A.H. Systems AK-571-4 Horn Antenna Kit
dBmW/m
2
= dB
V/m - 115.8
The constant in this equation is derived following similar logic. First, consider the Poynting Vector
which relates the power density (W/m
2
) to the electric field strength (V/m) by the following equation.
P=|E|
2
/
Where
is the free space characteristic impedance equal to 120
. Transforming this equation to
decibels and using the appropriate conversion factor to convert dBW/m
2
to dBmW/m
2
for power
density and dBV/m to dB
V/m for the electric field, the constant becomes 115.8
dB
V/m = dB
V + AF
Where AF is the antenna factor of the antenna being used, provided by the antenna manufacturer or
a calibration that was performed within the last year.
V/m = 10
{[(dBuV/m)-120]/20}
Not much to this one, just plug away!
dB
A/m = dB
V/m - 51.5
To derive the constant for the above equation, simply convert the characteristic impedance of free
space to decibels, as shown below.
20Log
10
[120
] = 51.5
A/m = 10
{[(dBuA/m)-120]/20}
As above, simply plug away.
dBW/m
2
= 10Log
10
[V/m - A/m]
A simple relation to calculate decibel-Watts per square meter.
dBmW/m
2
= dBW/m
2
+ 30
The derivation for the constant in the above equation comes from the decibel equivalent of the factor
of 1000 used to convert W to mW and vice versa, as shown below.
1
0Log
10
[1000] = 30
dBpT = dB
A/m + 2.0
In this equation, the constant 2.0 is derived as follows. The magnetic flux density, B in Teslas (T), is
related to the magnetic field strength, H in A/m, by the permeability of the medium in Henry's per
meter (H/m). For free space, the permeability is given as...
o
= 4
x 10
-7
H/m
Converting from T to pT and from A/m to
A/m, and taking the Log, the constant becomes...
240 - 120 + 20Log
10
[4
x 10
-7
] = 2.0
dBpT = dBuV + dBpT/uV + Cable Loss
dBuV/m = dBpT + 49.5 dB
A.H. Systems inc. – Apr 2020
REV C
12
