ESCO Technologies ETS-Lindgren 3147, Руководство пользователя, Страница: 37 / 49

Radiated Emissions Measurement | 37
In a 50- Ω system, the above equation becomes:
10 log
10
P = 20 log
10
V – 10 log
10
(50)
Converting from dB to dBm for power and from dB(V) to dB(µV) for voltage, the
added constant becomes:
30 – 120 – 10 log
10
(50) = –107
The constant in the second equation is obtained by considering the pointing
vector which relates the power density in (W/m
2
) to the electric field density in
(V/m) by:
P =
|E| 2
η
Where η is the free space characteristic impedance equal to 120 π Ω .
Transforming the previous equation to decibels and using the appropriate
conversion factors to convert dB(W/m
2
) to dB(mW/m
2
) for power density and
dB(V/m) to dB(µV/m) for the electric field, the constant becomes:
30 – 120 – 10 log
10
(120 π ) = –115.8
For the constant in the fifth equation, the magnetic field density is related to the
electric field density via the characteristic impedance of free space. When the
transformation is made to decibels, the constant becomes:
20 log
10
(120 π ) = 51.5
In the last equation, the magnetic flux density B in (T) is related to the magnetic
field density H in (A/m) the permeability of the medium in (H/m). For free space
the permeability is µo = 4 π 10
-7
H/m. Converting from (T) to (pT) and from (A/m)
to (µA/M) and taking the log, the constant becomes:
240 – 120 + 20 log
10
(4 π x10
-7
) = 2.0