34 lines
1.5 KiB
Markdown
34 lines
1.5 KiB
Markdown
---
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tags: [radio, ham-study]
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---
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Antennas have [impedance](./Impedance.md) as a property. The impedance of an
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antenna is affected by its size and positioning as well as the wavelength of the
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signal it is transmitting and receiving.
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The impedance of the **feed point** (the point where the [feeder](./Feeders.md)
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meets the antenna) should match the impedance of the feeder and transmitter.
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If the impedance of the antenna does not match that of the feeder, some energy
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will be reflected back down the antenna to the feeder creating "standing waves".
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Standing waves are formed when a wave encounters a boundary between two
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different mediums which allows the wave to
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[reflect](./Radio_wave_propagation.md). Although one source generated this wave,
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it results in two travelling waves, one outgoing and one reflected. This creates
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an interference pattern the same as two waves overlapping from two separate
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sources.
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A **Standing Wave Ratio** meter (SWR) tests whether there is a correct match
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between the impedance of the feeder and the antenna.
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A 1:1 ratio is ideal. Anything higher that 2:1 is a mismatch which may degrade
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the transmitter.
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In cases of mismatch, you can use an **Antenna Matching Unit** (AMU) (there are
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manual and automatic variants). This is also known as an **Antenna Tuning Unit**
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(ATU). This device reduces the SWR to ensure there is no damage to the
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transmitter.
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An AMU is mostly required when you are working "multi-band", that is: using a
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single antenna from multiple frequency bands.
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