P
D
'
E
2
Z
0
'
E
2
120B
'
E
2
377
P '
E
2
Z
0
'
E
2
50
' 50I
2
4-1.1
FIELD INTENSITY and POWER DENSITY
Sometimes it is necessary to know the actual field intensity or power density at a given distance from a transmitter
instead of the signal strength received by an antenna. Field intensity or power density calculations are necessary when
estimating electromagnetic interference (EMI) effects, when determining potential radiation hazards (personnel safety), or

In the 50S case, power and voltage are related by:
[2]
Conversions between measured power, voltage, and current where the typical impedance is 50 ohms can be obtained
from Table 2. The dBFA current values are given because frequently a current probe is used during laboratory tests to
determine the powerline input current to the system .
MATCHING CABLING IMPEDANCE
In performing measurements, we must take into account an impedance mismatch between measurement devices
(typically 50 ohms) and free space (377 ohms).
4-1.2
Table 1. Conversion Table - Field Intensity and Power Density
P = E /Z ( Related by free space impedance = 377 ohms )
D 0
2
E 20 log 10 (E) P 10 Log P
(Volts/m) (dBµV/m) (watts/m ) (dBW/m ) Watts/cm dBW/cm mW/cm dBm/cm dBm/m
6
D
2
D
2 2 2 2 2 2
7,000 197 130,000 +51 13 +11 13,000 +41 +81
5,000 194 66,300 +48 6.6 +8 6,630 +38 +78
3,000 190 23,900 +44 2.4 +4 2,390 +34 +74
4,000 186 10,600 +40 1.1 0 1,060 +30 +70
1,000 180 2,650 +34 .27 -6 265 +24 +64
700 177 1,300 +31 .13 -9 130 +21 +61
500 174 663 +28 .066 -12 66 +18 +58
300 170 239 +24 .024 -16 24 +14 +54
200 166 106 +20 .011 -20 11 +10 +50
100 160 27 +14 .0027 -26 2.7 +4 +44

-4
-4
-4
-5
-7
-8
-8
-8
-9
-4
-4
-4
-4
-6
70x10 97 1.3x10 -49 1.3x10 -89 1.3x10 -59 -19
-3
50x10 94 6.6x10 -52 6.6x10 -92 66x10 -62 -22
-3
30x10 90 2.4x10 -56 2.4x10 -96 24x10 -66 -26
-3
20x10 86 1.1x10 -60 1.1x10 -100 11x10 -70 -30
-3
10x10 80 2.7x10 -66 2.7x10 -106 2.7x10 -76 -36
-3
-5
-6
-6
-6
-7
-9

-10
-10
-10
-10
7x10 57 1.3x10 -89 1.3x10 -129 1.3x10 -99 -59
-4
5x10 54 6.6x10 -92 6.6x10 -132 66x10 -102 -62
-4
3x10 50 2.4x10 -96 2.4x10 -136 24x10 -106 -66
-4
2x10 46 1.1x10 -100 1.1x10 -140 11x10 -110 -70
-4
1x10 40 2.7x10 -106 2.7x10 -146 2.7x10 -116 -76
-4
-9
-10
-10
-10
-11
-13
-14
-14
-14
-15
-10
-12
-12
-12
-12
7x10 37 1.3x10 -109 1.3x10 -149 1.3x10 -119 -79

2x10 6 1.1x10 -140 1.1x10 -180 11x10 -150 -110
-6
1x10 0 2.7x10 -146 2.7x10 -186 2.7x10 -156 -116
-6
-13
-14
-14
-14
-15
-17
-18
-18
-18
-19
-14
-16
-16
-16
-16
NOTE: Numbers in table rounded off
Power received (P
r
) '
E
2
480B
2
c
2
f

Watts
(dBW)
volts/meter 132.8 12.8 -47.2
µv/meter 12.8 -107.2 -167.2
mW
(dBm)
volts/meter 162.8 42.8 -17.2
µv/meter 42.8 -77.2 -137.7

FIELD STRENGTH APPROACH
To account for the impedance difference, the antenna factor (AF) is defined as: AF=E/V, where E is field intensity
which can be expressed in terms taking 377 ohms into account and V is measured voltage which can be expressed in terms
taking 50 ohms into account. Details are provided in Section 4-12.
POWER DENSITY APPROACH
To account for the impedance difference , the antenna’s effective capture area term, A relates free space power
e
density P with received power, P , i.e. P = P A . A is a function of frequency and antenna gain and is related to AF
D r r D e e
as shown in Section 4-12.
SAMPLE CALCULATIONS
Section 4-2 provides sample calculations using power density and power terms from Table 1 and Table 2, whereas
Section 4-12 uses these terms plus field intensity and voltage terms from Table 1 and Table 2. Refer the examples in
Section 4-12 for usage of the conversions while converting free space values of power density to actual measurements with
a spectrum analyzer attached by coaxial cable to a receiving antenna.
Conversion Between Field Intensity (Table 1) and Power Received (Table 2).
Power received (watts or milliwatts) can be expressed in terms of
field intensity (volts/meter or µv/meter) using equation [3]:
[3]
or in log form: 10 log P = 20 log E + 10 log G - 20 log f + 10 log (c /480B ) [4]
r

300 49.5 169.5 1800 32.5 62.5 135.5
200 46.0 166.0 800 29.0 59.0 132.0
100 40.0 160.0 200 23.0 53.0 126.0
70 36.9 156.9 98 19.9 49.9 122.9
50 34.0 154.0 50 17.0 47.0 120.0
30 29.5 149.5 18 12.5 42.5 115.5
20 26.0 146.0 8 9.0 39.0 112.0
10 20.0 140.0 2 3.0 33.0 106.0
7 16.9 136.9 0.8 0 29.9 102.9
5 14.0 134.0 0.5 -3.0 27.0 100.0
3 9.5 129.5 0.18 -7.4 22.5 95.6
2 6.0 126.0 0.08 -11.0 19.0 92.0
1 0 120.0 0.02 -17.0 13.0 86.0
0.7 -3.1 116.9 9.8 x 10 -20.1 9.9 82.9
0.5 -6.0 114.0 5.0 x 10 -23.0 7.0 80.0
0.3 -10.5 109.5 1.8 x 10 -27.4 2.6 75.6
0.2 -14.0 106.0 8.0 x 10 -31.0 -1.0 72.0
0.1 -20.0 100.0 2.0 x 10 -37.0 -7.0 66.0
-3
-3
-3
-4
-4
.07 -23.1 96.9 9.8 x 10 -40.1 -10.1 62.9
.05 -26.0 94.0 5.0 x 10 -43.0 -13.0 60.0
.03 -30.5 89.5 1.8 x 10 -47.4 -17.7 55.6
.02 -34.0 86.0 8.0 x 10 -51.0 -21.0 52.0
.01 -40.0 80.0 2.0 x 10 -57.0 -27.0 46.0
-5
-5

-9
-10
-10
7 x 10 -84.1 36.9 9.8 x 10 -100.1 -70.1 2.9
-5
5 x 10 -86.0 34.0 5.0 x 10 -103.0 -73.0 0
-5
3 x 10 -90.5 29.5 1.8 x 10 -107.4 -77.4 -4.4
-5
2 x 10 -94.0 26.0 8.0 x 10 -111.0 -81.0 -8.0
-5
1 x 10 -100.0 20.0 2.0 x 10 -117.0 -87.0 -14.0
-5
-11
-11
-11
-12
-12
7 x 10 -104.1 16.9 9.8 x 10 -120.1 -90.1 -17.1
-6
5 x 10 -106.0 14.0 5.0 x 10 -123.0 -93.0 -20.0
-6
3 x 10 -110.5 9.5 1.8 x 10 -127.4 -97.4 -24.4
-6
2 x 10 -114.0 6.0 8.0 x 10 -131.0 -101.0 -28.0
-6
1 x 10 -120.0 0 2.0 x 10 -137.0 -107.0 -34.0
-6
-13
-13