Dielectric and Thermal Properties
of Materials at Microwave Frequencies
Answers to RF Cafe Quiz #45
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This quiz is based on the information presented in Handbook of Dielectric and Thermal Properties of Materials at Microwave Frequencies, by Theodore Anderson. This book is a wealth of information of dielectric characteristics of biological tissues in plants, animals, as well as for ceramics, soils and minerals. Particular attention is paid to properties in the ISM frequency bands. Extensive sources for similar studies are provided.
Also consider: USAFSAM-TR-85-73
Radiofrequency Radiation Dosimetry Handbook (4th Edition)
This is the mother of all such references for SAR studies.
1. What is a serious reason for uneven distribution of the thermal field in lossy media?
c) Thermal runaway
"Variations of dielectric and thermal properties of materials versus temperature impact the uniformity of microwave heating. One of the serious reasons for uneven distribution of the thermal field in lossy media is known as the thermal runaway phenomenon." (page 6)
2. What is the mechanism for generating heat in dielectric materials via microwaves?
a) Dipole polarization where polar molecules try to align themselves with the applied field
"Most microwave technologies... are intended for thermal processing of diverse dielectric materials such as food, wood, ceramics, polymers, and so forth. All these materials absorb microwave energy due to the dipole polarization phenomenon when their molecules try to align themselves to the external electromagnetic field." (page 13)
3. What name is given to a resonant cavity method for measuring complex dielectric permittivity of lossy materials?
d) Perturbation method
"Resonant cavity methods are also widely utilized in measuring complex dielectric permittivity of lossy materials. The most popular resonant cavity method is the perturbation method (PM)." (page 20)
4. What does heat capacity of a material measure?
a) Ability to accumulate heat by increasing its temperature
"Heat capacity characterizes the ability of media to accumulate heat by increasing its temperature." (page 23)
5. What are the main mechanisms for interaction between EM waves and biological (e.g., brain) tissue?
a) Oscillation of polar water molecules and ions
"The main mechanism for the interaction between EM waves and biological tissue is the same as in foodstuff: oscillation of polar water molecules and ions." (page 69)
6. Which list represents materials ordered from least to most polar high frequency dielectric properties.
c) Polyethylene, rubber, PVC
"According to the classification proposed in [reference], resins and plastics are divided into three main groups: (1) no polar high-frequency (HF) dielectrics including polyethylene, polypropylene, polyester, and others, (2) weak polar and polar HF and low frequency (LF) dielectrics including paraformaldehyde, rubbers, polybutadiene, and so forth, and (3) polar LF dielectrics such as polyamide, epoxide, and polyvinylchloride." (page 91)
7. Which property is very important in affecting temperature distribution in liquids heated by microwaves?
"Viscosity is a very important parameter that influences temperature distribution when liquid chemical substances are heated with microwave radiation. (page 127)
8. Which body component has the highest conductivity in the 915 MHz ISM band?
|Selected values from Table 4.5|
a) Cerebrospinal fluid
Conductivity of cerebrospinal fluid is 2.4187 S/m. See Table 4.5 (page 73)
9. Which body component has the lowest conductivity in the 915 MHz ISM band?
c) Bone marrow
Conductivity of bone marrow is 0.0406 S/m. See Table 4.5 (page 73)
10. What typically happens to conductivity of body tissues as frequency across the ISM bands is increased?
a) It increases
Per measured data in Table 4.2, most, if not all, tissues increase in conductivity and decrease in permittivity as frequency is progress from 433 MHz, to 915 MHz, to 2,450 MHZ, to 5,800 MHz.