| TECH INFO - USEFUL DATA - FIELD STRENGTH VS. RADIATED POWER |
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 Field Strength vs. Radiated Power
Calculations and tables for field strength vs. radiated power
The effective radiated power (ERP) presents the power which should be transmitted into an isotropic antenna to maintain the same field strength. By focussing ERP in a fixed direction an effect higher than actually delivered can be obtained in the given direction.
The below table shows how much ERP can be obtained in an antenna when the input power is 1, 10 and 100 watt.
The gain in the radiation direction is stated in 0, 3, 6, 10, 13, 16 and 20 dB. In the table just below the gain figure in dB it is written how much gain the stated dB figure has.
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 | This makes it easier to calculate if another input power is requested than the one stated.
Explanation of terms:
G: antenna gain [dB]
ERP: effective radiated power [W]
d: distance from antenna [m]
e: electric field strength [V/m]
ERP – Effective Radiated Power:
The effective radiated power represents the power that you would have to put into an antenna in order to obtain the same field strength.
An antenna with a gain of 3 dB concentrates the radiated power in a given direction so that the power density in this direction is 3 dB higher than it would have been using an dipole antenna.
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As a power increase of 3 dB is equal to the power being doubled, the effective radiated power for a 3 dB antenna is two times the power input of the antenna.
Below there are two examples showing the principle behind the table. The difference between the two examples is feeder loss.
Example 1:
The output of the transmitter is 10 watt and because there is no feeder loss this is also the input of the antenna. The antenna has 3 dB gain. At table look-up below 10 watt – 3dB the figure 20 is shown meaning that the effective radiated power (ERP) is 20 watt in the maximum direction of radiation.
Example 2:
Due to feeder loss, only half of the power leaving the transmitter arrives at the antenna input (20 W). This antenna has a gain of 6 dB (x4) so, the effective radiated power will be 80 W (in the direction of maximum radiation).
The table “field strength versus ERP” is created to show the field strength at a given ERP and the distance from the antenna. The formula behind the table is as follows:
- where e is field strength and d is the distance from the antenna.
The table is applicable for local EMC (electro magnetic compatibility) problems for instance. If it is the distance or ERP that are missing the formular can be shifted around. Below the formular shows the distance and ERP, respectively.
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