MICROWAVE & RADAR ENGINEERING
Roll no: 32?
1. Medical and civil applications of microwave frequency.
There are a variety of unique applications of the microwave in the areas of basic science, remote sensing, heating methods, and medical diagnostics and treatment mostly because various molecular, atomic, and nuclear resonances occur at these microwave frequencies.
We may divide medical applications of microwaves into three basic groups as per the purpose or on how they are used:
i) Used for treatment using thermal or non-thermal or sometimes both of the effects of microwave energy and/or technique.
ii) Used for diagnostics of diseases, for instance by the aid of permittivity measurements, attenuation measurements etc.
iii) Used as a part of a treatment or diagnostic system.
Medical applications of microwaves mostly are the treatment methods, called microwave thermotherapy, based on its thermal effects. It can further be divided into three different modalities as per the goal temperature level or interval:
• diathermia: heating up to 41 C
• hyperthermia: heating in the range of 41-45 C
• thermo-destruction / thermo-ablation: over 45 C
Applications of these can be found in physiotherapy (treatment of rheumatic diseases), oncology (cancer treatment), urology (BPH treatment), cardiology (heart stimulations), surgery, growing implants, ophthalmology (retina corrections). E.g. thermotherapy is usually used in cancer treatment, in the combination with some of the other modalities used in the clinical oncology. Microwave thermotherapy of BPH and hyperthermia for cancer treatment can be termed as the most important applications of microwaves in medicine.
Medical applications of the microwave, based on so-called non-thermal effects are also being researched. It is said to be used to reduce pain feeling (analogically to analgesics), to increase human being’s reaction capabilities, etc.
For the treatment of patients with superficial or subcutaneous tumor, applicators that work at 434 MHz and at 2450 MHz are used.
Typical range of 100 MHz -30 GHz frequencies are used in
• Diagnostic applications like that of tumor detection based on differences in tissue electrical properties, regional hyperthermia integrated with MRI
• Therapeutic applications based on local heating which can prostate hyperplasia, heart and other tissue ablation, angioplasty.
• MRI (; fMRI), radiometry, telemetry, motion detection are other application not yet reviewed.
The soft tissues in the human body have a substantial difference in their dielectric properties. The dielectric properties of malign or cancerous tissues are thus different from the healthy tissues. This principle is being utilized to make microwave imaging, one of the main applications of the microwave frequencies in the medical field. There are two approaches to creating microwave images
• Radar-based technology
The detection and imaging of a malignant tumor can be done through a tomography based microwave system and signal processing. The electrical property distributions in a human body are being efficiently utilized, enabling medical applications of microwave images. The internal structure of an object will be observed by means of electromagnetic fields at microwave frequencies of range 300?MHz–30?GHz. The detection of the tumor is then performed by the use of feasible signal processing techniques. The signal processing is done through the extraction of tumor information from background information and then reconstructing the image using certain algorithms. The imaging process is said to be rapid, sensitive, and specific i.e., it detects most tumors in the breast and detects the cancerous tumors only. It is anticipated to offer a comfortable and safer examination procedure to the patients.
Ultra-wideband (UWB) microwave imaging can be used to detect early-stage breast cancer by analyzing the large contrast in electrical parameters between malignant tumor tissue and the surrounding normal breast tissue. Breast cancer can be detected using microwave imaging. It is based on the contrast in electrical properties of cancerous tissues and the normal tissues. The transmitter placed at one side of the breast, give out the microwave energy which travels through it. This is detected at receivers, located on the other side of the breast. The reflections at the transmitter may be recorded, simultaneously. The incident microwaves scatter as it experiences a change in material dielectric property when it travels through the tumor. The energy detected at the receivers and the transmitter is different due to this scattering. This information is then used to form the images.
A 3D model will be developed on an electromagnetic simulation tool using the fields formed by the scattering of the UWB microwave signal in the breast, containing a malignant tumor with characteristic electrical properties of tissues. The scattered electromagnetic field distribution in the breast is thus obtained from the simulation. Suitable signal processing techniques are used to reconstruct the breast image. This involves the confocal method by delay and sum algorithms. The waves relating to the tumor can thus be identified.
The photon energy of microwave is sufficiently low making it safer compared to X-rays that are used conventionally for this purpose. However, the dielectric properties of a number of biological tissues, both healthy and malign need to be accurately determined so as to make use of the full potential of this research field. Only then can the contrast between dielectric properties of two types of tissues be found.
RF power of typical frequency range from 500 MHz to 40 GHz, depending on the type of treatment, are increasingly developed as an energy source in medical equipment used in treatments such as RF ablation, which can also be used to treat some heart disorders as well as some beauty treatments. Power levels can typically vary from 0 to +50 dBm. A 1300-W NXP RF amplifier operating at 2.45 GHz is an example of a part used for ablation.
Tissue motions, for instance, in the human vocal tract during voiced speech, can be measured by using a low power, radar-like EM wave sensors. It operates in a homodyne interferometric mode. Desiccation of tissue can be done by using microwave ablation, without the excessive charring and nerve damage associated with RF ablation. Treatment of large tumors or removal of unwanted tissue masses are the various applications of the same.
Small wavelength of a microwave has made it suitable for various applications.
• Large volume ablations can be made at typical frequencies 915 MHz and 2.45 GHz.
• Higher frequencies in the range 5.8 GHz – 10 GHz can be used to create shallow penetration of energy, for precise ablations which are suitable for treatments like skin cancer, ablation of the heart to treat arrhythmia, corneal ablation (vision correction), spinal nerve ablation (back pain), uterine fibroids, varicose vein treatment, verrucae treatment, multiple small liver metastases and many other specific treatments.
• Microwaves can even be used to coagulate bleeding especially, in highly vascular organs such as the liver and spleen.
• MW ablation, in the field of oncology, can be used as a tool to fight cancer, providing new opportunities to save many lives.
The microwave techniques find application in civil engineering as well. It is being used in a number of areas of civil engineering applications:
• Remote sensing – weather observation, planetary observation, below ground probing
• Geophysical prospecting
• Archeological applications
• Air traffic control – used to control air traffic and in the mapping of rain in the vicinity of airports and weather
• Law enforcement and highway safety – Radar speed meters used by police to enforce speed limits
• Nondestructive evaluation and testing
The various objects in the underground and the concrete structure have different dielectric properties. These can thus be differentiated and this, in fact, the basic idea behind the microwave nondestructive testing and remote sensing.
The microwave moisture-measurement (aquametry), uses the microwave or high-frequency electromagnetic signals for the measurement of moisture as well as in non-destructive testing and evaluation (NDT;E), mainly for concrete. A non-contact sensor for concrete (NSC) is designed to be applied in concrete mixers above the moving mix, using a cross-polarized, active back-scatter. It is based on microwave double transmission-reflection type-free-space-two parameter complex vector measurement. A new microwave NDT-method and instrument (sensor for timber, SFT) are also developed for measurement of the moisture content of timber for real-time. This timber is intended to be used as fuel in wood-fired power plants, renewable energy source. For direct and inverse modeling, corresponding to the type of the firewood, the moisture content in it, microwave attenuation and phase shift, etc, the problem-specific software was developed. A proposal will be given to the multi-frequency, free-space measurement setup, after proper validation.
2. The phenomenon of tunneling, tunnel diode, its potential impact in microwave applications.
Tunneling is the phenomenon of the passage of minute particles through force barriers that seem impassable. It is hence, also called barrier penetration. The phenomenon was first found in case of alpha decay, in which alpha escape from atomic nuclei of certain radioactive elements. The nuclear constituents are held together by a force that the alpha particles cannot overcome. The tunneling of these particles is hence contrary to conventional physics and hence requires explanation in terms of quantum mechanics.
On the quantum mechanics basis, the particles of subatomic size cross the barriers even though they have energies, too small to carry them over, according to conventional physics. Such particles follow undulations of a quantum-mechanical (de Broglie) wave. It accumulates wherever the wave grows and thins out wherever it diminishes. Thus, the alpha particles, which seem to be tunneling through an impenetrable barrier, really penetrate it as if it is a natural consequence owing to their wave properties. In electronics, tunneling is defined as a direct flow of electrons crossing the small depletion region from the conduction band on the n-side into the valence band on the p-side. The direct images of the atomic structure of surfaces are created by a scanning tunneling microscope using this tunneling of electrons.
Tunnel diodes can be termed as one of the most significant solid-state electronic devices. It is a highly doped semiconductor device that works on the principle of Tunneling effect. Leo Esaki discovered the electron tunneling effect used in these diodes and received the Nobel Prize in Physics in 1973 for the same. The diode is hence also called as Esaki diode.
The tunnel diode, as shown in the figure, is a two terminal device with p-type semiconductor acting as anode and the n-type as the cathode. The material usually used to make tunnel diodes is Germanium. The gallium arsenide and silicon materials can be used as well.
The Tunnel diode is heavily doped with impurities and hence it exhibits negative resistance in their operating range, i.e., the current flowing through the diode decreases as the voltage is increased, and hence used as an amplifier, oscillators and in any switching circuits. The diode is capable of very fast operations and is used in frequency detectors and converters. It is used mainly for low voltage high-frequency switching applications.
Unlike the p-n junction diode, the difference in the energy levels of the valence band and conduction band is very high in tunnel diode. Because of this, the conduction band of the n-type overlaps the valence band of the p-type material.
The depletion layer of tunnel diode is very small, in the order of nanometers and hence the electrons can penetrate directly through the barrier from n-side conduction band into the p-side valence band.
Unlike ordinary diodes, tunnel diodes only need a small voltage, less than the built-in voltage of depletion region, to produce an electric current as the electrons don’t have to overcome the opposing force from the depletion layer to produce it.
Working of tunnel diode:
Unbiased tunnel diode
When a small voltage applied to the tunnel diode:
When the applied voltage is slightly increased
When the applied voltage is further increased
When the applied voltage is largely increased
The characteristics of Tunnel diode and normal P-N junction diode are different from each other.
Under a forward bias condition, as voltage increases, the current decreases due to negative resistance. This is the most important operating region for a Tunnel diode. Further increase in the voltage will make the tunneling diode operate as a normal diode where the conduction of electrons travels across the P-N junction diode.
Under the reverse condition, the diode will act as a back diode or backward diode and can act as a fast rectifier with the offset voltage, zero. Tunnel diode can acts as an excellent conductor because of its high doping concentrations.
The discovery of tunneling diodes has had an impact on the application of microwave frequencies as well. The tunnel diodes are useful in many circuit applications in microwave amplification, microwave oscillation as the diode exhibits a negative-resistance characteristic in the region between peak current Ip and valley current Iv and also because of their lightweight, low-power operation, high speed, low noise, low cost, and high peak-current to valley-current ratio.
I-V characteristic along with the load lines
There are three points a, band c where each of the load lines intersects the characteristic curve. Points a and c are stable points, and the point b is unstable. If the voltage and current vary about b, the final values of I and V would be given by point a or c, but not by b. Since the tunnel diode has two stable states for this load line, the circuit is called bistable, and it can be utilized as a binary device in switching circuits. The second load line intersects the I-V curve only at point b, which is stable and shows a dynamic negative conductance that enables the tunnel diode to function as a microwave amplifier or oscillator.
1 Microwave Devices and Circuits SAMUEL LIAO