Aaronia SPECTRAN HF-2025, Руководство пользователя, Страница: 60 / 72

17.0 Units of measurement
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SPECTRAN offers several physical units to choose from. These can be selected
at any time in the “Unit” menu. Possible choices are:
dBm , dBµV , V/m and
A/m . In “Exposure limit calculation mode”, additionally also W/m² .
17.1 Measuring transmission power [dBm; dBµV]
Measurements of communications equipment often show extremely big diffe-
rences between levels. Therefore, it is useful to express signal levels in loga-
rithmic units to avoid “drowning” readings in an endless number of zeros.
Thus, our SPECTRAN HF-2025 e.g. already comes up with a measurement
range of -80dBm to 0dBm. As amplification increases 10 fold once every 10dB,
“readings” of 0 to 100.000.000! would need to be displayed. Though, these
“numeric giants” would be nearly unreadable and result in steady, drastic chan-
ges of the displayed values. So, we use the much easier to handle
logarithmic
unit dB:
As logarithms are dimensionless, logarithmic readings are always relative to a
certain
reference level, i.e., a relationship between two levels is established. A
common reference level that has become a standard in radio technology is 1 m W
or 1 µV . So, logarithmic level readings are expressed in dB milliwatts [dB m ] or
dB microvolts [dB µV ].
17.2 Field strength [V/m; A/m] and power density [W/m²]
If you don’t feed signals directly and instead perform measurements using an
antenna , you will mostly want to measure the so-called field strength or power
flux density instead of power. In professional measurement technology, mostly
only the “manageable” electric field strength is measured, using the unit
V/m . As
with dBm, V/m produces manageable readings without endless rows of zeroes.
In contrast, cheap broadband devices often
only provide readings of power
density (mostly in µ W/m²) to confuse the customer with allegedly huge changes
in “electrosmog” strength. This is just as if you measured distances on motor-
ways in mm:
Frankfurt to Munich = 400.000.000mm! Certainly “impressive”,
but just pure showiness, as it’s just 400km. Manageability and readability is left
behind, as the display constantly changes in huge steps. Of course, SPECTRAN
can also display power flux density in µW/m² or other “Giga-units”. However, in
contrast to the aforementioned cheap devices, SPECTRAN offers a practical
Autorange feature in W/m²-mode , e.g. instead of displaying an endless num-
ber of zeroes, it just displays the corresponding abbreviation f , p , n , µ or m befo-
re the unit.