III. Wireless LAN


Antenna Theory, Gain & Bands

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What is 'gain'?

Many users of Wireless LAN (WiFi) equipment will have heard the word 'gain' in relation to the aerials (antennae) on your wireless router or access point. It is generally assumed that the more 'gain' an aerial provides, the stronger your signal will be and thus your coverage range will increase. Therefore you might assume that for maximum range, you should use the highest gain aerial you can find.

That is true to an extent, but of course, increased power doesn't appear from nowhere - an aerial is a passive device - it cannot increase the output of your router. If the router transmission power is, say, 100mW, it's always going to be 100mW, regardless of your aerials.

Note that we will refer to transmission range, but of course Wireless devices need to receive signals as well as transmit them, so the principals of wireless gain which apply to transmission range also apply to receiving sensitivity, i.e. the ability to receive signals from further away, so anywhere here we refer to transmission range, we equally mean receiver sensitivity.

It is true that a higher gain aerial will (or can) increase the range of a transmitted signal, however this 'gain' in range is achieved by reducing the radiation pattern (or sensitivity pattern) of the aerial, i.e. at the expense of spread. Put simply, given a fixed power output, you could choose to transmit the signal in all directions, or restrict the radiation pattern so that the signal is output in a far more restricted pattern. Therefore, when we refer to aerial gain, we are referring to the gain in a specific direction, compared to if the signal was to be transmitted in a perfectly spherical pattern.

Omni vs. Unidirectional

The two most common types of wireless aerials are Omnidirectional (all directions, 360 °) and Unidirectional (one direction, for example 50 ° in one direction). A unidirectional aerial which concentrates its signal in one direction will radiate almost nothing behind it:

    Omnidrectional Aerial Signal Spread    
    Unidrectional Aerial Signal Spread    

Torus & Doughnuts, Cones and Beam Angles

Most Wireless LAN (WiFi) aerials provide some gain, because no standard aerials have a spherical transmission pattern (known as an isotropic spread). An isotropic aerial would spread its signal in all directions evenly, providing no gain in any specific direction. For a omidirectional aerial, the standard aerials you get with most routers and access points, the beam pattern will be in the shape of a torus - a 'doughnut' shape - as shown in the diagram above (Mmmmmm, doughnuts!). The signal pattern radiates out in the X axis (H-plane) evenly in all directions, but reduces the higher/lower that you go so that very little signal goes straight up or down (the E-plane). It's not a perfect doughnut, it's normally a very round one. A unidirectional aerial radiates its signal in a cone shape, instead of a torus, as shown in the diagram, above right.

It is the aerial's design which dictates the signal's spread. All aerials will have a specification stating its horizontal beam; known as its H-plane. That would be 360 degrees (a complete circle) in the case of an omnidirectional aerial. Aerials may also state their vertical beam angle (the E-plane). From the above, you will see that if you increase the gain of an aerial of an omnidirectional aerial, you have to reduce either the vertical or horizontal beam angle (or both). For example 5dBi aerial might have a vertical beam angle of 40 °, whereas a 7dBi aerial might have a vertical beam angle of 22 °. By concentrating the signal in a reduced pattern, the signal becomes stronger in the covered area - that is 'gain'. In the diagram below you can see that the aerials with the beam concentrated in a narrower spread, the signal reaches further:

WiFi Aerials Gain Beam Pattern

Adjusting an aerial's gain is analogous to putting your finger over the end of a water hose. The same amount of water is flowing, but by restricting the outlet size with your finger the jet is more concentrated - the spray's spread is less, and so the jet will reach further.

In the diagrams below, you can see a 3D rendering of example beam patterns, with a cross section (slice) removed (marked with the red/green cross-hairs). In all examples, we have the same transmission power input but you can see the range increases as we limit the spread pattern.

WiFi Aerials Gain Beam Pattern H-Plane and E-Plane

Choosing and positioning your aerial

You can hopefully now see why you don't necessarily want to select the aerial with largest gain - it will limit your coverage pattern vertically which may be desirable in some scenarios, and not others. For example, in a single floor office, you don't need any signal transmitted up or down to adjacent floors, but in a 2-storey house, you probably do. You should also now see why aligning your aerials can make a real difference to your coverage and performance - i.e. your aerials don't necessarily want to be in the transitional 'straight up' orientation. Experimentation is always recommended.

Band Matching

Aerials have to be tuned to match the frequency of the transmission (e.g. 5Ghz). You don't need to do that yourself; they are designed and manufactured tuned for a particular band. Wavelength and frequency are directly proportional so a full wave matched aerial for 2.4Ghz is 125mm long but for 5.8Ghz is only 51mm (that doesn't mean your aerial will be that length; an aerial can be any multiple or division of a full wave, as long as it is tuned). If you are replacing your aerial, you must therefore ensure that it is suitable for the frequency band in use. Dual-band aerials, ones which can operate at both 2.4 and 5Ghz are also available.

Antenna Diversity & MIMO

Most modern routers and wireless access points have two or more aerials. There are two reasons for this. Firstly, the 802.11n (and later) wireless systems use a method called MIMO - Multiple In and Multiple Out. This enables them to transmit and receive multiple signals simultaneously, thus increasing performance.

The second reason for having more than one aerial is to provide diversity. Wireless LAN (WiFi) signals are often not received within line of sight and signals bounce off walls, travel through ceilings and have to pass by or through various materials when travelling between the transmitter and receiver. All of this can introduce interference in the form of phase shifts, attenuation and distortion which affect the quality of the signal, even if the signal strength itself remains strong. Diversity is a method of countering this interference by using more than one aerial so that the signal can be received from different positions. i.e. by observing the same received signal from two or more locations, you can determine any changes which might have taken place in transmission by comparing the differences and thus deduce a more accurate signal.

Some wireless devices also have multiple aerials because they operate in more than one band (e.g. 2.4Ghz and 5Ghz) and use separate aerials (antennae) for each band. Other routers use dual-band aerials so the same aerial can simultaneously serve both transmitters/receivers.

Local Regulation

This page is provided as a guide only and is not intended to provide specific guidance on the permitted power output and gain permitted in all geographic areas. It is the users' responsibility that they comply with all local regulation and this document should not be used as the basis for confirming compliance - please check your own local and up-to-date regulations for permitted band-plans, equipment certification and permitted power output.

You may also be interested in our article on The difference between dB, dBi and dBm.

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