diff --git a/zk/Difference_between_frequency_and_bandwidth.md b/zk/Difference_between_frequency_and_bandwidth.md
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+---
+tags: [radio, ham-study]
+---
+
+# The difference between frequency and bandwidth
+
+Think of a radio transmission as being like a lane on the motorway. The
+frequency is the specific motorway that you are travelling on. A frequency like
+95.8 is the "center frequency".
+
+In fact, a licensed radio station has a range of frequencies at its disposal.
+These other frequencies exist around the center frequency. The total available
+frequencies constitute the station's bandwidth.
+
+The different frequencies are like different lanes on the motorway. Not all of
+the data will be transmitted on the center frequency. Nearby frequencies will
+also be used such as 95.7, for example.
+
+When you tune to 95.8 you are not only tuning to that frequency, you are tuning
+to it as the center frequency encompassing the other frequencies.
+[FM](./Frequency_modulation.md) has 200kHz bandwidth per station whereas AM has
+much less bandwidth resulting in poorer sound quality. An FM station can use the
+additional frequencies to transmit sound in stereo, for example.
+
+A radio receiver is designed to process the data accross the frequency spectrum
+for the station and unify it into a reconstruction of the original audio signal.
+
+The more information you want to transmit, the more "lanes" or bandwidth you
+require.
diff --git a/zk/Electrical_energy_equations.md b/zk/Electrical_energy_equations.md
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+---
+tags: [radio, ham-study]
+---
+
+# Electrical energy equations
+
+## Voltage, current, resistance
+
+![](../img/voltage-equation-ham.png)
+
+### Voltage
+
+$$
+V = I \times R
+$$
+
+### Current
+
+$$
+I = \frac{V}{R}
+$$
+
+### Resistance
+
+$$
+R = \frac{V}{I}
+$$
+
+## Power, voltage, current
+
+![](../img/power-equation-ham.png)
+
+### Power
+
+$$
+P = V \times I
+$$
+
+### Voltage
+
+$$
+V = \frac{P}{I}
+$$
+
+### Current
+
+$$
+I = \frac{P}{V}
+$$
diff --git a/zk/Radio_transmitters_and_receivers.md b/zk/Radio_transmitters_and_receivers.md
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+---
+tags: [radio, ham-study]
+---
+
+# Radio transmitters and receivers
+
+## Transmitters
+
+![The radio transmission process](../img/radio-transmission-process.png)
+
+Radio transmission comprises four stages:
+
+1. Audio stage or "amplification"
+
+- The weak signals from the microphone are amplified
+
+2. Frequency generation via an oscillator
+
+- The [carrier wave](./Frequency_modulation.md) is generated
+
+3. Modulation
+
+- The carrier wave and audio signal are combined
+
+4. RF power amplification
+
+- The combined signal is increased and fed through to the antenna
+
+The following are important considerations:
+
+> Incorrect setting of the oscillator can result in operation outside of the
+> amateur band and cause interference to other users
+
+> The microphone gain should not be set too loud as this will cause distortion.
+> Furthermore, too loud a signal causes **over-modulation** on AM and
+> **over-deviation** on FM which again could cause inteference.
+
+> The RF power amplifier must be connected to a correctly matched antenna to
+> work properly. If not, damage can be caused to the transmitter.
+
+## Receivers
+
+![The radio reception process](../img/radio-receiver-process.png)
+
+Radio reception comprises three stages:
+
+1. Tuning/RF amplifier
+
+- The receiver is tuned to the required frequency and the weak incoming signal
+  is amplified so that it can be used
+
+2. Demodulator
+
+- Extracts the original audio signal from the modulated radio signal, separating
+  the signal from the carrier.
+- Each type of modulation type (FM, AM, SSB etc) has its own demodulator
+
+3. Audio amplifier
+
+- Amplifies the recovered signal and feeds it to a loudspeaker or headphones
diff --git a/zk/Sudoers_file.md b/zk/Sudoers_file.md
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+---
+tags:
+  - Linux
+  - procedural
+---
+
+# Sudoers file
+
+Control users' `sudo` priveleges and against which services/programmes they can
+run `sudo`.
+
+Access with `sudo visudo`.
+
+Example:
+
+```
+deploy ALL=(ALL) NOPASSWD: /bin/systemctl daemon-reload, /bin/systemctl restart eolas-api.service
+```
+
+This gives the user `deploy` the ability to run certain restricted `systemctl`
+commands with no password. The second two limit him to running `systemctl`
+against specific `systemd` services, not all of them.
diff --git a/zk/Timed_systemd_units.md b/zk/Timed_systemd_units.md
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+---
+tags:
+  - Linux
+  - procedural
+  - systemd
+---
+
+# Timed `systemd` units
+
+In order to use [systemd](./systemd.md) in the manner of [cron](./Cron.md) (but
+with added benefits, such as being able to set the user to run as, environment
+variables and so on), you first create a timer template, e.g.:
+
+```
+# everyhour@.timer
+[Unit]
+Description=Run %i every hour
+
+[Timer]
+OnCalendar=*-*-* *:00:00
+Persistent=true
+Unit=%i.service
+
+[Install]
+WantedBy=timers.target
+```
+
+You can then pass in specific units using this template to `systemctl`.
+
+As an example, this unit runs a simple bash script that pings a server:
+
+```
+# ping-eolas-api.service
+
+[Unit]
+Description=Ping Eolas API to check it is up
+
+[Service]
+User=thomas
+Group=thomas
+Type=oneshot
+ExecStart=/home/thomas/self-host/scripts/ping_eolas-api.sh
+```
+
+Then when enabling the service, do the usual:
+
+```sh
+systemctl daemon-reload
+```
+
+But when enabling/starting, invoke the timer template, viz:
+
+```sh
+systemctl enable everyhour@ping-eolas-api.timer
+systemctl start everyhour@ping-eolas-api.timer
+```
+
+The schema is:
+
+```
+[timer_template_name][service_name].template
+```
diff --git a/zk/Whats_running_on_port_x.md b/zk/Whats_running_on_port_x.md
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index 0000000..e69de29