Blog Posts

What is the best type of Antenna ?

Posted by John Barker on January 06, 2015

A lot of customers ask us that question regarding an outdoor hot spot installation they are planning. Well, the answer depends on the characteristics of the site, and where the antenna will be located. The best type of antenna will certainly not be an omni-directional antenna although omnis are very popular. We stock one high gain omni for customers who insist, but we stock a lot more patch and sector antennas. A sector array is appropriate for many outdoor hot spot sites.

Lets see first of all why an omni-directional antenna has limitations for many outdoor hot spot applications. The diagram below sums up the main problem

 

 

The omni-directional antenna radiates electromagnetic energy at a right angle to the antenna. The field strength is 360 degrees around the antenna. If the antenna is mounted on a mast then the field strength is way above users. If the antenna is mounted at ground level then obstacles between the antenna and the user will attenuate the signal strength.

The ideal antenna for the outdoor application shown in the diagram is a down-tilt omni directional: there are no obstructions between the user and the antenna, and the user is in the path of maximum field strength. You can buy a down-tilt omni-directional antenna: it is constructed using sectorized antennas. Another name for the down-tilt omni-directional antenna is a sector array. All cellphone towers are down-tilt omni-directional antennas: usually built using three sector antennas, each with 120 degrees of arc. Three antennas give 360 degrees of arc.
The unique characteristic of the sector antenna is the electromagnetic radiation pattern. The diagram below attempts to show the radiation pattern.

The horizontal radiation pattern is determined by the antenna design. Sector antennas are constructed for 90 degrees, 120 degrees and 180 degrees; therefore 4, 3 and 2 antennas respectively are needed to provide a 360 degree radiation pattern.

The vertical radiation pattern is determined by the gain of the antenna: a 14dBi antenna will have approximately 10 degrees vertical angle between the –3dB points, and a 20dBi antenna will have approximately 7 degrees vertical angle.

If the hot spot antenna is going to be placed in the center of the area of coverage like a cell phone tower, then it makes sense to have 360 degrees of coverage. Most hot spot applications however require a much smaller arc of coverage.

Lets take one example: we have customers who install hot spots in marinas. The sector antenna needs to be mounted as high as possible, this ensures that the boats closer to the antenna don’t block the access of the boats at the furthest point of the marina. In many cases the highest point in a marina is a dry stack storage building, where boats are taken out of the water for the winter and stored. A dry stack building can be 50 or more feet high and is usually located in one corner of the marina. Marinas usually approximate to a rectangular shape. Take a look at the diagram below.

The sector antenna can be mounted on the corner of the dry stack building shown. The correct arc of coverage will be 120 degrees for this marina.  The antenna should be mounted as high as possible and then tilted down to provide a good signal strength for the farthest boat. The sector antenna installation is shown in the diagram below.

The access point that powers the antenna should be selected for power output and receiver gain depending on the size of the marina. An outdoor access point should be should be mounted close to the antenna to avoid losses in the RF cable. A short piece of LMR-400 cable with two N connectors can attenuate the signal 3 or 4 dB (i.e. reduce the signal strength below half!!)

For the marina shown in the diagram, the access point can have +26dBm or +28dBm output and provide good coverage. For a larger marina it will be necessary to have an amplified access point with an output of +30dBm and a receiver amplifier that provides an additional 20dB of gain.

We know of a number of marinas that have used this type of installation very successfully. Customers are happy with the area of coverage and the cost for the marina owner is reasonable.

The principles described above can be applied to many other situations such as inside a large airport terminal building, or coverage of a large public park.

Setting up Indoor Hot Spots

Posted by John Barker on January 06, 2015

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.