eolas/zk/Feeders.md

---
tags: [radio, ham-study]
---

**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")

![](../img/feeder-types.png)

### Coaxial cable

![](../img/coaxial-diagram.jpg)

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](./Ground.md) whereas the central conductor has a varying
voltage relative to ground.

### Twin feeder

![](../img/ladder-line-feeder.jpg)

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](./Voltage_sources.md), they swap
over their potential difference at each cycle, making them effectively
interchangeable.

![](../img/feeder-balance.png)

## 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).