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