Long-term effects of electromagnetic (EM) radiation from cellular and Personal Communications Services (PCS) handsets are still unknown. Prudent practices include the use of measurement tools to detect unusually high levels of EM radiation from, for example, broken gaskets in a cellular handset package. Another safety measure is to measure the radiation levels from wireless handsets using a low-cost radiation checker as proposed in this article. This report will present the design, review its principles of operation, and demonstrate its effectiveness in measuring cellular-handset radiation.

The radiation checker consists of an antenna (two identical elements), a plane small reflector, a detector including a matching circuit, a moving-magnet meter, and a plastic cover. The antenna is placed about one-tenth of a wavelength away from the reflector to receive maximum energy from EM radiation source. The received radiated energy by the antenna passes through a detector and a matching circuit to provide appropriate current for the movement of the magnet. The deflection of moving magnet is proportional to the received current. A substitution method has been used to verify the calibration of the radiation checker in mW/cm².

The biological effects of UHF and microwave frequencies have been a concern for years. Recently, Lai et al.^1-3 and Phillips et al.⁴ have reported that the EM radiation emitted from a cellular/PCS phone is dangerous for animal/human cells. Their results show that there is DNA damage in rats due to electromagnetic radiation below l mW/cm². Another group of scientists ^5-8 have reported that the EM radiation did not increase the number of single-strand DNA breaks in rats. Recently, Kumar⁹ provided a review on the biological effects of low power radiation on human and animals and why groups differ on DNA damage.

There are many large and expensive commercial radiation monitors. This new radiation checker should not be confused with those larger radiation monitors. A radiation checker is used for a quick read of EM radiation and not for precision measurements of EM radiation. Still, the radiation checker does provide full-scale sensitivity of 0.001mW/cm² for detection of low-level radiation.

The radiation checker measures the total EM radiation incident on the human head in the near-field. Kumar ^10,11 and Chung¹² have reported that, due to body absorption effect, only about 29 to 72 percent of the antenna input power is radiated into free space for various-head distances (1 to 5 cm). The closer the distance, the less the antenna input power is radiated into free space. Thus, strong EM coupling at close distances increases the body absorption effect. It has also been reported that the head absorbs more than 80 percent of the whole-body absorbed power since the antenna is located in front of the head. Since it is compact, a radiation checker can provide a good indication of absorbed power into the head.

The radiation checker's antenna works on the principle of the cavity resonator.¹³ One end of the dipole is connected to the structure's positive terminal and other end is connected to the negative terminal (Fig. 1). Figure 2 shows a simplified version of a backfire antenna model, which consists of a large reflector,¹³ a small reflector, and a dipole antenna. The large and the small reflectors are parallel to each other, at a distance c. The planes of both reflectors are transverse to the longitudinal axis. The dipole radiator is placed closed to the small reflector. The dipole antenna is placed at a distance d from the small reflector. According to the diagram of Fig. 1, two surface-wave-modulated structures lie between the two reflectors. The first is connected to the positive end and the second to the negative end, with a diode connected between the positive and negative terminals.

When an EM wave is incident upon the large reflector, it travels directly from the large reflector to the small reflector, through the modulated-surface-wave structure, and a resonant cavity is formed. The dipole structure picks up the EM wave, which passes through a diode detector. The diode detector converts the alternating-current (AC) EM wave to direct current (DC), which passes through the positive and negative ends of the dipole to the coil. The coil and magnet are placed in the back of the large reflector. There are no electrical contacts between the large reflector and the modulated-surface-wave structure.

In Figure 2, the center of the large reflector is placed at the origin of a Cartesian coordinate system with its y-axis lying along the cavity antenna axis. The dipole primary radiator is parallel to the z-axis with half-length h and radius a, and its center is located at y = d along the positive y-axis. The small reflector is parallel to the x-z plane with its center along the positive and negative y-axis at y = c.

Integral equations for the induced current excited in the dipole and on the surface of the small reflector were formulated based on the boundary condition that the tangential electric field vanishes at all conducting surfaces. In calculating the received radiation field on the antenna, the large reflector currents are truncated by using the actual size of the reflector. Image techniques determine the currents induced in the primary dipole antenna and on the surface of the small reflector.

The radiated field emitted by the source is captured by the combination of the plane reflector (λ/6) and the curve surface (λ/8). The extension of the curve surface supports the assumption that the large reflector is infinite in extent in theoretical calculations of the E-field. The EM field E(r) at any point in space, maintained by all the currents in the antenna system is:

where:

β_o = 2π/λ_o, the free-space wave number,

ξ_o = (εo /µ_o)^1/2, the free-space wave number,

R(r, r′) = |r –r′| is the distance between the field point at r and a source point at r′, and

V = the volume including all the currents in the antenna system.

The term J(r′)dV′, in this case, consists of the current z I_z(z′)dz′ in the dipole, the surface current K_r(r′)ds′ = [xK_rx (x′, z′) + xK_rz (x′, z′)]ds′ on the small reflector as well as the effects of their images. Currents xI_z and K_r are imaged into the y = 0 plane to replace the effects of K′s on the large reflector and concurrently satisfy the boundaries conditions at its surface. The surface current is excited on the conducting plane, which is determined from the electromagnetic field maintained by the currents in the dipole and on the small reflector, and their images (Figs. 2 and 3). The large reflector has diameter of 6 cm while the small reflector has diameter of 1.8 cm, with 3.5-cm distance between the two.