additional ham notes

zk/Feeders.md

@@ -46,9 +46,23 @@ interchangeable.
 
 ![](../img/feeder-balance.png)
 
+### Better way to understand balanced/unbalanced
+
+> Twin feeder has equal and opposite signals on each wire. Coax has the full
+> signal on the inner conductor.
+
 ## Why balanced/unbalanced feeder matters
 
 Some antennas are suited to unbalanced feeder whilst others are suited to
 balanced feeder. If you want to use an unbalanced feeder with a balanced antenna
 (or vice versa) you can do so, but this requires a Balun (balanced-to-unbalanced
 transformer).
+
+## Feeder loss
+
+All feeder cables exhibit loss because some of the RF energy is converted to
+heat by the resistance of the feeder. This loss applies on transmit and receive.
+
+Longer feeders have greater loss. The loss increases with frequency.
+
+For this reason, low loss feeders should be chosen for VHF aand UHF operation.

zk/Frequency_modulation.md

@@ -12,6 +12,11 @@ A radio signal comprises two parts:
 The process of adding information (an audio signal) to a carrier is called
 **modulation**.
 
+> There are two simple ways of modulating a carrier. The first is to vary the
+> amplitude of the carrier _in time with the audio signal_. This is called
+> 'Amplitude Modulation'. The other way is to vary the frequency of the carrier
+> _in time with the audio signal_. This is called 'Frequency Modulation'.
+
 ## Amplitude modulation (AM)
 
 The message is combined with a carrier on a set frequency.
@@ -22,7 +27,7 @@ varies depending on the volume of the audio signal.
 ## Frequency modulation (FM)
 
 Again the audio signal is combined with a carrier on a set frequency however the
-amplitude of the resulting signal remains constant. The freequency changes
+amplitude of the resulting signal remains constant. The frequency changes
 relative to the volume of the audio signal. When the volume of the audio signal
 is louder, the waves are closer together. In other words, the frequency of the
 waves per second increases.

zk/Radio_transmitters_and_receivers.md

@@ -49,7 +49,7 @@ Radio reception comprises three stages:
 - The receiver is tuned to the required frequency and the weak incoming signal
   is amplified so that it can be used
 
-2. Demodulator
+2. Demodulator (or "detector")
 
 - Extracts the original audio signal from the modulated radio signal, separating
   the signal from the carrier.
@@ -58,3 +58,6 @@ Radio reception comprises three stages:
 3. Audio amplifier
 
 - Amplifies the recovered signal and feeds it to a loudspeaker or headphones
+
+> The demodulator must match the the modulation used to create the signal, e.g.
+> AM, FM or SSB (USB or LSB).