Electromagnetic radiations and your health
Low-Frequency Fields (II)
Recently, much concern about EMR has focused on low-frequency energy rather than RF. Amateur Radio equipment can be a significant source of low-frequency magnetic fields, although there are many other sources of this kind of energy in the typical home, without to forget HV power lines.
Magnetic fields can be measured relatively accurately with inexpensive dosimeters and EM-field strength meters (see last page) that are made by many manufacturers.
The two first tables show typical EM-field strengths of various common devices.
In natural fields, the Electrical field can reach 200 V/m and the magnetic flux density at ground level can reach about 70 µT (700 mG). In homes not located near HV power lines, the background magnetic flux density may reach about 0.2 µT (2 mG) and locally several µT. Typical home environments (not close to appliances or power lines) are in the range of 1-5 mT (10-50 mG).
At the usual working distance, most devices (excepting some handhelds) are inside the guideline limit for the general public fixed at 100 µT and 5 kV/m (see Table 3). In addition, as for RF radiations, house walls substantially reduce the electric field levels from those found at similar locations outside the house.
Table 1 - Sources WHO and Federal Office of Radiation Safety, Germany, 1999
The Federal Office for Radiation Safety in Germany recently (1999) measured the daily exposure to magnetic fields of about 2000 individuals across a range of occupations and public exposures. All of them were equipped with personal dosimeters for 24 hours. The measured exposure varied widely but gave an average daily exposure of 0.10 µT. This value is a thousand times lower that the standard limit of 100 µT for the public and 200 times lower than the 500 µT exposure limit for workers. Furthermore, the exposure of people living in the centres of cities showed that there are no drastic differences in exposure between life in rural areas and life in the city, although the rumor seems to state the contrary. Even the exposure of people living in the vicinity of high voltage power lines differs very little from the average exposure in the population. But the risk is know and quantified in specific conditions. We will come back on these risks on the last page.
To check : PCE Instruments
Measuring and control systems, including EM field detector
Most GSM working at 900 MHz and dual band GSMs emitting on 900/1800 MHz use an antenna offering a gain of about 1.7 dBi. A 900-MHz GSM emits with a power varying according to the distance to the relay station, either 0.1 Watt or 2 or 3 Watts.
Portable outdoor antennas can offer a 16-dBi gain but some models to put on cars hardtop can reach 26 dBi. At 1 meter away from such antennas (3 W) the power density reaches 262 mW/cm2, which is very harmful at term knowing that FCC set the threshold at 5 mW/cm2 in controlled environment and at 1 mW/cm2 only in uncontrolled environments.
When connecting to the network, the antenna of a classic GSM (1.7 dBi, 3 W) offers a power density that can exceed 1000 mW/cm2 at 1 cm from the ear but that falls down to 1 mW/cm2 at 30 cm away. If you place it close to your ear during this procedure, you are exposed to levels of radiation exceeding largely safety values. It is for this reason that we recommend to let the GSM a few tens of centimeters away when it is establishing this connection.
During a conversation, most GSM produce an electric field strength below 1 V/m when placed close to the ear. This level is far below the Swiss Standard (Regulation about Protection against Nonionizing Radiation, Swiss Federal Council, 1999) that accepts a level of 4 V/m at 900 MHz and 6 V/m at 1800 MHz (the penetration depth of RF decreasing as the frequency increase), and up to 8.5 V/m for broadcast radio (and probably TV). However, their scientific basis is doubtful.
About their nocivity, the european regulation is very lax and recommends to relay antennas to not exceed 41 V/m for the 900-MHz network, and 58 V/m for the 1800-MHz network, while some european countries adopted levels between 1 and 6 V/m. It is obvious one more time (as it was the case about the nuclear, GMOs, etc), that European commission has not been impartial and was influenced by lobbies that make pressure on ministeries. Indeed, telephony operators have a market to protect and it is without any doubt that their advice influenced the european decision.
Possible nocivity of Wi-Fi systems and other Airport that permit to establish short distance wireless communications is not very studied and there is not official figures. Many authors claim that they are harmless but they show no figure to valid their comments what does not reinforce their credibility.
Only thing sure, a Wi-Fi system like the one that each of us can purchase to connect his computing devices emits with a power between 0.0032 and 0.01 W and up to 0.25 W in burst mode. An indoor antenna displays a gain between 5 and 8 dBi and emits in a radius of 100 meters. Bu there are exceptions.
Some outdoor antennas like the Dlink ANT24-1500 model sustains up to 50 W (in CW) and shows a 15-dBi gain. It can connect a device at 300 meters away if there is no obstacle.
Wi-Fi systems work at a speed of 11 Mbps in a frequency band close to 2.4 GHz (802.11b standard). The fatest systems works at the frequency of 5 GHz (802.11a standard).
The signal intensity varies as the coulomb or Newton law in 1/r2 : if you double the distance to the antenna, its power becomes 4 times weaker and its nocivity decreases as much. It is valid for any field, that it is electric, magnetic or electrostatic among others.
An indoor Wi-Fi antenna offering a 8-dBi gain for a power of 0.25 W in burst mode placed 1 meter away from the operator displays a power density of 0.04 mW/cm2.
At 20 m away, an outdoor Wi-Fi antenna ANT24-1500 (50 W) displays a power density of 0.07 mW/cm2. It falls down to 0.02 mW/cm2 at 40 m away.
These values are much lower than the 5 mW/cm2 limit recommended by FCC. These figures show a priori that Wi-Fi networks induce much less risks for the health than GSMs.
For short, we note that many home devices show a risk higher than the one of Wi-Fi systems. However, this does not allow us to conclude that Wi-Fi systems are harmless at long term. The hazard is probably much lower compared to the one of other devices.
What about amateur radio equipment, and specially antennas ? Unfortunately, determining the power density of the RF fields generated by an amateur station is not as simple as measuring low-frequency magnetic fields. Although sophisticated instruments can be used to measure RF power densities quite accurately, they are costly and require frequent recalibration. Most amateurs don’t have access to such equipment, and the inexpensive field-strength meters that we do have are not suitable for measuring RF power density. The best we can usually do is to estimate our own RF power density based on measurements made by others or, given sufficient computer programming skills, use computer modeling techniques.
As write ARRL, "because these EM fields dissipate rapidly with distance, *prudent avoidance* would mean staying perhaps 30 to 45 cm (12-18") away from most Amateur Radio equipment (and 60 cm or 24" from power supplies with 1-kW RF amplifiers) whenever the AC power is turned on. The old custom of leaning over a linear amplifier on a cold winter night to keep warm may not be the best idea! There are currently no non-occupational US standards for exposure to low-frequency fields. However, some epidemiological evidence suggests that when the general level of 60-Hz fields exceeds 20 mT (2 mG), there is an increased cancer risk in both domestic environments and industrial environments". IRPA confirms this order of magnitude.
Table 3 - Measurements made by the FCC and EPA, 1990
As this third table indicates, a good antenna well removed from inhabited areas poses no hazard under any of the various exposure guidelines. However, the FCC/ EPA survey also indicates that amateurs must be careful about using indoor or atticmounted antennas, mobile antennas, low directional arrays or any other antenna that is close to inhabited areas, especially when moderate to high power is used.
Ideally, before using any antenna that is in close proximity to an inhabited area, you should measure the RF power density.
If that is not feasible, the next best option is make the installation as safe as possible by observing the safety suggestions listed below. Other values can be read in this IRPA document (PDF).
Table 4 - RF Awareness Guidelines from ARRL
At last, recall that at the very high frequencies at which work EME traffic as well as ionospheric sounding (e.g. at 1260 MHz, 2230 MHz and above) the fact to stay for long time in line-of-sight or standing "in" a large dish antenna when it is in emission with powers higher than 10 kW, can cause sterility.