3.4. CONCEPTS OF MEASUREMENTS



Three kinds of electromagnetic measurement techniques are of primary interest: the electric field, the magnetic field, and the SAR. The basic concepts underlying these measurement techniques are discussed in this section. More detailed information is given in Chapter 7.

3.4.1. Electric-Field Measurements

Devices for measuring an E-field usually consist of two main components: a small antenna or other pickup device that is sensitive to the presence of an E-field, and a detector that converts the signal to a form that can be registered on a readout device such as a meter. The pickup is typically a short dipole. The dipole can be two short pieces of thin wire (Figure 3.41(a)) or two short strips of thin metal as on a printed circuit (Figure 3.41(b)). Sometimes the dipole is flared out to look like a bow tie (Figure 3.41(c)) to improve the bandwidth of the dipole.

Figure 3.41.
Short dipole used to sense the presence of an electric field.

The detector is usually a diode or a thermal sensor. A diode rectifies the signal so that it can register on a dc meter. A thermal sensor responds to heat produced in some lossy material that absorbs energy from the E-field. The heat produces a voltage or current that can be registered on a meter. An example of a thermal sensor is a thermocouple, which consists of two junctions of dissimilar metals. The two junctions produce a voltage proportional to the temperature difference between them.
Leads are required to transmit the voltage or current from the detector to the meter or other readout devices, as illustrated in Figure 3.42. The leads often cause problems because they themselves can be sensitive to the presence of an E-field and may produce erroneous readings through unwanted E-field pickup. To overcome this problem, high-resistance leads are often used in E-field probes. The sensitivity of the pickup element is roughly proportional to its length compared to a wavelength of the E-field to be measured. At low frequencies, where the wavelength is very long, short elements are sometimes not sensitive enough; however, if the element is too long it may perturb the field to be measured. To avoid field perturbation, the element should be short compared to a wavelength; thus the tradeoff between sensitivity and perturbation is difficult.

Figure 3.42.
Simple electric-field probe with a diode detector.

The dipole element is sensitive only to the E-field component parallel to the dipole; an E-field perpendicular to the dipole will not be sensed. This can be understood in terms of the force that the E-field exerts on the charges in the dipole, for that is the basic mechanism by which the dipole senses the E-field. An E-field parallel to the dipole produces forces on charges that tend to make them move along the dipole from end to end, which amounts to a current in the dipole. An E-field perpendicular to the dipole, however, tries to force the charges out through the walls of the dipole, which produces essentially no current useful for sensing the E-field. In practice, three orthogonal dipoles are often used, one to sense the E-field component in each direction. By electronic circuitry, each component is then squared and the results are added to get the magnitude of the E-field vector.
Although commercial instruments for measuring E-field are based on the simple concepts described here, they are very sophisticated in their design and fabrication. Some of them are described in Chapter 7.

3.4.2. Magnetic-Field Measurements

Devices for measuring B-field also consist of two basic components, the pickup and the detector. For the B-field the pickup is usually some kind of loop, as shown in Figure 3.43. The loop is sensitive only to the B-field component perpendicular to the plane of the loop, as indicated. A time varying B-field produces a voltage in the loop that is proportional to the loop's area and the rapidity (frequency) of the B-field's time variation. Thus at low frequencies the loop must be large to be sensitive to weak fields. As with the E-field probe, making the probe large to improve the sensitivity yet small enough to minimize the perturbation of the field being measured requires a tradeoff.

Figure 3.43.
Loop antenna used as a pickup for measuring magnetic field.

Diode detectors are commonly used with B-field probes, although some thermal sensors have been used. Leads can also cause unwanted pickup of fields in B-field measurements. Another problem with the loop sensors is that they may be sensitive to E- as well as B-field. Special techniques have been used to minimize the E-field pickup in loops used with commercial B-field probes. Some of the available commercial B-field probes are described in Chapter 7.

3.4.3. SAR Measurements

Usually only research laboratories make SAR measurements because they are relatively difficult and require specialized equipment and conditions (see Chapter 7). Three basic techniques are used for measuring SARs. One is to measure the E-field inside the body, using implantable E-field probes, and then to calculate the SAR from Equation 3.49; this requires knowing the conductivity of the material. This technique is suitable for measuring the SAR only at specific points in an experimental animal. Even in models using tissue-equivalent synthetic material, measuring the internal E-field at more than a few points is often not practical.
A second basic technique for measuring SAR is to measure the temperature change due to the heat produced by the radiation, and then to calculate the SAR from that. Probes inserted into experimental animals or models can measure local temperatures, and then the SAR at a given point can be calculated from the temperature rise. Such calculation is easy if the temperature rise is linear with time; that is, the irradiating fields are intense enough so that heat transfer within and out of the body has but negligible influence on the temperature rise. Generating fields intense enough is sometimes difficult. If the temperature rise is not linear with time, calculation of the SAR from temperature rise must include heat transfer and is thus much more difficult. Another problem is that the temperature probe sometimes perturbs the internal E-field patterns, thus producing artifacts in the measurements. This problem has led to the development of temperature probes using optical fibers or high-resistance leads instead of ordinary wire leads.
A third technique is to calculate absorbed power as the difference between incident power and scattered power in a radiation chamber. This is called the differential power method (see Section 7.2.5).
Whole-body (average) SAR in small animals and small models can be calculated from the total heat absorbed, as measured with whole-body calorimeters. Whole-body SARs have also been determined in saline-filled models by shaking them after irradiation to distribute the heat and then measuring the average temperature rise of the saline.



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Last modified: June 24, 1997
October 1986, USAF School of Aerospace Medicine, Aerospace Medical Division (AFSC), Brooks Air Force Base, TX 78235-5301