134 lines
4.3 KiB
Markdown
134 lines
4.3 KiB
Markdown
---
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tags: [radio, ham-study]
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---
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> Antennas convert electrical signals into radio waves and radio waves into
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> electrical signals
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Different sized antennae are used for different
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[frequency](./Frequency_modulation.md) bands. In general, **longer wavelengths
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(hence lower frequencies) require larger antennas**.
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There are five antenna that you need to know for the exam:
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- the half wave dipole
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- the quarter wave
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- the five-eighth wave
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- the end-fed long wire
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- the Yagi (beam) antenna
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## The half wave dipole antenna
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Typically used by amateur operators.
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Its total length is equal to half a wavelength. This informs the length it needs
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to be in order to pick up a certain frequency.
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For example, if 15MHz has a wavelength of 20 meters, your dipole should be 10
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meters long (because 20/2 = 10).
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The signal from a dipole radiates as follows:
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## The quarter wave ground plane antenna
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This antenna is vertical and is omnidirectional, radiating the signal in all
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directions.
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It's length corresponds to 1/4 of a wavelength, hence used for VHF and UHF
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bands.
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The radials at the bottom form the ground plane that boosts the signal
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horizontally in addition to the vertical reach of the main part of the antenna.
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## The five-eighth wave ground plane antenna
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This antenna is also vertical and omnidirectional and used for VHF and UHF
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bands. It has a coil at the base in addition to the radials.
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## End-fed long wire
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The length is random compared to the other antennae. The length used determines
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the wavelengths that can be picked up and it needs to be matched to a specific
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frequency.
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The most important thing to remember about this antenna is that it is more
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susceptible to EM interference than other types.
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## Yagi antenna
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In contrast to the others, the Yagi is directional and focused in one direction
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- typically the direction of the station you are trying to pick up.
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It basically works as a dipole with a reflector behind it. This helps to push
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the waves in the horizontal direction of travel, assisted by the director spoke
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in front.
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### Gain and ERP
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Antennas like the Yagi have **gain**. This means they can increase the power
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that they receive from the transmitter via the [feeder](./Feeders.md).
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Gain is measured in decibels:
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| Antenna gain | Power muliplied by a factor of... |
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| ------------ | --------------------------------- |
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| 3dB | 2 |
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| 6dB | 4 |
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| 9dB | 8 |
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| 10dB | 10 |
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ERP is the **Effective Radiated Power**. This is the product of the power that
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the antenna is receiving from the feeder and the gain level.
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> ERP (Watts) = power feed x antenna gain
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Some example calculations:
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| Gain | Gain times | Power to antenna | ERP |
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| ---- | ---------- | ---------------- | --------- |
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| 3dB | x2 | 10 watts | 20 watts |
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| 6dB | x4 | 10 watts | 40 watts |
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| 9dB | x8 | 10 watts | 80 watts |
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| 10dB | x10 | 10 watts | 100 watts |
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### ERIP
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ERP is calculated relative to the half-wave dipole. This is the benchmark. The
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power that would be generated using this type of antenna.
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But, as noted above, the dipole has a distinct radiation pattern - diffusing in
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poles at right angles to the horizontal plane of the antenna.
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By being tied to the dipole, ERM is therefore not the most universal or
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objective unit of measurement.
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To remedy this, there is another unit: ERIP. This stands for **Effective
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Isotropic Radiated Power**. It's the same calculation but it uses a theoretical
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antenna that would radiate equally in all directions as the benchmark.
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Because of the equal radiation pattern in all directions, EIRP will always be
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higher than ERP. Roughly speaking, 10 Watts in EIRP would be 6 Watts in ERP.
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## Antenna polarisation
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**Polarisation** means whether the antenna is positioned on a vertical or
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horizontal plane.
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Of the antennas covered at foundation level, only dipoles and Yagis can be
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positioned at variable polarities.
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VHF and UHF are received most effectively when the transmitter and receiver have
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the same antenna polarisation.
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