How tightly the antenna concentrates the energy it is given - not how much energy it produces.

An antenna is passive. It radiates exactly what the transmitter supplies, shaped into a pattern. Gain expresses how much stronger the signal is in the preferred direction compared with a reference that radiates equally everywhere.

So higher gain means a longer reach in one direction and less coverage in others. A high-gain omnidirectional antenna, for example, flattens the doughnut - reaching further horizontally while covering less above and below, which is a real problem when mounted on a high ceiling above the users.

That is why 'more gain' is not automatically better. For a room, a modest gain with a broad pattern covers it properly. For a link to a building a kilometre away, high gain is exactly what is needed.

Regulations also limit total radiated power, so beyond a point extra antenna gain must be offset by reducing transmitter power - the antenna shapes the coverage rather than increasing the total.

Omnidirectional for central positions, sector for edges and corners, directional for links and specific targets, downtilt for covering ground from height.

An omnidirectional antenna mounted centrally in an open area gives even coverage around it. It is the default for indoor access points and for a mast in the middle of a yard.

A sector antenna covers an arc - typically 60, 90 or 120 degrees - and belongs at the edge of the area it serves. Three sectors from one mast cover a full circle with more gain and less mutual interference than one omnidirectional.

Directional antennas concentrate along a line. Patch antennas suit medium distances and covering a specific area from one end; yagi and dish types suit long point-to-point links.

Downtilt is a variation rather than a type: a pattern angled below the horizontal, so a high-mounted antenna covers the ground near it instead of radiating over the heads of the people it is meant to serve.

Because a mismatch between the two ends of a link costs a large amount of signal, and nothing about the installation looks wrong.

Radio waves are polarised - the orientation of the electric field is vertical, horizontal or circular depending on the antenna. A receiving antenna aligned with the transmitting one captures the energy efficiently; one at ninety degrees to it captures very little.

On a point-to-point link, getting this wrong can cost twenty decibels or more, turning a well-engineered link into a marginal one. The equipment reports a poor signal and everything else looks correct.

Many modern systems use dual polarisation deliberately, transmitting two independent streams on the two orientations to double capacity over the same path. Both ends must then be matched and correctly oriented.

Mounting is where it goes wrong: a panel antenna rotated ninety degrees to fit a bracket changes its polarisation. Check the orientation marking rather than the aesthetics of the fixing.

Enough to matter, and enough that a poor cable choice can waste all the gain the antenna provided.

Coaxial cable loses signal per unit length, and the loss increases with frequency - so the same cable that is acceptable at 2.4 GHz is much worse at 5 GHz and worse again at 6 GHz.

A long run of thin cable can easily lose more than the antenna gains, which means the installation performs worse than the integrated antenna it replaced. This is one of the most common and most avoidable errors.

The remedies are to keep runs short, use low-loss cable of adequate diameter, and minimise connectors, each of which adds loss.

The better answer is usually to move the radio rather than extend the cable - mount the access point at the antenna and run Ethernet, which loses nothing over 90 metres.

Calculate the loss for the specific cable and length before ordering. Manufacturers publish loss per metre by frequency.

Yes. Multiple-input systems need every element connected, and leaving one unused degrades throughput even where the signal appears strong.

Modern equipment uses several antennas simultaneously to send independent data streams over the same channel. The number of streams achievable is limited by the number of usable antennas.

Leaving one disconnected reduces the streams available, so the link negotiates a lower rate. The signal strength reading may still look good, which is why the fault is usually attributed to something else.

An unterminated connector can also affect the transmitter, and on outdoor equipment it is an obvious water ingress path.

Spacing matters too. The antennas need physical separation to receive sufficiently different versions of the signal for the technique to work - mounting several elements tightly together reduces the benefit.

Where fewer antennas are genuinely intended, terminate the unused connectors properly and expect the corresponding reduction in throughput.

Yes, and it is one of their most useful applications - shaping coverage away from sources and neighbours rather than trying to overpower them.

Interference is a matter of signal relative to noise. Increasing power raises both if the noise comes from the same direction. Shaping the pattern raises the wanted signal without raising the unwanted one.

A directional antenna pointed at the area to be served rejects signals arriving from other directions - so a link across a busy site can be made reliable by narrowing the beam rather than shouting louder.

Downt: an antenna angled downward from a mast covers the ground and radiates far less toward the horizon, which both improves local coverage and reduces interference with distant installations.

Sector antennas in a multi-tenant building can contain coverage within a floor or a tenancy, reducing mutual interference between networks.

Surveying to establish where the interference comes from is the prerequisite - shaping the pattern only helps if the wanted and unwanted signals arrive from different directions.

Weatherproofing at the connector, ultraviolet-stable materials, wind loading, and surge protection on the feed.

The connector is the usual failure. Water entering a coaxial connector wicks along the cable, raises the loss and eventually destroys it, and the degradation is gradual. Proper weatherproofing with a tape system, and a drip loop so water runs off before reaching the connector, are the standard measures.

Radomes and housings must be ultraviolet-stable or they become brittle and crack within a few years, admitting water.

Wind loading on a large antenna is substantial and must be part of the mounting assessment - and movement in wind degrades a narrow-beam link even without structural failure.

Surge protection belongs on any outdoor antenna feed, particularly on masts. An arrestor at the point of entry, bonded to a proper earth, protects the radio and everything behind it.

Corrosion at coastal and industrial sites attacks connectors and fixings, so stainless hardware and periodic inspection are worth specifying from the start.