18.2 Calculating the “close-up range” in [m]
If you want to perform a measurement, you need to do this
outside
the so-cal-
led close-up range of the transmitter. The close-up range depends on the trans-
mitter frequency. According to the aforementioned formula, the close-up range
can be determined quickly. Here, you simply need to multiply the resulting wave-
length by a factor of 10 (there are some sources which only use a factor of 3).
The result is the close-up range, i.e. the minimum distance to the transmitter in
which a sensible measurement can be achieved.
Example: For a “900MHz cellphone”, the wavelength is:
Hence, the close-up range is 0,33m * 10 =
3,3m
.
It is easy to see that e.g. measurements taken directly next to the cellpho-
ne will definitely result in WRONG readings
!
Notice:
Inside the close-up range, E and H fields would need to be measured
separately. However, outside the close-up range, both field types are coupled
tightly to each other, so knowing the strength of one of them will be enough to
derive the other.
18.3 Calculating power density [W/m²] from power [dBm]
ONLY
if you know the transmitter frequency you can calculate power density
[W/m²] from power [dBm]. You also need the antenna gain of the used antenna.
The formula you need looks like this:
S represents the power density [W/m²], p is the measu-
red power [dBm], is the wavelength of the tranmitter
frequency [m] and G the antenna gain [dBi] (dBd-values
can be converted to dBi by adding 2,15).
You get the dBi antenna gain of the HyperLOG antennas easily by looking at the
“antenna.ini”-file used with our “LCS” PC-Software.
Example:
Pretend you measure -40dBm at 950MHz with a HyperLOG 7025 antenna (this
antenna offers 4,9dBi at 950MHz).
The result is:
Also see the following conversion table!
18.0 Some high-frequency math
63
= 0,00000458 W/m² = 4,08µW/m²
Test Equipment Depot - 800.517.8431 - 99 Washington Street Melrose, MA 02176
Содержание SPECTRAN HF-2025
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