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Feeder is the cable that connects the transmitter/receiver to the antenna.
As the EM energy travels through the feeder, some energy is lost as heat.
- The longer the feeder, the greater the loss
- The higher the frequency being transmitted, the greater the loss
This is why for VHF and UHF thicker, low-loss feeder is needed
Types of feeder
The two types of feeder you need to know for the exam:
- coaxial
- twin feeder (a.k.a "ladder line")
Coaxial cable
The inner conductor carries the signal. The screening keeps the signal within the inner cable preventing loss.
It is unbalanced because the outer shield is at ground potential whereas the central conductor has a varying voltage relative to ground.
Twin feeder
Here we have two wires with spacers maintaining seperation. The two wires carry equal and opposite signals.
Twin feeder is balanced because neither wire is grounded. Both have the same impedence to ground in a symmetrical fashion. If wire A is at +5V, wire B will be at -5V. Because of alternating current, they swap over their potential difference at each cycle, making them effeectively interchangeable.
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).