RF Wireless Characteristics · Topic 2 of 4
Antenna Theory
Learn how antenna gain, beamwidth, polarisation and radiation patterns decide coverage, and how to choose omni, sector or directional antennas for a link.
- 4 min
- Gain is measured in dBi, and it comes from focusing energy, not from extra power.
- Higher gain means a narrower beam.
- An omni antenna covers all around, a sector covers a wedge and a directional antenna covers a narrow beam.
What it is
An antenna turns electrical signal into radio waves and back again, and it shapes where those waves go.
An antenna does not create power. It focuses the power the radio gives it, so a gain in one direction means less signal in others.
Choosing the right antenna is often the cheapest way to improve a wireless network.
How it works
Gain, beamwidth, polarisation and pattern together describe an antenna.
Gain
Gain is how much an antenna concentrates energy in its main direction. It is measured in dBi, against an ideal antenna that radiates equally in all directions.
Higher gain makes a narrower beam. A very high gain antenna reaches far in one direction but covers a small area.
Beamwidth
Beamwidth is the angle over which the antenna radiates most of its power. It is given for the horizontal plane and for the vertical plane.
An omnidirectional antenna has a wide horizontal beamwidth. Raising its gain squeezes the pattern flat like a doughnut, so the vertical beamwidth becomes narrow.
A high gain omni antenna mounted high on a building may miss clients directly below it, because the vertical beam is too narrow.
Antenna types
| Type | Pattern | Typical use |
|---|---|---|
| Omnidirectional | All around the antenna | Indoor ceiling access points and open areas |
| Sector | A wide wedge, often 60 to 120 degrees | Covering one side of a tower or a large area in sections |
| Directional (panel, yagi, dish) | A narrow beam | Point to point links and long distance client links |
Several sectors together can cover a full circle with more gain than one omni antenna. Each sector then needs its own channel to avoid interference.
Polarisation
Polarisation is the orientation of the radio wave, usually vertical or horizontal. Two antennas talk best when they use the same polarisation.
A mismatch between horizontal and vertical causes a large signal loss. Many MikroTik directional units carry two chains, one vertical and one horizontal, so the link works at either orientation and can use both at once.
Radiation pattern, lobes and front to back
The radiation pattern shows where the signal goes. Besides the main lobe, an antenna has small side lobes and some signal behind it.
The front to back ratio compares the strength in front with the strength behind. A good directional antenna has a high ratio, so it ignores interference from behind it.
Line of sight and the Fresnel zone
A long link needs more than a visible path. The radio wave spreads in an oval area around the straight line, called the Fresnel zone.
Obstacles inside this zone weaken the link even if the straight line looks clear. Aim to keep most of the zone free. Raising the antennas often solves it.
Cable loss and connectors
Cable and connectors lose signal. A long cable between the radio and the antenna reduces the power that reaches the antenna and the signal it sends back.
Keep cables short and use the right type. Count the cable loss in the link budget.
Telling the router about the antenna
Where the antenna is external, tell RouterOS the gain so that the power limit is applied correctly.
/interface wireless set wlan1 antenna-gain=12
/interface wireless align interface=wlan1antenna-gainis the antenna gain in dBi.- The alignment tool on the legacy wireless package helps point a client at an access point by showing the signal while you move the antenna.
What to remember for the exam
Seven facts cover antenna questions.
- Gain is measured in dBi, and it comes from focusing energy, not from extra power.
- Higher gain means a narrower beam.
- An omni antenna covers all around, a sector covers a wedge and a directional antenna covers a narrow beam.
- Both ends of a link should use the same polarisation.
- A dual chain unit has one vertical and one horizontal polarisation.
- The front to back ratio shows how well an antenna ignores signals from behind.
- Keep most of the Fresnel zone clear on long links.
A very high gain omni antenna can leave a coverage hole below it, because its vertical beam is narrow.
Common mistakes
These four errors appear in antenna questions.
Thinking gain adds power. The antenna only focuses the same power into a smaller area.
Mixing polarisations. A vertical antenna facing a horizontal one gives a poor link.
Choosing the highest gain by default. A narrow beam is hard to aim and does not cover nearby clients.
Ignoring cable loss. A long cable can cancel the benefit of a better antenna.
Check yourself
Work through these three questions.
Why does a higher gain antenna have a narrower beam?
Gain comes from focusing the same power into a smaller area. A narrower beam means more signal in one direction and less in others.
Which antenna type suits a long point to point link, and why?
A directional antenna suits it. It has a narrow beam, so it sends a strong signal to the far end.
What does matching polarisation mean for a link?
Both antennas should use the same orientation, vertical or horizontal. A mismatch causes a large signal loss.
Topic 2 of 4 in RF Wireless Characteristics
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