2.4 Radio Frequency Interference
| Twisted pair cable and other in-house wiring are used to carry signals from one end, within an office or a home, to another end. Part of the signal energy is lost during transmission; becoming heat and electromagnetic radio waves. At high frequencies, the signal loss is relatively heavy, and the amount of radiation could become quite significant to the degree that it could affect other sensitive radio transmission systems occupying the same frequency band. The radio interference caused by twisted pair cable or in-house wiring can be considered as an extension of the crosstalk effect. Crosstalk is an electromagnetic phenomenon between two pairs of cables laid next to each other. The same electromagnetic wave that causes crosstalk also propagates through open space. The fact that there is no electromagnetic shield on a twisted pair cable or on most in-house wirings makes the radio propagation worse. In traditional terms, the pair causing the crosstalk is called the disturbing pair, and the radio emission from a cable is called the egress. Therefore, the egress from a twisted pair or an in-house wiring is from the disturbing pair. On the other hand, the same electromagnetic mechanism causes a twisted pair cable or in-house wiring to pick up noises from an interference field from other radio transmission systems. Again, in traditional terms, the pair receiving the crosstalk is called the disturbed pair, and the radio emission to a cable is called the ingress. Therefore, the ingress to a twisted pair or in-house wiring is picked up by the disturbed pair. In practice, a twisted pair cable or in-house wiring is used to transmit and receive signals acting as both a disturbing and disturbed pair simultaneously. A twisted pair cable or an in-house wiring needs to deal with both egress and ingress issues. For the egress, the amount of radio emission from a twisted pair cable or in-house wiring needs to be below the limits set forth by the Wireless Telecommunications Bureau [10] under the Federal Communications Commission. Specifically, some radiation limits have been imposed by Part 15 (Radio Frequency Devices) of the FCC rules and regulations, which is labeled Code of Federal Regulation (CFR) Title 47. In Part 15 section 209 of Title 47, the radiation limits are defined for four frequency bands as shown in Table 2.5. In Table 2.5, the field strength is measured 3 m from the subject except for the frequency band of 1.705 to 30 MHz, which is measured 30 m from the subject. Notice that no specific measurement bandwidth is given and that the measurement of the whole signal spectrum is assumed. These limits are derived for narrow band radio transmission systems with a maximum bandwidth of 9 kHz. For radio transmission systems with wider bandwidths, an additional 13 dB of signal level is allowed above these limits. We have expressed the noise power in terms of decibels, which is the power ratio against 1 milliwatt. On the other hand, the strength of a radio electromagnetic field is expressed in terms of decibel microvolts per meter (dBµV/m), which is the ratio against 1 µV/m. An antenna is normally used to pick up a signal from an electromagnetic field. In the receiving process, an antenna converts the field strength to electrical power, which is usually expressed in terms of decibel microvolts on a particular termination impedance. The difference between the field strength and the electrical power is expressed as the antenna gain in terms of decibels. In addition, for a terminal impedance of 100 ohms, the equal power conversion factor between decibels and decibel microvolts is expressed as Equation 2.54
For example, an isotropic antenna has an antenna gain of 5 dB [11]. For this particular isotropic antenna case, the dBm-to-dBµV/m conversion factor is 0 dBmV/m The RFI loss can also be used to calculate field strength, caused by a twisted pair cable or an in-house wiring carrying a signal of a particular power, at a certain distance. We can estimate the field strength using the following expression: Equation 2.55
where E is the field strength measured in terms of dBµV/m, PSD is the power spectrum density of the signal on a twisted pair cable or in-house wiring measured in dBm/Hz, B is the bandwidth of the signal measured in hertz, R is the terminal impedance measured in ohms, r is the distance from the cable or wiring measured in meters, and RFI is the RFI loss measured in decibels. Let us do a calculation example by assuming P = 85 dBm/Hz, B = 2.2 kHz, R = 100 ohms, r = 30 m, and RFI = 40 dB. We have Equation 2.56
The bandwidth of 2.2 kHz corresponds to that of a HAM radio. A HAM radio receiver usually has a sensitivity of about 0 dBµV/m. Therefore, a twisted pair cable or in-house wiring carrying a signal with a power spectrum density of 85 dBm/Hz will not cause any harm to a HAM radio located 30 m away. Even at a distance closer than 3 m, the field strength is about 2.1 dBµV/m, which is still below the sensitivity of a HAM radio receiver. |
110 5 = 115 dBm. On the other hand, the dBmV/m-to-dBm conversion factor is 0 dBmV/m 
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