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Setting up Indoor Hot Spots

Posted by John Barker on January 06, 2015 0 Comments

Most hot spots are installed inside buildings. Coffee bars and restaurants alone will account for a few tens of thousands of hot spots. A common problem when trying to connect to hot spots is that the signal to noise ratio is poor which reduces the bandwidth, or in the worst case, an IP cannot be obtained. It is clear that many access points are installed without regard to the RF propagation characteristics of the antenna. In a lot of cases domestic access points are used such as the Linksys models. Linksys access points are popular because software (Open-WRT; DD-WRT) can be installed on them providing many hot spot features. Linksys devices are low cost because they have a low power output (+15dBm): the cheap standard rubber duck antenna (3dBi gain) is a poor directional radiator.

The effective range of a low power access point can be greatly improved by replacing the rubber duck antenna with a directional antenna, such as a patch antenna. Patch antennas can be purchased with the correct RF connector already fitted. Remember to use a very short RF cable, which means mounting the access point close to the antenna. 6dB attenuation over the length over an RF cable is not unusual. A 6dB loss means that the signal strength is reduced to a quarter (25%) at the other end of the cable.

The radiation characteristics of the rubber duck (omni-directional) and patch antennas are compared in the diagrams below.

The omni-directional antenna radiates perpendicular to the axis of the antenna, and the radiation is a circular pattern around the antenna. The gain of the antenna determines the RF field strength perpendicular to the antenna compared to other points around the antenna. Therefore an omni-directional antenna with a higher gain will transmit further, providing that the receiving antenna is in a plane perpendicular to the transmitting antenna. The receiving antennas will probably be grouped within one small arc of the 360 degree circumference of radiation. This means that RF energy is wasted because it is being sent in a direction that has no receiver antennas.

It is clear that the rubber duck omni-directional antenna is only effective if located in the center of the room, and positioned vertically at the same height as the laptop computers that will be receiving the RF from the antenna. This location is not practical for 99% of installations. In practice the access point is located in some back room or above a drop ceiling. It is obvious why the signal strength is poor in the building. Due to poor performance a rubber duck antenna should not be used for a hot spot.

The FCC gives advice to unlicensed spectrum (hot spot) users and recommends that transmitting devices should avoid causing interference with other users. A simple way to do this is avoid radiating RF where there are no receiving antennas.

If the rubber duck omni-directional antenna is bad, then what are the characteristics of a good antenna? The answer is an antenna that radiates RF energy in a similar way that a flashlight radiates light. This means that the antenna should have a beam of energy that spreads out at some angle determined by the construction of the antenna. A flashlight beam spreads out at an angle determined by the construction of the reflector. Several antenna designs have the characteristics desired, however the patch antenna is the simplest to install and has the lowest cost.

The radiation pattern of a patch antenna is shown in the diagram below. The point of highest RF density is located at a point perpendicular to the plane of the antenna. The signal strength reduces away from that center point. As the signal strength reduces to half (-3dB point) then a shape (e.g. circle) can be plotted around the center point that shows the angle for the RF ‘beam’. A patch antenna that has a gain of 8dBi will have a ‘beam angle’ of about 70 degrees.

This 70 degree beam dispersion angle of an 8dBi patch antenna is very convenient if the antenna is mounted in the corner of a room: the beam angle approximates to the 90 degree angle of the corner. The RF will radiate across the room, with little radiation outside the room.

If the patch antenna is mounted in the corner at the same height as the users computers then the users close to the antenna will attenuate the signal and the user furthest away will get a very weak signal. This problem is solved by mounting the patch antenna as high as possible in the corner and then point it downwards towards the customers at the back of the room. See the diagram below

With the antenna in the upper corner of the room the RF will pass over the heads of customers close to the antenna and therefore the signal strength will still be high at the back of the room. This principle of installation applies to a 100 square foot coffee bar, or a 100,000 square foot airport terminal. The patch antenna can also be mounted above a drop ceiling as ceiling tiles have minimal attenuation.
Remember that antennas are polarized and the patch antenna should be mounted with vertical polarization as laptop antennas are more sensitive to vertical polarization. The antenna polarization is usually noted on a label on the rear of the patch antenna.

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