Cellmax Tilt Paper_Rev 2

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Prepared by: Bo Jonsson Date 2011-01-14 Page: 1(4) Doc.no: CM:10-013 Rev. 2 CellMax Technologies AB Djursholmsvägen 30 SE-183 52 Täby SWEDEN Tel: +46-(8)-755 12 80 Fax: +46-(8)-755 12 81 www.cellmax.se Org.no: 556614-4316 V.A.T. no: SE556614431601 Tilt and vertical beam width Introduction In the cell planning process of radio networks, antenna radiation patterns and tilt settings are two very important parameters. However seemingly simple, both parameters needs to be understood and applied with care. In this short paper we will discuss the impact of high gain antennas with narrow vertical beam width, the sharper roll-off curve they provide and the opportunities and challenges this offers in regards to tilt. Tilt Why do we use down tilt? This is a very important question and as usual it has many answers. But very often the reason for down tilt is to control the cell-border and the level of interference. The intention is therefore to reduce the radiation towards the horizon, meaning the next cell, or the cells were the frequencies are to be reused again. While at the same time maintain a strong and good coverage level within the intended cell. How much tilt shall we use When tilting an antenna we need to consider the vertical beam width and the so called roll- off from maximum gain. We must also know the level and position of the upper side lobes. The specified vertical beam width, VBW, is the angle between the two points where the gain is 3 dB lower than the gain peak. The roll-off is how fast the gain is dropping relative to the angle from maximum gain. Each antenna has its own roll-off curve, but in general one can say that the narrower the beam width always means an increasingly steeper roll-off curve. Let us look at a few antenna radiation patterns to see how the radiation towards the horizon drops down as we tilt. And at what angle we get the lowest radiation. Below are 4 different plots of vertical radiation patterns for two brands of 7 degree antenna on the left and two brands of 4 degree antennas on the right.

Transcript of Cellmax Tilt Paper_Rev 2

Prepared by: Bo Jonsson

Date 2011-01-14

Page: 1(4)

Doc.no: CM:10-013

Rev. 2

CellMax Technologies AB Djursholmsvägen 30 SE-183 52 Täby SWEDEN

Tel: +46-(8)-755 12 80 Fax: +46-(8)-755 12 81 www.cellmax.se

Org.no: 556614-4316 V.A.T. no: SE556614431601

Tilt and vertical beam width

Introduction In the cell planning process of radio networks, antenna radiation patterns and tilt settings are two very important parameters. However seemingly simple, both parameters needs to be understood and applied with care. In this short paper we will discuss the impact of high gain antennas with narrow vertical beam width, the sharper roll-off curve they provide and the opportunities and challenges this offers in regards to tilt. Tilt Why do we use down tilt? This is a very important question and as usual it has many answers. But very often the reason for down tilt is to control the cell-border and the level of interference. The intention is therefore to reduce the radiation towards the horizon, meaning the next cell, or the cells were the frequencies are to be reused again. While at the same time maintain a strong and good coverage level within the intended cell. How much tilt shall we use When tilting an antenna we need to consider the vertical beam width and the so called roll-off from maximum gain. We must also know the level and position of the upper side lobes. The specified vertical beam width, VBW, is the angle between the two points where the gain is 3 dB lower than the gain peak. The roll-off is how fast the gain is dropping relative to the angle from maximum gain. Each antenna has its own roll-off curve, but in general one can say that the narrower the beam width always means an increasingly steeper roll-off curve. Let us look at a few antenna radiation patterns to see how the radiation towards the horizon drops down as we tilt. And at what angle we get the lowest radiation. Below are 4 different plots of vertical radiation patterns for two brands of 7 degree antenna on the left and two brands of 4 degree antennas on the right.

Prepared by: Bo Jonsson

Date 2011-01-14

Page: 2(4)

Doc.no: CM:10-013

Rev. 2

CellMax Technologies AB Djursholmsvägen 30 SE-183 52 Täby SWEDEN

Tel: +46-(8)-755 12 80 Fax: +46-(8)-755 12 81 www.cellmax.se

Org.no: 556614-4316 V.A.T. no: SE556614431601

First we can see that most antennas have upper side lobe suppression around 20 dB. We can also see that we reach this minimum point at around one VBW out from the center of the main lobe. And that this is valid both for 7 and 4 degree VBW antennas irrespective of brands. By not simply “clicking” on a specific antenna in the simulation toll but too actually take a look at the “Planet file”, it is a simple text file so it is easily done, we can there see how the radiation is reduced as the angle from bore sight is increased. We can also easily see the minimum radiation points and find out where the radiation actually starts growing with increased tilt.

Prepared by: Bo Jonsson

Date 2011-01-14

Page: 3(4)

Doc.no: CM:10-013

Rev. 2

CellMax Technologies AB Djursholmsvägen 30 SE-183 52 Täby SWEDEN

Tel: +46-(8)-755 12 80 Fax: +46-(8)-755 12 81 www.cellmax.se

Org.no: 556614-4316 V.A.T. no: SE556614431601

Below we can see two extracts from “Planet files”, that is the radiation patterns for simulators. The table to the left is for a Kathrein 742215 18 dBi antenna were the 3 dB point is around 3,25 degrees and maximum rejection is 15 dB at 7,5 degrees. To the right is a CellMax 4 degree VBW, were we can see the -3 dB point at 2 degrees and the minimum point of -19 dB at 4,5 degrees above bore sight.

First conclusion What conclusion can be drawn from those facts and figures? The most important conclusion is that the radiation in the horizontal direction can only be reduced by around 15 - 20 dB by utilizing down tilt and that this is reached at a tilt around one VBW. A larger tilt than the VBW will actually have an adverse effect as next upper side lobe will come down below the horizon and increase the unwanted radiation, the interference! Next interesting thing to look at is how far away the main lobe will hit ground? This is a simple case of geometry and a table can easily be prepared, see below. If we assume a 20 meters tower and a flat earth the point where the main lobe hits the ground is simply; Tower height /Tang α. Were α is our tilt angle. If a 10 meter tower is used just multiply the distance with (10/20) = 0,5 and correspondingly with a 30 meter tower, multiply by (30/20) = 1,5. Tilt angle, degrees Distance, meters 0 ∞ 1 1.145 2 573 3 382 4 286 5 229 6 190 7 163 8 142

Prepared by: Bo Jonsson

Date 2011-01-14

Page: 4(4)

Doc.no: CM:10-013

Rev. 2

CellMax Technologies AB Djursholmsvägen 30 SE-183 52 Täby SWEDEN

Tel: +46-(8)-755 12 80 Fax: +46-(8)-755 12 81 www.cellmax.se

Org.no: 556614-4316 V.A.T. no: SE556614431601

From the table above we can make up a few examples:

1. The 18 dBi antenna (7 degrees VBW) at 4 degrees down tilt. We can see that the center of the main lobe will hit the ground 286 meters from the tower, which is quite close. We can also see that at one degree below the horizon, that is 1.145 meters away from the tower, the gain is still over 15 dBi as we are still within the VBW. Even towards the horizon the gain is close to 15 dB (18 -3 dB) and we suffer a great risk of high interference levels. (See diagram below)

2. The CellMax 21 dBi antenna (4 degree VBW) at 4 degrees down tilt. The center of the main lobe will still hit the ground 286 meters from the tower, but now with a 3 dB stronger signal due to the higher gain! We also have 18 dBi of gain at -6 degrees, that is 190 meters away from the tower, so there are no “weak spots” underneath the main lobe Looking at one degree below the horizon, we are now around 10 dB down and at the horizon we are close to the minimum point. This is almost 15 dB lower than the 7 degree antenna when it comes to interference rejection due to the steeper roll of. Also note that there is not much more rejection to gain by using more down tilt Graphical examples Below is a CellMax 21 dBi antenna with 5 degree tilt described graphically.

Prepared by: Bo Jonsson

Date 2011-01-14

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Doc.no: CM:10-013

Rev. 2

CellMax Technologies AB Djursholmsvägen 30 SE-183 52 Täby SWEDEN

Tel: +46-(8)-755 12 80 Fax: +46-(8)-755 12 81 www.cellmax.se

Org.no: 556614-4316 V.A.T. no: SE556614431601

And below the 18 dBi Katherine antenna, red color, at 7 degree tilt as an overlay over the CellMax, blue color, at 5 degrees tilt.

For a typical 7 degrees 18 dBi antenna compared to CellMax 4 degree 21 dBi antenna we can see that although the CellMax antenna has it’s 15 dBi point 75% further out, it still provides a lower interference level towards the horizon and the same -12 dB at 1 degree below the horizon (that is 1200 meters from the tower). This is possible thanks to the sharper roll of in the Cellmax high gain antenna. The graph below describes how the level drops relative to the distance for a very good 18 dBi antenna compared to the CellMax 21 dBi antenna. Note that the high gain antenna maintains a higher level for 90% of the coverage, only very close to the site a small spot can be found were the signal is weaker.

3. If we now look at the situation where we are trying to maximize the coverage and set the 4 degree antenna at 1 degree down tilt. The main lobe will hit the ground 1.145 meters away but we will also have close to full power at the horizon, one degree up.

Prepared by: Bo Jonsson

Date 2011-01-14

Page: 6(4)

Doc.no: CM:10-013

Rev. 2

CellMax Technologies AB Djursholmsvägen 30 SE-183 52 Täby SWEDEN

Tel: +46-(8)-755 12 80 Fax: +46-(8)-755 12 81 www.cellmax.se

Org.no: 556614-4316 V.A.T. no: SE556614431601

Below the main lobe, 4 degrees below the horizon, we will have at 18 dBi of gain, 3 dB down from maximum gain. This will be at 382 meters from the tower were this antenna still have the same radiation as an 18 dBi antenna with 3 degrees down tilt. The first null will come at around 5 degrees, or 190 meters away. A null that in the CellMax antenna has a very well balanced null fill and still will provide a level well above most design targets. A conclusion is therefore that maximum coverage installations will benefit from 4 degree antennas with tilt between 0 and 2 degrees if interference is not a limiting factor. We can also see that there are now week spots under the main lobe if the antenna has a good null fill, something many antennas do not have.

Conclusion In order to decide what tilt angle to use a few things needs to be considered:

1. Vertical beam width of the antenna 2. Interference situation 3. Desired cell range

• “Lowest” radiation is normally reached around one VBW up from the centre of the main lobe. This means that the antenna gain is 16-20 dB lower at one VBW direction above the main lobe.

• The interference situation must be considered in order to know what radiation that can be accepted towards the affected cell.

• Down tilting will make the main lobe hit the ground closer to the tower, 2 degrees at 573 meters for example.

• Excessive down tilt can make the second upper side lobe to come down below the horizon and thus increase unwanted radiation instead of reducing it.