HW V4 / © 2005 – 2013 Aaronia AG, DE-54597 Strickscheid
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Manual Spectran V4
Physical units
11.3. Interesting facts regarding high-frequency formula
In publications the writers often jump around between units. Consequently,
the casual reader often loses track of the relationships between units
completely.
Spectran offers functionality to convert between units or directly display
measurements in the unit of your choice. Furthermore, we have collected
several tables for simple comparison and, correspondingly, conversion of
units and values. We also included the most important basics and formulas
(chapter 12.1.), dealing with high frequency radiation:
Power density of high-frequency signals is oftentimes measured in W/m²
(primarily in Europe) or W/cm² (primarily in the USA). Conversion between
these units is simple:
1µW/cm² = 0.01 W/m² = 0.001 mW/cm² or
10 µW/cm² = 0.1 W/m² =0.01 mW/cm² as so on. (Table 2)
Less frequently, values in dBm/m² and dBW/m² can be found. Conversion is
similarly simple.
1µW/cm² = 10dBm/m² = -20 dBW/m² or.
10µW/cm² = 20 dBm/m² = -10 dBW/m² etc., respectively.
However, the most frequently used unit is V/m (see table 3). Here you should
remember that power flux density S (measured in W/m²) really consists of
two separate fields: The electric field E (measured in V/m) and the magnetic
field H (measured in A/m). However, as these do not exist separately
anymore at high frequencies, their separate indication is not really interesting
for RF exposure. However, this is only the case outside the so called close-
up range. How you calculate the close-up range and how the relationship
between the different radiations works is described below.
Power flux density S (also called electromagnetic field or power density) is
calculated as follows:
=
∗
If „H“ is unknown, the following formula still allows a calculation of S,
assuming 377 Ohms as the so-called „field resistance“ of air:
=
²
377
