diff --git a/zk/Foundation_licence_conditions.md b/zk/Foundation_licence_conditions.md
index a3428de..f742067 100644
--- a/zk/Foundation_licence_conditions.md
+++ b/zk/Foundation_licence_conditions.md
@@ -24,7 +24,7 @@ tags: [radio, ham-study]
 - in cases of reported interference, ask you to log the transmissions you have
   made over a specified time period
 
-## The following types of activity are not permitteed
+## The following types of activity are not permitted
 
 - Business or commercial activity
 - The use of encryption or codes, other than Morse Code
diff --git a/zk/Impedance.md b/zk/Impedance.md
new file mode 100644
index 0000000..62fe281
--- /dev/null
+++ b/zk/Impedance.md
@@ -0,0 +1,20 @@
+---
+tags: [electricity, radio, ham-study]
+---
+
+Impedance is not the same thing as [resistance](./Resistance.md) although it is
+a related concept.
+
+Resistance is opposition to the flow of current **in both
+[AC and DC](./Voltage_sources.md)** circuits. It is a quantity that **remains
+constant**. Resistance in a circuit causes electrical energy to be dissipated as
+heat.
+
+In contrast, impedance is not constant. It changes based on the frequency of the
+current and it only applies to AC circuits.
+
+Impedance is calculated as the resistance _plus_ the frequency-dependent
+opposition to alternating current from components such as capacitors and
+inductors.
+
+> Both resistance and impedance are measured in Ohms
diff --git a/zk/Impedance_and_antennas.md b/zk/Impedance_and_antennas.md
new file mode 100644
index 0000000..666dbb5
--- /dev/null
+++ b/zk/Impedance_and_antennas.md
@@ -0,0 +1,34 @@
+---
+tags: [radio, ham-study]
+---
+
+Antennas have [impedance](./Impedance.md) as a property. The impedance of an
+antenna is affected by its size and positioning as well as the wavelength of the
+signal it is transmitting and receiving.
+
+The impedance of the **feed point** (the point where the [feeder](./Feeders.md)
+meets the antenna) should match the impedance of the feeder and transmitter.
+
+If the impedance of the antenna does not match that of the feeder, some energy
+will be reflected back down the antenna to the feeder creating "standing waves".
+
+Standing waves are formed when a wave encounters a boundary between two
+different mediums which allows the wave to
+[reflect](./Radio_wave_propagation.md). Although one source generated this wave,
+it results in two travelling waves, one outgoing and one reflected. This creates
+an interference pattern the same as two waves overlapping from two separate
+sources.
+
+A **Standing Wave Ratio** meter (SWR) tests whether there is a correct match
+between the impedance of the feeder and the antenna.
+
+A 1:1 ratio is ideal. Anything higher that 2:1 is a mismatch which may degrade
+the transmitter.
+
+In cases of mismatch, you can use an **Antenna Matching Unit** (AMU) (there are
+manual and automatic variants). This is also known as an **Antenna Tuning Unit**
+(ATU). This device reduces the SWR to ensure there is no damage to the
+transmitter.
+
+An AMU is mostly required when you are working "multi-band", that is: using a
+single antenna from multiple frequency bands.
diff --git a/zk/Impedence.md b/zk/Impedence.md
deleted file mode 100644
index e69de29..0000000