车厢内移动电话电磁场

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Electromagnetic Field of a Mobile Phone in a Car

A transmitting mobile phone is a strong electromagnetic field source in the radio-frequency (RF) spectrum. A controversy is associated with the maximal permissible field levels that can be considered as safe for humans. For a transmitter operating at 900 MHz, standards in various countries worldwide typically permit an electric field in the range 20-50 V/m (corresponding to the power density levels of about 1-6 W/m2), though there are some standards (e.g., in Russia) that are much more restrictive. The permissible levels depend on the exposure time, the part of the human body exposed to the electromagnetic field, and other conditions. The rational for the quoted standards is that the only well-established effect on the human body is heating of the tissues. However, there are also claims that the RF electromagnetic field, although classified as a non-ionizing radiation, may cause other effects. To add to the controversy, the maximal permissible electric-field levels for standard electronic equipment are in the range 1-5 V/m, which is much lower than for humans.

We do not have an intention of disputing the question of the maximal permissible field levels. Rather, the objective of this study is to demonstrate the advantage of car-mounted antennas in reducing the electromagnetic field to which a person is exposed when using a mobile phone in a car.

The average power radiated by a mobile phone depends on the system (analog or digital) and the level to which the transmitter is set to provide an adequate quality of the link to the base station. Due to this, the results presented here should be interpreted in terms of relative levels (ratios), not as absolute field levels.

A mobile phone is used in a car in two typical technical arrangements. The first one (prohibited by law in many countries, due to traffic safety reasons) is to hold the phone in a hand. Thereby, the antenna built into (or onto) the mobile phone is used. This antenna is located very close to the operator's head (within less than 50 mm). The second arrangement is to use a hands-free disposition, where a car antenna is used that is typically located on the car roof or on the windshield.

If the antenna of a mobile phone is located outside a car, it provides a better transmission and reception then if the antenna is located within a car. This is due to a partial shielding of the electromagnetic field by the metallic car body. This shielding strongly depends on the position of the antenna within the car, in particular, the location with respect to the window through which the radio wave propagates. The measured decrease of the field level due to the shielding is typically in the range 5-10 dB. Hence, the transmitter of a mobile phone may switch to a higher power level when the antenna is located in the car, i.e., when the mobile phone is held next to the operator's head, than if the an external antenna is used. However, in this study, we shall not take into account differences in the power level. We shall assume a transmitter power level of 1 W. To stress again, this power level is arbitrary, and it should be considered only as a normalization level.

A mobile phone can be approximately modeled by a resonant (half-wave) dipole. The field in the meridian plane of the dipole (excluding the influence of nearby objects) decays with the distance from the antenna (x). At a sufficient distance from the antenna (in the so-called far-field region) this decay is as 1/x. The computed electric field (using program AWAS, published by the Artech House), as a function of the distance from the antenna (x), is shown below. Note logarithmic scales for both the field intensity and the distance.[/td][/tr]
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[tr][td]This study presents some measured and computed results for the electric field levels. The electric field was measured using a half-wave dipole as a probe. The signal induced in this dipole is practically proportional to the intensity of the electric field in which the dipole is located. To illustrate the behavior of the measurement equipment, shown below is the transfer function (the transfer scattering parameter) between two identical parallel dipoles (the Dynaflex antennas 586), as a function of the distance between them. This situation corresponds to measuring the electric field of a mobile phone (simulated by one dipole) as a function of the distance (where the other dipole acts like a field probe).[/td][/tr]
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The shapes of the curves for the electric field and for the scattering parameters are similar, giving a decrease by a factor of about 10 (i.e., 20 dB) when the distance is increased from 50 mm to 1000 mm.

Suppose one person in the car uses a hand-held mobile phone. The above curves can be used to estimate the field level to which other persons in the car are exposed, taking into account that the distance in a typical passenger car is up to about 1000 mm. For example, if the driver uses a hand-held phone, the signal level measured at 50 mm would correspond to his head (-15 dB), and the signal measured at 500 mm (-30 dB) would correspond to the average position of other passengers in the car.

Note, however, that a car is a large metallic structure. Within the car cabin, the electromagnetic field exhibits the character of a standing wave, due to the field reflections from the roof, floor, and walls. The standing wave means that the field has pronounced minima and maxima. Measured field levels at these extreme points differ typically for about 10-20 dB (3-10 times). The distance between a minimum and an adjacent maximum is relatively small, about 80 mm (quarter wavelength). Hence, the levels given in this report are averaged to cancel out the influence of the standing waves.

The first example of a car-mounted antenna is a 586, located on the front windshield, as indicated below.

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The measured levels (at the height of persons' head and shoulders) are:

· -30 dB at the driver's seat (front-left seat);

· -35 dB at the front-right seat;

· -40 dB at the back seats.

Assuming the driver using a hand-held mobile phone, the glass-mounted antenna reduces the field level at the driver's head for about 15 dB (about 5 times), and at the heads of other passengers the field reduction is much smaller, of the order of 5-10 dB (about 2-3 times).

The second example is a Glassflex, mounted on the rear windshield, as indicated below.[/td][/tr]
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The measured levels are:

· -45 dB at the front seats;

· -35 dB at the back seats;

· -30 dB at the top of the rear windshield, near the antenna.

Assuming the driver using a hand-held mobile phone, this arrangement provides a field reduction of about 30 dB (30 times) for the driver, and a small reduction of about 5 dB (about 2 times) for other passengers.

The glass-mounted antennas provide a field reduction primarily due to the increased distance to the persons' heads (in particular, the distance between the antenna and the operator's head).

The third example of a car-mounted antenna is a Challenger or a Satflex, located on the car roof, 100 mm from the back windshield, as indicated below.[/td][/tr]
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The measured levels are:

· -50 dB at the front seats;

· -40 dB at the back seats;

· -30 dB at the top of the rear windshield.

Assuming the driver using a hand-held mobile phone, the roof-mounted antenna reduces the field level at the driver's head for about 35 dB (about 50 times), whereas at the heads of other passengers the field reduction is smaller, of the order of 10-20 dB (about 3-10 times).

The situation is similar if the antenna is mounted on the front part of the car roof, when the front seats are somewhat more exposed than the back seats.

Finally, if the antenna is mounted in the middle of the car roof (e.g., a Maxi Challenger antenna), the measured level at all seats is approximately -45 dB.

The effect of the roof-mounting yields a field reduction due to two effects. The first one is the increased distance to the person' heads. The second one is a partial electromagnetic field shielding due to the effect of the metallic surface of the roof. To illustrate this effect, we analyze theoretically a monopole antenna mounted on the car roof (using program WIPL, published by the Artech House). The car roof is modeled by a flat metallic plate, and the antenna is assumed mounted in the location typical for Satflex. The computer simulation model is shown below. The Cartesian coordinate origin is in the center of the plate.[/td][/tr]
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Shown below is the intensity of the electric field, at the level z=-300 mm (i.e., 300 mm below the metallic plate), for x>0, and for x and y extending beoynd the dimensions of the plate. Two things are obvious. Firts, the standing-wave pattern of the field. Second, the shielding effect of the metallic plate, as the field intensity has very pronounced large values at points close to the antenna, in the region corresponding to the top part of the rear windshield.

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