If you are designing a system to provide Wifi coverage to a nearby community like a local village or maybe you want to provide Wifi coverage to a boating community in a marina or a local bay area. You will be interested in how to find the right Wifi Sector Antenna Coverage required without wasting radiating signal power in areas where it is not required. We have created a Wifi Sector Antenna Coverage Tool for this purpose. The general scenario can be seen in the following diagrams.

Sector Antenna Basics

A sector antenna is designed to deliver its signal in a pattern of radiated power that is optimised to reach its target area. In this way no signal energy is wasted in areas where it is not needed. This leads to increased efficiency and reduced interference with other signals. All antennas are characterised by their horizontal and vertical beam patterns. The diagram above shows a typical sector pattern where you can see the signal distribution in the horizontal and vertical planes. In this example the antenna provides a specified gain of 14 dBi in a horizontal sector of 120 degrees. It also has a vertical sector providing 14 dBi gain in a zone of 10 degrees in the vertical plane. Now lets see how to use such an antenna and work out what antenna sector angle is required in our example.

Wifi Sector Antenna Coverage Tool using a sailing zone example to calculate the required Sector Antenna

Take a look at the image and lets take the example of yacht club based on the shore that wants to monitor the progress of individual sail boats with a 802.11n speed wifi connection. The yacht club plan to monitor, in real time, video feeds provided by mobile phone cameras located on each sailboat. This way people connected by Wifi mobile phones to the local wifi router in the yacht club can monitor the view seen by individual sailboats as desired.

The Benefits of Using a Sector Antenna

The benefit of this system is that it overcomes the problem of for example trying to see details of the race while looking from the shore at a fleet of racing sailboats 2 or 3 kilometers away.

Lets use the illustration in the picture and assume that the boats are racing at a maximum distance (X) Km from the shore. Also assume that the sailing course is length (Y). This is measured perpendicular to the line of length (X) from the proposed shore antenna to the end of the sailing zone. We are considering the use of a sector antenna to reach the sailing zone to ensure that the wifi signal energy available is radiated only where it is needed and none is wasted. That is we intend to established a sectored wifi beam that covers the sailing zone.

The Formula for Calculating the Sector Antenna Horizontal Beam Width

The formula for calculating the required sector coverage by our antenna is calculated using the formula below. Note the Arctan function is the inverse of the Tan function. This function is mathematically expressed as Tan ^(-1) and is often seen on calculators keyboards with this symbol.

HBW (Horizontal Beam Width) = 2 x Arctan(Y/2X)

For your convenience we have created a Wifi Sector Antenna Coverage Tool. So using the calculator lets plug in some numbers. Please note you can use any distance units you like in the calculator as long as you use the same units for entering distances.

Lets assume: Distance X = 4Km and Distance Y = 5Km the result is 64 degrees

Using Wifi Sector Antenna Coverage Tool we get a result as shown in the screen shot here.

What we Need to do Next

The Wifi Sector Antenna Coverage Tool has shown us that we require a 64 degrees sector to cover our sailing zone. A standard 60 degree horizontal sector Antenna would be a good choice for our needs. Note that the small discrepancy of 2 degrees either side of the zone will only make very slight differences here as the gain will fall very slightly in these regions but not radically for such small angles so a 60 degree sector antenna is a good choice.

Antenna beam footprints and Antenna Gain

But this is only one part of the problem. We now need to look at some antenna specification sheets to see what gain they achieve and what their horizontal and vertical beam footprint looks like. So we start with these horizontal and vertical beam footprints for a typical sector antenna.

The diagrams show the relative signal strength of a typical sector antenna. What these diagrams show is how the relative signal changes with angular position. Lets take the horizontal pattern. We see the reference point marked as 0 on the right on the centre line. It is here we can expect a maximum signal level from the antenna. If we then measure the signal level at different points of the compass moving round in a circle we can plot points at different power levels thus creating the red line representing the antenna footprint. On the centre line at a point marked as (-3) is a circle which intersects the red line above and below the centre line.

The half power point used for Gain measurement

In the diagram the (-3) point intersects the red line at compass points 60 degrees either side of the centre line ensuring that the half power point indicated by the (-3) meaning minus 3 dB ensures that in this sector all points can expect to receive half the maximum power achieved at the reference point (0). By convention the antenna gain is measured at these end points. So in this case the manufacturer has specified a gain of 14 dBi is achieved in a horizontal sector of 120 degrees. A similar argument applies to the vertical sector footprint which in this case indicates that the design gain of 14 dBi is achieved in in a vertical sector of 15 degrees. This is also an important measurement as we will use this to ensure that when the antenna is mounted we can apply some tilt to the antenna to focus the signal in the desired direction. More on that later.

Sector Antenna Electrical Specification

The electrical specification of a typical antenna is show here and from it we can see that this antenna properties do not exactly match what we are looking for. Antennas with reasonable gains (10 or more dBi) are required for longer links depending on the driving power to them. So a gain of 14 in this case would be OK. But with 120 degrees horizontal beam there is would be much wasted signal so this antenna would not suit our needs. We will find a suitable candidate later so read on.

Sector Antenna Mechanical Specification

The mechanical specification of the antenna is also important. We want an antenna for outdoor use with good weather proofing characteristics. We want to ensure the material covering the antenna is UV stable and won’t break down in time. We also want a light weight antenna where possible to reduce bending moments causing shifts in the signal direction. The wind loading on the antenna mounted on poles is affected by the physical dimensions of the antenna surface area, the smaller the better. In this case the antenna is tall and slim as seen in the picture below.

Calculating the Antenna Mounting Height

Now how high should we mount the antenna? Surprisingly the earths curvature has a bearing on the ability to see a transmitting antenna over the apparent horizon. A 6 foot tall individual standing on the sea shore when looking out to sea can only see the horizon at a distance of about 3 miles. We now turn to a very useful tool also on this site on our page about wifi in marine applications Marine wifi antenna boosters Sighting Distance Calculator at Sea and enter the data for the minimum mounting height until we clear our range of 5000 meters. In our case we will assume that the receiving antennas are mobile phones and that they are 0 meter above sea level. In our sailing club example this is very easy and any height above 2 meters will be OK. We may need to mount the antenna higher to clear obstructions and avoid tampering but this is fine and any reasonable value say between 3 to 15 meters would be fine.

Sailing club – Antenna mounting height (h)

Using the information in our sailing club example (X=4000 meters) and the vertical beam is 15 degrees as an example. If we said it was (h=5 meters) then using the vertical tilt angle calculator we get a result of 8 degrees for the tilt angle. But if our antenna had a vertical beam width of 60 degrees the tilt angle would the be 30 degrees. Note that in these cases the Rx antenna height would be the height of the receiving antenna on the sailing boats relative to the sea level and we have assumed 0 meters for this.

Calculating the Vertical Sector Antenna Tilt Angle

Here is a sector antenna with an adjustable mounting bracket for changing the vertical tilt of the antenna which will be required to maximise the signal propagation in the target zone. Note that in a point to point application between antennas there would be no need to calculate a tilt angle if the antennas were at the same height. If not we would calculate a tilt angle but change the calculation slightly to ensure the maximum signal was carried directly on the line of sight between the antennas.

We have created a calculator to calculate the Tilt Angle for you. You can try the Tilt Angle Calculator Here. Using the calculator for our sailing club where we assume a bigger vertical beam width of 60 degrees the screen shot shows the results of 30 degrees tilt angle.

Designing Wifi Systems with a Sector Antennas

So far we have determined in our sailing club example that:

• Antenna horizontal beam width required is 60 degrees
• Antenna vertical beam width of 60 degrees for example needs a tilt angle of 30 degrees
• Antenna gain of 10 dBi or more is in the acceptable gain range for this type of application
• We need to stream video and therefore need a high capacity wifi link feed for real time video

Now Lets continue with our sailing club project. The next step is to consider the link budget to see the required signal power to drive our antenna. The receiver sensitivity is very important here as it will be a factor to determine the transmitter power. The problem that will be a limiting factor in the proposal will be the available transmit power from mobile phones to reach our access point. But we will do the calculations below and see what can be acheived. In our design we also need to minimise signal loss elements like cable and connector losses. Another factor is how much of the antennas radiated power is actually getting to the target area. In this case we shall assume for our calculations the following data.

• Receiver (mobile phone) Tx power is 21 dBm
• Receive Antenna (mobile phone) Sensitivity is -90 dBm
• RX sensitivity of AP: 802.11n: –92 dBm @ MCS0 to -72 dBm @ MCS7 40MHz
• TX power of AP: 802.11n: 32dBm @ MCS0 to 28dBm @ MCS7
• We shall use POE to feed the AP transmitter to minimise cable loss
• We shall assume a fade margin of 12dB signal loss

Calculating the Wifi AP Tx Power to drive our Sector Antenna

We now turn to a another very useful tool also on this site Wifi Link Budget Calculator and enter the data we have assumed above. We are trying to find the right transmit power to meet our needs. So try this calculator with different values to get optimum results.

Using the example data we can see from the screen shot we come up with a transmitter power of 30 dBm. This calculation is based on data for modern smart phones like Iphone 5. The typical theoretical receiver sensitivity about -99 dBm when operating at WIFI speeds on 802.11n 2.4 Ghz. However we have made a conservative sensitivity setting of -90 dBm. We also include a large gain margin to compensate for a number of factors. First because of the need to cover the sailing zone we will loose 3 dB in signal power directly into the sea. In addition because of the environment over the sea in average conditions we make another considerable loss compensation of 9 dB which corresponds to half the typical losses in urban and rural locations for mobile phones. This is because we should expect better signals in our sailing club example as we have mainly unobstructed views between boats and the central AP.

The calculation is based on typical device power for high end Wifi transmit power device. In our sailing club example we only need a maximum range of 4 Km’s so in this case we could afford less transmit power. But a bit over kill is not a bad idea here to cover changing weather conditions. We can control the output power of our AP so its not a problem to turn it down a bit. As mentioned earlier the power from the mobile phones will be the limiting factor and not the power available from the AP.

Calculating the Wifi Power required by mobile phones on the sailboats

In the screen shot we can see that we will need about 21 dBm output power from our phone. This is on the high side of average signal power for a smart phone but by no means a maximum (400 mx) and is achievable but will lead to quicker battery drain. We have set the receiver sensitivity of our AP to -92 dBm to get the maximum distance consistent with a manageable link speed. Once again the distance covered by the link is about 4 Km’s which just about covers the required distance in the sailing club example.

Calculation of Link Speed available

Below you will find a possible solution to our problem. We will use data from the selected device to compute available link speed operating at 802.11n. But first look at this image which shows the standard links speeds, modulation schemes, channel widths etc in various combinations. We will use this for reference when we look at our AP transmitter specification.

Now from our previous specification requirements we have said. The AP should be able to achieve the following.

• RX sensitivity of AP: 802.11n: –92 dBm @ MCS0 to -72 dBm @ MCS7 40MHz
• TX power of AP: 802.11n: 32dBm @ MCS0 to 28dBm @ MCS7

From this we see that at -92 dBm we can achieve data rates of MCS0 and on a sliding scale with a sensitivity of -72 dBm it is possible to achieve speeds of MCS7. From our link budget power requirements we needed a sensitivity at our AP of -92 dBm. This corresponds to MCS0 channel speeds on 802.11n. This implies a links speed from 7 to 15 Mb’s. for supported modulation schemes which in turn depends on the quality of the mobile phone. Really modern phones like the I phone 5 can support WIFI at 5 Ghz and 2.4 Ghz and also supports 64-QAM modulation but also others can. Many other high end smart phones have similar specifications and could support these signal levels and link speed. We adopt the more common 2.4 Ghz band as many more phones work in this band.

Video Stream Bandwidth Requirements for our Marine Sailing Application

Applications like Skype and Quicktime allow video streaming and employ h.264 video codecs among others to compress camera images. Skype in particular is also very adaptive to channel capacity. However a 1.5 Mb’s link would be quite adequate for a high definition video call sampling at 15 frames a second. Skype has a huge user base and is ported for most mobile operating systems like Android. So if we assume 2 Mb’s per boat link. The AP will need an uplink speed of 40 Mb’s to cope with 10 concurrent boat links.

Putting all the design criteria together

We now have a list of requirements for our yacht club project

• Sector Antenna for outdoor use with good weather proofing characteristics
• Sector Antenna that is weather proof, UV stable and won’t break down over time
• Light weight antenna where possible to reduce bending moments
• Antenna with low wind loading
• Antenna with pole mounting and the ability to tilt the antenna
• Sector Antenna horizontal beam width is 60 degrees
• Sector Antenna vertical beam width of up 60 degrees would provide a good footprint
• Antenna gain of at least 10 dBi or more is in the acceptable gain
• We need to stream video and therefore need a high capacity wifi link feed for real time video using 802.11n
• AP RX sensitivity: 802.11n: –92 dBm @ MCS0 to -72 dBm @ MCS7 40MHz
• AP TX power: 802.11n: 32dBm @ MCS0 to 28dBm @ MCS7
• Modern Mobile smart phone supporting : 802.11n: @ MCS0 Tx power 21 dBm +
• Modern Mobile smart phone supporting Receive Sensitivity -90 dBm
• POE feed for AP transmitter to minimise cable loss
• Assume a fade margin channel losses for link of 12 dB
• AP sensitivity of -92 dBm

Thats a big list! Well where do we find all that? The answer is right here on this site. You will find the MikroTik SXT G 2HnD Outdoor CPE Sector AP a complete solution. It is a perfect match for our needs as it combines all the ingredients in one package and ticks all the boxes in our shopping list. The MikroTik SXT G 2HnD Outdoor CPE Sector AP has a built in antenna and radio transmitter. It is a fantastic buy offering so much at such an incredible price. So if this is the kind of project you are considering look no further than the MikroTik SXT G 2HnD Outdoor CPE Sector AP.