Enter the Tx Power, Tx Gain, Rx Gain, Wavelength and the Distance to calculate the power received at the antenna.

The Friis transmission equation gives the power received by an antenna from another antenna that is transmitting a known amount of power at a distance under ideal conditions. The formula was derived by Herald T. Friis at Bell Labs in 1945.

**The Derivation of the Formula can be seen below:**

**Assumption:** Assume that the Transmitting antenna sends out a power (P_{t}). We assume that the transmit antenna is omni-directional, lossless and that the receive antenna is in the far field.

**Step 1:** The Power Density of the plane wave incident on the receive antenna which is at a distance ® from the transmitting antenna, can be calculated by:

**Step 2:** We assume that the transmit antenna has a gain (G_{t}), then the power density becomes:

**Step 3:** We assume that the receive antenna has an effective aperture of A_{e}. Then the power received by the receive antenna is (P_{r}):

**Step 4:** The Effective Antenna Aperture A_{e}, can also be defined as:

**Step 5:** The Final formula for the power received at the antenna can be denoted by the **Friis Transmission Equation**:

OR – If you prefer to use frequency over wavelength