For as long as there have been cell phones, there’s been speculation that they might be hazardous to your health. Here’s one solution to minimizing the concern.
When a cell phone is held against the user’s ear in a normal operating mode, the RF energy emitted from the phone can raise the temperature within the ear by approximately 0.1oC. This rise in temperature is caused by RF energy radiated from the phone’s antenna, where a relatively large electromagnetic-field strength of approximately 25 V/m is generated.
The use of a hands-free headset partially reduces the electromagnetic-field strength at the head.1 However, the cable not only transmits the intended low-frequency voice signal, but also couples a portion of the cell phone’s RF energy onto the cable and subsequently to the user’s head. The localized field strength of this RF energy depends on the frequency (either 900 MHz or 1,900 MHz), the length of the cable (normally about 25²), and the standing waves this creates.
The cable from the phone, typically attached to the user’s belt, to the headset usually is about 1² or 2² from the body. Attempting to represent the electromagnetic-field distribution along the cable only can be simulated with great difficulty in the laboratory.
The following experimental setup permits an estimation of the resonance processes on the cable and provides an indication of the field strength at the cable end.
Experimental Setup
A 25² long cable is placed approximately 2² from and parallel to a metallic plate. Both ends are fixed with a high-frequency N-type connector (Figure 1). At one end, an RF signal is applied; on the other end, there is a 4.7-pF capacitor attached. This represents the capacity of the headset to the head.
At the capacitor end of the cable, the applied signal is measured. This signal is an indication of the electromagnetic field at the head of the user of the hands-free headset.
Measurement Results
Figure 2 shows the relation in decibels of the applied signal to the received signal at the second N-connector. The frequencies around 900 MHz and 1,900 MHz receive special attention. If you use a conventional cable such as a PVC insulated cable 4 × AWG26, the attenuation at 900 MHz and 1,900 MHz reaches approximately 8 dB and 4 dB, respectively. Resonances occur over the entire frequency range on the line.
If the standard cable is replaced with a ferrite-coated cable, the wave that propagates along the cable is greatly attenuated by the ferrite layer (Figure 3). No more resonances occur above 400 MHz, and a considerably higher attenuation at 900 MHz and 1,900 MHz is realized in comparison to the uncoated cable. This is demonstrated in Figure 3 where the attenuation reaches 30 dB and 55 dB, respectively. The difference in comparison to the conventional cable is approximately 20 dB and 50 dB at the two considered frequencies.
The measured attenuation shows that the RF energy is absorbed in the course of its propagation on the line, and only a small part of the energy is finally transferred to the cable end.
SAR Testing
To confirm this practical application in the use of ferrite-coated cables, actual headsets were manufactured using ferrite-coated cables and submitted to an independent test lab for both Body SAR and Head SAR testing in accordance with the measurement procedures specified in FCC/OET Bulletin 65 Supplement C (2001) and IEEE 1528-200X (draft 6.4, July 2001).
Head SAR
Two of the tests were for Head SAR using both AMPS and CDMA modulation schemes. These tests compared a cell phone (models CPBTSCP and CPBTPHN, respectively) held in the normal operating mode (against the ear) and the same cell phones using a hands-free headset manufactured with a ferrite-coated cable.
Even though both the phone and the headset pass the FDA-imposed safety limit of 1.6 W/kg, the results demonstrate the dramatic margin in the difference in SAR: 1.380 vs. 0.0006 W/kg in AMP modulation and 0.9187 vs. 0.0006 W/kg in PCS/CDMA modulation.
Body SAR
Similarly, Body SAR testing was performed using the same modulation schemes but comparing a standard headset to a ferrite-coated style. Again, the results were similar. Both the nonferrite and the ferrite headsets passed the imposed limits; however, the ferrite headsets typically had four to five times better (lower) SAR ratings than headsets made with standard cables.
Conclusion
When using a ferrite-coated cable for this application, a significantly smaller electromagnetic-field strength results at the head of the headset user in comparison to a headset made with a conventional cable. This would suggest there is an easy-to-apply solution to everyone’s concern over the uncontrolled exposure of RF energy to the human brain and body through the use of cell phones.
Further Applications
Many devices used in medical technology and sensory technology are very sensitive to strong RF fields. The use of ferrite-coated cables is a practical solution to the improvement in the immunity of these devices.
In many applications, these cables do not need a metallic shield since the absorption in the ferrite layer attenuates the effect of the RF energy. In a similar vein, the high-frequency radiation of the device and its connecting cables is reduced. As a result, complex filtering techniques, which are particularly expensive at very high frequencies, can be omitted.
Reference
- Mobile Telephones and Health – The Latest Development; Conference; London, July 6-7, 2001.
About the Authors
William Watts, president of EMC Eupen, has nearly 40 years experience in the EMC industry. He founded EMC Technologists in 1984 and EMC Eupen in 1989. Mr. Watts is a life-long member of the IEEE/EMC Society, the IEEE Power Electronics Group, IEEE Microwave Electronics, and the dB Society. EMC Eupen, 5033 Industrial Rd. #6, Farmingdale, NJ 07727, 732-919-1100, e-mail: [email protected]
Manfred Kirschvink joined Kabelwerk Eupen in 1986 and currently is responsible for R&D on all special cables. Previously, he was with Balteau, Liege, Belgium, (a Schlumberger-owned company) and served as director of R&D. Mr. Kirschvink received the Diplom Ingenieur degree from Technische Hochschule Aachen, Germany for RF and Telecommunication Engineering in 1965 and has been a member of VDE for 30 years. Kabelwerk Eupen, Malmedyer Strasse 9, B-4700 Eupen, Belgium, e-mail: [email protected]
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December 2002
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