Antenna Types and Gain

What it is

An antenna turns the radio signal into a radiated wave and shapes where that wave goes.

An antenna does not create power. It only focuses the power that the radio gives it, so a higher gain means a narrower beam.

The right antenna depends on the job. A room needs wide coverage, while a long link needs a tight beam aimed at one point.

How it works

Antennas are described by their pattern, their gain, their beamwidth and their polarisation.

Main antenna types

An omnidirectional antenna sends energy in all horizontal directions. Raising its gain squeezes the pattern flatter, so it reaches further sideways but covers less up and down. A tall building next to a high gain omni can sit in a blind zone.

A sector antenna covers a wedge. Several sectors on one mast can cover a full circle, and each sector can use its own channel.

Gain and beamwidth

Gain is measured in dBi, which compares the antenna with an ideal radiator that sends equally in every direction. A higher gain gives a stronger signal in the main direction and a narrower beam. Beamwidth is the angle where the signal is still useful.

Every +3 dB is double the power, and every +10 dB is ten times the power. The gain is the same for sending and receiving, so a good antenna helps in both directions.

Polarisation

Waves are polarised in a plane, such as vertical or horizontal. The antennas at both ends should use the same polarisation. If one is vertical and the other horizontal, a lot of signal is lost.

A dual polarised antenna has two feeds at right angles. It suits a two chain card, because each chain uses one polarisation.

Line of sight and the Fresnel zone

A long link needs more than a straight line between the two antennas. The signal also uses an oval area around that line, called the Fresnel zone. If trees or buildings cut into that area, the signal weakens even when you can see the far end.

Raising the antennas is the usual cure. The zone is wider at lower frequencies and at greater distance.

Losses and EIRP

The signal loses strength in the cable and connectors before it reaches the antenna. The legal limit is set on the radiated power, called EIRP (effective isotropic radiated power):

EIRP = transmit power - cable loss + antenna gain

For example, 20 dBm from the radio, 2 dB of cable loss and a 10 dBi antenna give 28 dBm EIRP. Many countries limit EIRP, so a high gain antenna can force you to reduce transmit power.

The wireless interface has an antenna gain setting where you tell RouterOS what antenna is attached:

/interface wireless set wlan1 antenna-gain=10

With a country regulation mode, RouterOS can use this value to keep the output inside the legal limit. The details are in the country topic.

Free space path loss

A signal weakens with distance and with frequency. Doubling the distance costs about 6 dB, and a higher frequency loses more than a lower one over the same path. This is why 5 GHz needs more gain or less distance than 2.4 GHz for the same result.

Choosing an antenna

Think in this order:

• Decide the coverage: a point, an area or a single far site.
• Pick the type: omni, sector, directional or dish.
• Check the legal limit and reduce transmit power if needed.
• Match the polarisation and the number of chains.
• Plan the mounting height and the clear path.