HW V4 / © 2005 – 2013 Aaronia AG, DE-54597 Strickscheid
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Manual Spectran V4
Physical units
11. Physical units
11.1. Measuring transmission power (dBm; dBµV)
Measurements of communications equipment often show extremely big
differences between levels. Therefore, it is useful to express signal levels in
logarithmic units to avoid “drowning” readings in an endless number of zeros.
Thus, our Spectran HF-2025E e. g. already features a measurable level
range of -80dBm to 0dBm. As amplification increases tenfold 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 changes 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 1mW or 1µV. So, logarithmic level readings are expressed in
dB
milliwatts
[dBm] or dB
microvolts
[dBµV].
11.2. 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 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 instruments often only provide
readings of power density (mostly in µW/m²) to confuse the operator with
allegedly huge changes in “electrosmog” strength. This is just as if you
measured distances on motorways in mm: Frankfurt to Munich = 400,000,00
mm. 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 number of zeroes; it
just displays the corresponding abbreviation f, p, n, µ or m before the unit.
