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.

-Posted on the The BASE Board!, early 1998.

The energy around an antenna is in the form of electromagnetic radiation (EMAG). Amplitude Modulated (AM) and Frequency Modulated (FM) radio waves, as well as microwaves are considered non-ionizing radiation. These are the types of radiation emitted from most antennas we would consider a legitimate site. This means that the EMAG's we subject our bodies to at an antenna site are not energetic enough to break chemical bonds in the molecules of our bodies. There have been numerous epidemiological studies conducted that attempt to correlate elevated incidence of brain tumors, leukemia and other human body cancers with electromagnetic radiation. There was also a study conducted by the government that looked at military personnel who worked closely around RADAR, and radio sources in order to determine if there were occupational dangers. The majority of studies find no conclusive link between non-ionizing radiation and human body cancers. There were some studies however that do suggest correlation.

Unlike ionizing radiation that can destroy the molecules that make up our bodies, the only plausible mechanisms for harm due to non-ionizing radiation are thermal heating and induced cellular currents . The heating around most sites is negligible. The induced currents around our cells is on the order of a magnitude smaller than those that naturally occur in our bodies already.

On the face of it there seems to be little to worry about on most antenna sites. There is however enough doubt, in my mind any way, to exercise a little prudence. We can reduce the amount of exposure (dose) we incur on a jump by taking advantage of some of the laws of physics. The energy transmitted by an antenna decreases with the square of the distance we are from the antenna. This means that at ten meters from the source, the energy is 100 times less than it is at the source, 100 meters from the antenna the energy is 10,000 times less, etc. Furthermore, some antennas are directional. This means that the energy emitted is concentrated in certain directions. The antennas you describe on the site you are presently doing the ground work for sound like microwave antennas. You mentioned that they looked like barrels. Are they barrels lying on their sides, sort of like kettle drums? If so they are omni-direction; which means they only emit in the direction the "drum-head" is facing.

Finally there are government guidelines that set the Specific Absorption Rate (SAR) to 400 milli Watts/ kilogram of body mass. Most companies limit their emissions to a fraction of this limit to comply with these guidelines.

-Posted on the The BASE Board!, early 1998.