1.8 KiB
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Antennas like the Yagi have gain. This means they can increase the power that they receive from the transmitter via the feeder.
Gain is measured in decibels:
| Antenna gain | Power muliplied by a factor of... |
|---|---|
| 3dB | 2 |
| 6dB | 4 |
| 9dB | 8 |
| 10dB | 10 |
ERP is the Effective Radiated Power. This is the product of the power that the antenna is receiving from the feeder and the gain level.
ERP (Watts) = power feed x antenna gain
Some example calculations:
| Gain | Gain times | Power to antenna | ERP |
|---|---|---|---|
| 3dB | x2 | 10 watts | 20 watts |
| 6dB | x4 | 10 watts | 40 watts |
| 9dB | x8 | 10 watts | 80 watts |
| 10dB | x10 | 10 watts | 100 watts |
ERIP
ERP is calculated relative to the half-wave dipole. This is the benchmark. The power that would be generated using this type of antenna.
But, as noted above, the dipole has a distinct radiation pattern - diffusing in poles at right angles to the horizontal plane of the antenna.
By being tied to the dipole, ERM is therefore not the most universal or objective unit of measurement.
To remedy this, there is another unit: ERIP. This stands for Effective Isotropic Radiated Power. It's the same calculation but it uses a theoretical antenna that would radiate equally in all directions as the benchmark.
Because of the equal radiation pattern in all directions, EIRP will always be higher than ERP. Roughly speaking, 10 Watts in EIRP would be 6 Watts in ERP.