.
In recent years they have also been used for license-free error-tolerant communications applications such as wireless LANs and Bluetooth:
900 MHz band (33.3 cm) ( also GSM communication in India )
2.45 GHz band (12.2 cm)
IEEE 802.11b wireless Ethernet also operates on the 2.45 GHz band
The MAGNETRON (A), is a self-contained microwave oscillator that operates differently from the linear-beam tubes, such as the TWT and the klystron. View (B) is a simplified drawing of the magnetron. CROSSED-ELECTRON and MAGNETIC fields are used in the magnetron to produce the high-power output required in radar and communications equipment.
The magnetron is classed as a diode because it has no grid. A magnetic field located in the space between the plate (anode) and the cathode serves as a grid. The plate of a magnetron does not have the same physical appearance as the plate of an ordinary electron tube. Since conventional inductive-capacitive (LC) networks become impractical at microwave frequencies, the plate is fabricated into a cylindrical copper block containing resonant cavities that serve as tuned circuits. The magnetron base differs considerably from the conventional tube base. The magnetron base is short in length and has large diameter leads that are carefully sealed into the tube and shielded.
The cathode and filament are at the center of the tube and are supported y
the filament leads. The filament leads are large and rigid enough to keep the
cathode and filament structure fixed in position. The output lead is usually a probe
or loop extending into one of the tuned cavities and coupled into a waveguide or
coaxial line. The plate structure, shown in figure 2 -18, is a solid block of copper.
The cylindrical holes around its circumference are resonant cavities. A narrow
slot runs from each cavity into the central portion of the tube dividing the inner
structure into as many segments as there are cavities. Alternate segments are
strapped together to put the cavities in parallel with regard to the output. The
cavities control the output frequency. The straps are circular, metal bands that are
placed across the top of the block at the entrance slots to the cavities. Since the
cathode must operate at high power, it must be fairly large and must also be able
to withstand high operating temperatures. It must also have good emission
characteristics, particularly under return bombardment by the electrons. This is
because most of the output power is provided by the large number of electrons
that are emitted when high-velocity electrons return to strike the cathode. The
cathode is indirectly heated and is constructed of a high-emission material. The
open space between the plate and the cathode is called the INTERACTION
SPACE. In this space the electric and magnetic fields interact to exert force upon
The basic addition to the mobile phone is going to be the rectenna.
A rectenna is a rectifying antenna, a special type of antenna that is used to directly convert microwave energy into DC electricity. Its elements are usually arranged in a mesh pattern, giving it a distinct appearance from most antennae.A simple rectenna can be constructed from a schottky diode placed between antenna dipoles. The diode rectifies the current induced in the antenna by the microwaves.
Rectennae are highly efficient at converting microwave energy to electricity. In laboratory environments, efficiencies above 90% have been observed with regularity. Some experimentation has been done with inverse rectennae, converting electricity into microwave energy, but efficiencies are much lower--only in the area of 1%.
With the advent of nanotechnology and MEMS the size of these devices can be
brought down to molecular level. It has been theorized that similar devices, scaled down
to the proportions used in nanotechnology, could be used to convert light into electricity
at much greater efficiencies than what is currently possible with solar cells. This type of
device is called an optical rectenna. Theoretically, high efficiencies can be maintained as
the device shrinks, but experiments funded by the United States National Renewable
Energy Laboratory have so far only obtained roughly 1% efficiency while using infrared
Another important part of our receiver circuitry is a simple sensor. This is simply used to identify when the mobile phone user is talking. As our main objective is to charge the mobile phone with the transmitted microwave after rectifying it by the rectenna, the sensor plays an important role.
The whole setup looks something like this.
THE PROCESS OF RECTIFICATION
Studies on various microwave power rectifier configurations show that a bridge
configuration is better than a single diode one. But the dimensions and the cost of that
kind of solution do not meet our objective. This study consists in designing and
simulating a single diode power rectifier “in hybrid technology” with improved
sensitivity at low power levels. We achieved good matching between simulation results
and measurements thanks to the optimisation of the packaging of the Schottky diode.
Microwave energy transmitted from space to earth apparently has the potential to provide environmentally clean electric power on a very large scale. The key to improve transmission efficiency is the rectifying circuit. The aim of this study is to make a low cost power rectifier for low and high power levels at a frequency of 2.45 GHz with good efficiency of rectifying operation. The objective also is to increase the detection sensitivity at low levels of power.
Different configurations can be used to convert the electromagnetic wave into DC signal, the study done in showed that the use of a bridge is better than a single diode, but the purpose of this study is to achieve a low cost microwave rectifier with single Schottky diode for low and high power levels that has a good performances.
This study is divided on two kind of technologies the first is the hybrid technology and the second is the monolithic one.
The goal of this investigation is the development of a hybrid microwave rectifier with single Schottky diode. The first study of this circuit is based on the optimization of the rectifier in order to have a good matching of the input impedance at the desired frequency 2.45GHz. Besides, the aim of the second study is the increasing of the detection sensitivity at low levels of power.
The sensor circuitry is a simple circuit, which detects if the mobile phone receives any message signal. This is required, as the phone has to be charged as long as the user is talking. Thus a simple F to V converter would serve our purpose. In India the operating frequency of the mobile phone operators is generally 900MHz or 1800MHz for the GSM system for mobile communication. Thus the usage of simple F to V converters would act as switches to trigger the rectenna circuit to on.
A simple yet powerful F to V converter is LM2907. Using LM2907 would greatly serve our purpose. It acts as a switch for triggering the rectenna circuitry. The general block diagram for the LM2907 is given below.
Thus on the reception of the signal the sensor circuitry directs the rectenna circuit to ON and the mobile phone begins to charge using the microwave power.
Thus this paper successfully demonstrates a novel method of using the power of the microwave to charge the mobile phones without the use of wired chargers. Thus this method provides great advantage to the mobile phone users to carry their phones anywhere even if the place is devoid of facilities for charging. A novel use of the rectenna and a sensor in a mobile phone could provide a new dimension in the revelation of mobile phone.
Tae-Whan yoo and Kai Chang, " Theoretical and Experimental Development of
10 and 35 GHz rectennas" IEEE Transaction on microwave Theory and
Techniques, vol. 40.NO.6.June.1992.
5 Hawkins, Joe, et al, "Wireless Space Power Experiment," in Proceedings of the
9th summer Conference of NASA/USRA Advanced Design Program and
Advanced Space Design Program, June 14-18, 1993.
MW Medley Jr and MW Medley, 'Microwave and RF circuits: analysis, synthesis,
and design', Artech House, Norwood, MA, 1993.
Falcone, Vincent J., "Atmospheric Attenuation of Microwave Power," Journal of
microwave Power, 5(4), 1970.