Manual SPECTRAN V5
© Aaronia AG 2016 | www.aaronia.com
36
PHYSICAL UNITS
7.
Physical units
7.1. Measuring transmission power (dBm; dBµV)
Measurements of communications equipment often show extremely big differ-
ences 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-2025E e. g. already features a measurable level range of
-80dBm to 0dBm. As amplification increases tenfold once every 10dB, “read
-
ings” 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].
7.2. 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 pow
-
er 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 impres
-
sive, 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 end
-
less number of zeroes; it just displays the corresponding abbreviation f, p, n,
μ or m before the unit.
