diff --git a/zk/Antennas.md b/zk/Antennas.md
index 57b41e2..7fd945a 100644
--- a/zk/Antennas.md
+++ b/zk/Antennas.md
@@ -103,3 +103,32 @@ Some example calculations:
 | 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.
+
+## Antenna polarisation
+
+**Polarisation** means whether the antenna is positioned on a vertical or
+horizontal plane.
+
+Of the antennas covered at foundation level, only dipoles and Yagis can be
+positioned at variable polarities.
+
+VHF and UHF are received most effectively when the transmitter and receiver have
+the same antenna polarisation.