There are many parameters that define an RF personal monitor. Some of the newest models are rich in features while others provide basic functionality—they simply sound an audible alarm when field levels above a preset limit are detected. Although many of these features are important, the performance and rightfully, the value, all depends on the performance of the sensors. If the sensors are not detecting field levels accurately, the other features are of little value. Sensors are designed to detect either the electric (E) field or the magnetic (H) field. They can have either flat-frequency response or shaped-frequency response. All the modern models have shaped-frequency response, but some (the Nardalert XT models, for example) are far more accurate in conforming to the applicable standard.
Sensor Type: E-Field, H-Field, or Both
In the far field, electric field and the magnetic field levels are equal. Therefore, E-field and H-field sensors should yield the same results in the far field. The distance between the source of energy, such as the antenna, and the far field is dependent on several factors. The two most important are frequency and the type of the antenna. The major measurement standards use 300 MHz as the cutoff point for measuring both fields. Although it is not explicitly stated, the logic behind this is that under all but the most unusual circumstances, you will invariably be in the far field at frequencies above 300 MHz.
In reality, you can make measurements at frequencies as low as 50 MHz with little compromise in accuracy, providing that you remain half a meter away (about 20 inches). This is because the definition of far field conditions says that the fields must be equal in magnitude and the phases must be correct. Yet phase error has little impact on the sensors used in RF personal monitors and RF survey instruments.
Below 50 MHz, it becomes more likely that the E-field and the H-field will be of different magnitudes when you are anywhere near enough to the antenna to be concerned about RF radiation.
At these frequencies, the electric field is far more important biologically. Many of the newest standards have relaxed exposure limits for the magnetic field below 100 MHz. This is because RF heating efficiency, or the Specific Absorption Rate (SAR), drops off dramatically below the human resonance region. At the lower frequencies, electrostimulation (shocks and burns) is more of a concern than whole-body heating. And the potential for induced current and contact current is proportionate to the strength of the electric field. The magnetic field has limited effect on the body. See Biological Effects and Why and where do shocks and burns occur? for more information.
Therefore, a good electric field sensor does the job biologically, although it may not guarantee compliance with a standard that has the same exposure limits for both the E-field and the H-field. An H-field sensor alone is of little value.
RF personal monitors that contain both an electric field sensor and a magnetic field sensor—the RadMan series are the only models that currently have this feature—appear to provide the ideal solution. The problem is that the RadMan uses a simple dipole for the electric field. And dipoles do not work on the body at frequencies below about 30 MHz. The only electric field sensors that work well on the body at these lower frequencies employ surface area detection rather than dipole detection. This is the type of sensor used in the Nardalert XT to detect the lower frequencies. See Sensor Designs for more information.