回複：俺怎麽記得別人說EMF跟距離的平方成反比

The amount of radiation that we are exposed to from an antenna is usually expressed in power per unit area (watts/square centimeter, etc.). A watt is a unit of energy (1 Joule/sec). The radiation we absorb is a function of radiated power and distance from the source (antenna). Specifically it is:

S=PG/4PIR²

where S= radiated power from the antenna , P = power applied to the antenna, G = gain of the antenna, R = distance from the antenna. It is obvious that the most effective way to reduce your exposure is to maintain distance from the source. This is due to the R² term in the denominator of the formula. It means that at 2 meters from the antenna the power is 4 times less, at 3 meters it is 9 times less, at 4 meters it is 16 times less at 10 meters it is 100 times less... etc.

The formula above assumes an isotropic field, that is, the antenna radiates power equally in all directions. This is rarely the case. Your question about the "disk" antennas is specific to anisotropy in the field. It is governed by the gain (G) factor in the equation above. The gain is not equal in all directions. In the case of the "disk" antenna you are alluding to it is most likely a parabolic antenna which is highly directional. The gain is highest directly in front of the antenna. Consequently the relative field strength is much lower at all other points. Furthermore, this formula is specific to the far field region of an antenna. The far field region is determined by the point at which the radiated power is in the form of parallel waves emanating from the antenna where the power decreases with the distance squared. There are basically three regions adjacent to an antenna: the far field, the transition region, and the near field or Fresnel region. In the transition region the power decreases with distance (not the square of the distance), in the near field the power will reach a maxima before beginning to decrease with distance. On an antenna BASE jump is likely that we will be immersed in all three fields at one time or another.

An antenna jump is unique in the BASE endeavor in that additional to the obvious objective dangers is the consideration of the radiation factor relative to risk management. In order to precisely determine the dose of radiation that you will experience on an antenna jump you have to really do your ground work. You must:

1) determine the transmitted power of the object.
2) determine the antenna characteristics, i.e., the gain in all directions.
3) determine how long your body will be at each distance from the antenna on the way up and down.
4) determine the power at each point as you climb/descend.
5) integrate (sum) your exposure at each point over time - your accuracy will be determined by your measure of time as well as your distance interval.

One must also consider what non-ionizing radiation does (or does not do) to the human body. Although personally, as an engineer, I find a certain mount of satisfaction in working out the dosimetery of an antenna BASE jump, the average BASE jumper may find it too mundane. The real question is what does a given dose of energy do to your body? What is the added risk of BASE jumping in an electromagnetic field? There is a considerable body of epidemiological research on the subject. Realize that epidemiological studies only attempt to associated phenomenon. They do not consider mechanisms. The link of emags to ill-health effects is very dubious at this point from an epidemiological standpoint. Only recently has there been any serious research into the specific mechanisms of how emags might harm us.

My own beliefs are not important. I am by no means completely informed. What we need to do as individuals, in my humble opinion, is learn as much as possible about the objective dangers associated with BASE jumping (or life in general) and make informed decisions as to when to back-off and when to go for it.

--文學城www.wenxuecity.com--

所有跟帖： 

• 其實現實世界裏那些都不適用 -tty- ♀ (170 bytes) () 12/10/2003 postreply 08:45:00

• 你那裏冷麽？知道美地出版社麽？ -hamburg- ♀ (87 bytes) () 12/10/2003 postreply 09:50:00

• 十冬臘月下大雨，感冒的人無數，鬱悶。。。 -貼圖幫助- ♀ (0 bytes) () 12/10/2003 postreply 12:03:00