2026-01-26 19:24:52 +00:00
|
|
|
---
|
|
|
|
|
tags: [radio, ham-study]
|
|
|
|
|
---
|
|
|
|
|
|
|
|
|
|
> Antennas convert electrical signals into radio waves and radio waves into
|
|
|
|
|
> electrical signals
|
|
|
|
|
|
|
|
|
|
Different sized antennae are used for different
|
|
|
|
|
[frequency](./Frequency_modulation.md) 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
|
|
|
|
|
|
2026-01-27 19:23:48 +00:00
|
|
|
## The half wave dipole antenna
|
2026-01-26 19:24:52 +00:00
|
|
|
|
2026-01-27 19:23:48 +00:00
|
|
|
|
|
|
|
|
|
|
|
|
|
Typically used by amateur operators.
|
2026-01-26 19:24:52 +00:00
|
|
|
|
|
|
|
|
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
|
2026-01-27 19:23:48 +00:00
|
|
|
meters long (because 20/2 = 10).
|
2026-01-26 19:24:52 +00:00
|
|
|
|
|
|
|
|
The signal from a dipole radiates as follows:
|
|
|
|
|
|
2026-01-27 19:23:48 +00:00
|
|
|
|
|
|
|
|
|
|
|
|
|
## 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](./Feeders.md).
|
|
|
|
|
|
|
|
|
|
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 |
|
