68 lines
2.1 KiB
Markdown
68 lines
2.1 KiB
Markdown
---
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tags: [radio, ham-study]
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---
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**Feeder** is the cable that connects the transmitter/receiver to the antenna.
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As the EM energy travels through the feeder, some energy is lost as heat.
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- The longer the feeder, the greater the loss
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- The higher the frequency being transmitted, the greater the loss
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> This is why for VHF and UHF thicker, low-loss feeder is needed
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## Types of feeder
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The two types of feeder you need to know for the exam:
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- coaxial
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- twin feeder (a.k.a "ladder line")
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### Coaxial cable
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The inner conductor carries the signal. The screening keeps the signal within
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the inner cable preventing loss.
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It is **unbalanced** because the outer shield is at
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[ground potential](./Ground.md) whereas the central conductor has a varying
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voltage relative to ground.
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### Twin feeder
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Here we have two wires with spacers maintaining seperation. The two wires carry
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equal and opposite signals.
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Twin feeder is balanced because neither wire is grounded. Both have the same
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impedence to ground in a symmetrical fashion. If wire A is at +5V, wire B will
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be at -5V. Because of [alternating current](./Voltage_sources.md), they swap
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over their potential difference at each cycle, making them effectively
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interchangeable.
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### Better way to understand balanced/unbalanced
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> Twin feeder has equal and opposite signals on each wire. Coax has the full
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> signal on the inner conductor.
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## Why balanced/unbalanced feeder matters
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Some antennas are suited to unbalanced feeder whilst others are suited to
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balanced feeder. If you want to use an unbalanced feeder with a balanced antenna
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(or vice versa) you can do so, but this requires a Balun (balanced-to-unbalanced
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transformer).
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## Feeder loss
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All feeder cables exhibit loss because some of the RF energy is converted to
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heat by the resistance of the feeder. This loss applies on transmit and receive.
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Longer feeders have greater loss. The loss increases with frequency.
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For this reason, low loss feeders should be chosen for VHF aand UHF operation.
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