Antenna tiltguide

Antenna Downtilt Guideline
ANTENNA DOWNTILT GUIDELINE
1 (6) 1999-09-29

2 (6) 1999-09-29
TABLE OF CONTENTS
1 INTRODUCTION .............................................................................................. 3
2 ANTENNAS...................................................................................................... 3
2.1 The Antenna diagram ............................................................................................... 3
2.1.1 Gain .................................................................................................................... 3
2.1.2 Horizontal beamwidth .......................................................................................... 3
2.1.3 Vertical beamwidth .............................................................................................. 3
2.1.4 First null beamwidth............................................................................................. 5
2.1.5 Null-fill ................................................................................................................. 5
2.1.6 Back-lobe ............................................................................................................ 6
2.2 Mechanical versus electrical tilt .............................................................................. 6
2.3 Super high gain antennas ........................................................................................ 8
2.4 Theoretical tilt-effects............................................................................................... 8
3 MEASUREMENTS ......................................................................................... 12
3.1 Signal strength MEASUREMENTS in forward direction....................................... 13
3.1.1 18 dBi Antennas ................................................................................................ 13
3.1.2 15 dBi antennas ................................................................................................ 15
3.2 Signal strength MEASUREMENTS in side direction............................................. 17
3.3 Signal strength MEASUREMENTS in Backward direction................................... 19
4 RECOMMENDATIONS .................................................................................. 20
4.1 General recommendations..................................................................................... 20
4.2 Recommended tilt-values....................................................................................... 22
4.2.1 Areas with large cells......................................................................................... 22
4.2.2 Areas with small cells ........................................................................................ 22
5 CONCLUSION................................................................................................ 23

3 (6) 1999-09-29
1 INTRODUCTION
With an increasing capacity demand, and a limited frequency spectrum,
the operators are forced to utilise the frequency spectrum more
efficiently. High capacity frequency planning techniques are often based
on tight frequency reuse. The networks become interference limited, and
in order to maximise the capacity, every available technique to minimise
interference becomes important.
As the capacity demand increases, the network plans also become
tighter. A macro-cell site-site distance down to 400 meter or less is not
unusual. With shorter site-to-site distances, limiting the interference from
each cell becomes more and more important. When the cells are very
small, down tilt can be applied without loss of coverage. Compared to no
tilt at all, downtilt can even improve coverage in these dense networks.
A well-chosen overall tilt-strategy can lower the overall interference in
the network. A too aggressive down tilting strategy will however lead to
an overall loss of coverage. In addition to a general down tilt strategy,
applied in all cells, down tilt can be used to solve specific problems, for
example local interference problems or cells that are too large.
2 ANTENNAS
2.1 THE ANTENNA DIAGRAM
2.1.1 Gain
The antenna diagrams show the antenna gain, in a given direction,
relative an isotropic antenna. The maximum gain for an antenna can be
increased by narrowing the horizontal and/or the vertical beam width.
Typical for a three sector site is a 65° horizontal beam width with a
maximum gain of 15 or 18 dBi.
2.1.2 Horizontal beamwidth
The standard antennas for a three-sector site has a horizontal beam
width, also referred to as the “half power beam width”, of 65°. This
means that the gain is 3 dB less at +/- 32.5° (i.e. half power) than the
maximum gain in the 0° direction. At 60° (i.e. the theoretical cell border
between the sectors), the gain is suppressed typically 10 dB.
2.1.3 Vertical beamwidth
The most interesting part of the antenna pattern when it comes to tilting
is the vertical antenna-gain pattern in the forward direction. A 15 dBi
antenna usually has a vertical half power beam width of around 15° (i.e.

+/- 7.5 °). The high gain 18 dBi antennas have a narrower vertical beam
width, typically 6°-8° (i.e. +/- 3° - 4°).
Below is an example of two typical antennas with 15.5 and 18 dBi gain.
Vertical antenna gain in forward direction
5
0
-5
-25 -20 -15 -10 -5 0 5 10 15 20 25
-10
dB -15
GAIN 15 dBi
-20
-25
-30
-35
-40
Degrees
GAIN 18 dBi
Figure 1. Vertical gain for two typical 15 dBi and 18 dBi antennas.
If the antenna tilted for example 5°, the gain in the horizontal direction,
relative the maximum gain, equals the gain at –5° in the antenna
diagram. For the antennas above, a 5° downtilt would mean
approximately –1.5 dB for the 15.5 dBi antenna, and –8.5 dB for the 18
dBi antenna.
Below is an example of what an 18 dBi vertical antenna diagram will look
like with 0°, 5° and 8° down tilt.
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Vertical diagram for different tilts (18 dBi gain antenna)
-10 -5 0 5 10 15 20
5 (6) 1999-09-29
0
-5
-10
-15
-20
-25
-30
degrees
dB
GAIN 18 dBi, 5 degrees
GAIN 18 dBi, 0 degree tilt
GAIN 18 dBi, 8 degree tilt
Figure 2. Vertical antenna diagram for 18 dBi antenna. 0°, 5° and 8° tilt
applied.
Note that the antenna diagram is valid only in the forward direction.
When mechanical downtilt is applied, the tilt effect in directions other
than straight forward is different, which means that the horizontal
antenna diagram is changed (see further chapter 2.2).
2.1.4 First null beamwidth
The first null beam width is the angle between the nulls adjacent to the
main lobe. In the antenna diagram in Figure 1, it can be seen that the 15
dBi antenna has a First null Beam width of 32° (+/- 16°). The 18 dBi
antenna has a First null Beam width of 15° (+/- 7,5°). These figures may
vary a little bit for different antennas models, but the figures are roughly
the same for all 65° antennas with 15 or 18 dBi gain.
Tilting half of the First null Beam width will, at least in theory, suppress
the antenna gain towards the horizon with up to 20 dB or more.
2.1.5 Null-fill
Some antennas use “null-fill” in order to make the first null under the
horizon smaller. This is to limit the loss of signal strength that the mobile
may experience if it is located at a position where the vertical angle from
the basestation antenna corresponds to the first null under the horizon in
the antenna diagram. Such antennas do however tend to loose some of
its maximum gain. Most of the large antenna manufacturers such as
Kathrein, has a the first null specified to be > -25 dB relative the
maximum gain. This figure is however somewhat theoretical, since the
actual antenna diagram for these low power dips is effected by the
antenna mounting. Moreover, the reflections and diffractions in the wave
propagation will even out the dip in the antenna diagram, and the
receiving mobile will not experience such a dramatic decrease in signal
strength.

0
-5
155 160 165 170 175 180 185 190 195 200 205
-10
-15
-20
-25
-30
-35
-40
-45
-50
-55
6 (6) 1999-09-29
2.1.6 Back-lobe
The theoretical back-lobes for two typical 65° antennas are shown in the
picture below. However, the actual antenna gain for different vertical
directions is very difficult to estimate. Things like the mounting masts
and the near environment on the roof has a large impact on the radiation
in the backward direction. The actual signal strength behind the cell may
also be the result of reflections from the energy transmitted in the
forward direction. It is therefore difficult to theoretically predict the effect
that down tilting has on the signal strength in the backward direction.
Vertical antenna gain for Backlobe
-60
degrees
dB
GAIN 15 dBi
GAIN 18 dBi
Figure 3. Vertical gain for the back-lobe for two typical 15 dBi and 18 dBi
antennas.
2.2 MECHANICAL VERSUS ELECTRICAL TILT
Mechanical tilt
When using mechanical tilt, the antenna is mounted with adjustable
brackets in a way that the tilt can be adjusted on site.
Electrical tilt
Electrical tilt means an in-built tilt that lowers the vertical beam in all
horizontal directions. Electrical tilt can be combined with additional
mechanical tilt.

Electrical Downtilt vs. Mechanical Downtilt
The largest advantage of electrical antenna down tilt is that the
horizontal beam width is not affected. With mechanical down tilt, the
tilting effect is greater in the 0° direction. At for example +/- 60°, the
effective tilt angle becomes lower. This effect can be very difficult to
predict. With an overall, very high mechanical tilt level in the network, the
cells become shorter and wider, more comparable to maybe 90°
antennas. The frequency planning becomes more difficult, and the
overall interference level in the network becomes higher.
Electronical vs mechanical downtilt
0
-10 -5 0 5 10 15 20
-5
-10
-15
-20
-25
Figure 4. Comparison of vertical antenna gain for mechanical and
electrical down tilt. Note that the graph above is only valid in the forward,
0° horizontal directon.
7 (6) 1999-09-29
-30
degrees
dB
6 degrees EDT
18 dBi
6 degrees mechanical

Mechanical vs Electrical Downtilt
Horizontal gain at zero degree vertical angle
20
15
10
5
0
-90 -60 -30 0 30 60 90
-5
-10
-15
Degrees
8 (6) 1999-09-29
dB Gain
MDT
No tilt
EDT
Figure 5. Comparison of horizontal antenna gain at 0° vertical angle for
mechanical and electrical down tilt. Because no 3D antenna patterns are
was available, the Mechanical downtilt antenna diagram is estimated
from the vertical antenna pattern in the forward direction.
2.3 SUPER HIGH GAIN ANTENNAS
For 1800 MHz, there are extremely high gain (around 21 dBi) 65°
antennas available. These antennas have an even narrower vertical
beam width, around half the beam width of an 18 dBi antenna. These
antennas are larger than the standard 18 dBi 1800 MHz antennas. The
effect that these antennas have on coverage in urban areas has not
been verified, but with such narrow beam width, at least in theory an
even larger tilting effect can be achieved.
2.4 THEORETICAL TILT-EFFECTS
When selecting the optimum tilt angle, the goal is to have as high signal
strength as possible in the area where the cell should be serving traffic.
Beyond the serving area of the cell, the signal strength should be as low
as possible.
The basic theory is that down tilting an antenna increases the signal
strength in the area close to the site, whereas the signal strength
becomes lower at far distances. The relation between the signal strength
and distance from the site depends on:
· Down tilt angle
· Antenna type

· Antenna height
· Near environment (topography and obstacles)
In an open environment, the effects of antenna down tilting can be fairly
accurately estimated by calculating the vertical angle between the
antenna and the mobile at various distances from the site.
Example
Tilt effect, in terms of antenna gain experienced by a mobile, calculated
given the following circumstances:
· Effective antenna height: (antenna height – mobile height): 50 meter
· Distance, site – mobile: 500 meter
· Antenna down tilt: 8°
The vertical angle to the mobile is:
a=arctan(50/500) = 5.7°
It the antenna was not down tilted, the antenna gain for the mobile would
correspond to –5.7° in the antenna diagram. However, since the antenna
is down tilted, the corresponding angle in the vertical antenna diagram
is:
5.7° - 8° = -2.3°
In the figure below, the theoretical antenna gain for different distances
from the site have been calculated for a typical 18 dBi gain antenna. The
antenna gain has been added to a simple path propagation model in
order to show the signal strength in relation to the distance from the site
for different antenna tilt angles.
In the calculations, a 50 meter antenna height has been assumed. A
different antenna height will change the scale of the X-axis, but the
relative gain for the different antenna tilt angles will remain. In order to
make the figure easier to read, two different scales have been plotted.
9 (6) 1999-09-29

Theoretical signalstrength, 15 dBi antenna
-50 0 2000 4000 6000 8000 10000 12000
10 (6) 1999-09-29
-40
-50
-60
-70
-80
-90
-100
0 500 1000 1500
Distance from site (meter)
dBm
Max Gain
0 degrees
8 degrees
14 degrees
Figure 6. The theoretical signal-strength from a 50 meter high site, using
a 15 dBi Gain antenna. The Max Gain is the theoretical signal-strength if
a dipole antenna, with 18 dBi Gain was used. The Max Gain is included
in the graph in order to be able to see the effect that the vertical antenna
diagram has on the signal-strength for the different tilt-angles.
-70
-90
-110
-130
dBm
Max Gain
0 degrees
8 degrees
14 degrees
Figure 7. The theoretical signal-strength from a 50 meter high site using
a 15 dBi antenna. Compared to Figure 6, the scale is different in order to
see the effect at far distance.

-50 0 2000 4000 6000 8000 10000 12000
11 (6) 1999-09-29
-40
-50
-60
-70
-80
-90
-100
0 500 1000 1500
dBm
Max Gain
0 degrees
5 degrees
9 degrees
Figure 8. The theoretical signal-strength from a 50 meter high site, using
a 18 dBi Gain antenna.
-70
-90
-110
-130
dBm
Max Gain
0 degrees
5 degrees
9 degrees
Figure 7. The theoretical signal-strength from a 50 meter high site using
a 18 dBi antenna. Compared to Figure 6, the scale is different in order to
see the effect at far distance.
As can be seen from the figures, with a 50 meter antenna height and no
down tilt, a 18 dBi antenna will have its first null at around 400 meter
from the site, and a 15 dBi antenna will have its first null around 200
meter from the site. Down tilting the antenna moves the first null closer
to the site. At far distance, the signal strength is lower with down tilt,
which means less coverage (if the cell is serving there) or reduced
interference (if the cell is not serving). This is the basic theory behind all
antenna down tilting.

3 MEASUREMENTS
This chapter contains some measurements that where performed in a
large Asian City. The topography is very flat, and has no significant
impact on the results.
The area refereed to as “Urban” is dense, but not extremely dense. The
buildings are of various heights, including skyscrapers up to 100 meter
or more. A photo from a typical Urban site is shown in Appendix.
Most of these measurements are from non line-of-site. Areas referred to
as “Suburban” do not have that many high-rise buildings, and the
buildings are not as densely built.
Five sites were selected for the measurements. These sites where all 3-
sector, using 65° horizontal beamwidth antennas with no Electrical
downtilt. This means that all tilts were done using mechanical downtilt.
For each site, two cells where selected. Two of the sites had 18 dBi
antennas, the other three sites where equipped with 15.5 dBi antennas.
For each cell, the signal strength was measured for three different tilt
angels. If possible, the first measurement was always performed with 0°
tilt, as a reference. For some cells, the antenna mounting was however
such that 0° tilt could not be applied.
Measurement procedures
Prior to the measurements, the cells to be measured got the BCCH
frequencies configured with “clean” test-frequencies. The signal strength
was measured by a TEMS phone, using “Scan-mode”, and logged
together with GPS readings. After the measurements, the signal strength
was plotted in Map-info, and the result was analysed. In addition to this,
the measurements were also post-processed in Mat-lab. The signal
strength was filtered out for different directions, and plotted as a function
of distance from the site. These plots are presented in this chapter.
These kind of measurements are time consuming. In this measurement
project, a large number of cells have been prioritised rather than
performing a larger number of different tilt-angles for each cell. Due to
the fact that the different tilt-angles were not logged simultaneously, the
measurements are not exact enough to compare the signal-strength
very close to the site. The accuracy of the positioning is limited by the
GPS readings. Each drive-test was not performed with exactly the same
speed. This does also have an impact on the accuracy of the compared
signal strengths for the different tilt-angles. In the measurement graphs,
the signal-strength in each point is the middle value of all samples in a
50 x 50 meter square.
12 (6) 1999-09-29

3.1 SIGNAL STRENGTH MEASUREMENTS IN FORWARD DIRECTION
The signal strength from the three different tilt angles are plotted in
graphs, as a function of the distance from the site. Only the
measurement samples with a horizontal angle of +/- 40° has been used.
The tilt angles used for each cell can be seen in the graph labels. In
addition to the signal strength plots, the relative difference, compared to
0° tilt (or the lowest measured tilt where not applicable) is also plotted.
13 (6) 1999-09-29
3.1.1 18 dBi Antennas
Cell 1A: 18 dBi antenna, Semi Urban environment,
45 meter antenna height
0, 9 and 14 degree tilt, Forward direction (+/- 40 degrees)
20
10
0
-10
-20
-30
-40
-50
-60
-70
-80
-90
-100
-110
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5
Disance form site, km
dB
SS_0
SS_9
SS_14
SS_9 - SS_0
SS_14 - SS_0

Cell 1B dBi antenna, Semi-Urban environment,
Forward direction (+/- 40 degrees)
14 (6) 1999-09-29
20
10
0
-10
-20
-30
-40
-50
-60
-70
-80
-90
-100
-110
0 0.5 1 1.5 2 2.5 3 3.5 4
dB
SS_0
SS_4
SS_12
SS_4 - SS_0
SS_12 - SS_0
Cell 2A: 18 dBi antenna, Urban environment, 50 meter antenna
height
20
10
0
-10
-20
-30
-40
-50
-60
-70
-80
-90
-100
-110
0 0.5 1 1.5 2 2.5
dB
SS_5
SS_9
SS_14
SS_9 - SS_5
SS_14 - SS_5
Cell 2B: 18 dBi antenna, Urban environment, 50 meter antenna height
20
10
0
-10
-20
-30
-40
-50
-60
-70
-80
-90
-100
-110
0 0.5 1 1.5 2 2.5
dB
SS_0
SS_5
SS_12
SS_5 - SS_0
SS_12 - SS_0

15 (6) 1999-09-29
3.1.2 15 dBi antennas
Cell 3A: 15.5 dBi antenna, Urban environment,
30meter antenna heigth
20
10
0
-10
-20
-30
-40
-50
-60
-70
-80
-90
-100
-110
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5
dB
SS_0
SS_8
SS_14
SS_8-SS_0
SS_14-SS_0
Cell 3B: 15.5 dBi antenna, Urban environment,
20
10
0
-10
-20
-30
-40
-50
-60
-70
-80
-90
-100
-110
0 0.5 1 1.5 2 2.5 3 3.5
dB
SS_0
SS_8
SS_14
SS_8-SS_0
SS_14-SS_0
Cell 4A: 15.5 dBi antenna, Urban environment,
20
10
0
-10
-20
-30
-40
-50
-60
-70
-80
-90
-100
-110
0 0.5 1 1.5 2
Distance from site, km
dB
SS_0
SS_8
SS_14
SS_8-SS_0
SS_14-SS_0

Cell 4B: 15.5 dBi antenna, Urban
environme3n0t ,meter antenna height
16 (6) 1999-09-29
20
10
0
-10
-20
-30
-40
-50
-60
-70
-80
-90
-100
-110
0 0.5 1 1.5 2 2.5
Distance form site, km
dB
SS_1
SS_8
SS_14
SS_8-SS_1
SS_14-SS_1
Cell 5A: 15.5 dBi antenna, Urban environment
20
10
0
-10
-20
-30
-40
-50
-60
-70
-80
-90
-100
-110
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 6.5 7
dB
SS_0
SS_8
SS_14
delta_8
delta_14
Cell 5B: 15.5dBi antenna, Sub-urban environment
20
10
0
-10
-20
-30
-40
-50
-60
-70
-80
-90
-100
-110
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6
dB
SS_0
SS_8
SS_14
SS_8-SS_0
SS_14-SS_0

Evaluation of Signals strength measurements in forward direction
The measurements show that the effect of tilting was pretty much as
could be expected from the theoretical calculations. At far distances, the
signal strength becomes lower when the antenna is tilted, but not quite
as much as can be expected from the theoretical calculations based on
the antenna diagram. One reason for this could be that the signal
strength that reaches the mobile actually is transmitting more “straight
forward” (at a lower vertical angle), above the rooftops, and later
diffracted down to the mobile station.
For the 18 dBi antennas, down tilting increases the signal strength at
around 500 meter and closer to the site. For 15 dBi antennas, down
tilting increase the signal strength at closer than around 2 – 300 meter
from the site. It should be kept in mind that these sites where mostly
around 50 meter high. If the sites are lower, the “break point” where the
down tilted antenna is stronger, is closer to the site. At very high sites,
this “breakpoint” is further from the site.
The measurements show that an overall down tilt, of all cells in the
network, can give a positive effect on the signal-to-interference ratio, C/I.
This is however only true if the cell size does not exceed the distance
where down tilting will reduce the coverage. For typical Urban cellplans,
with sites that are 50 meter or lower, this means that the cell ranges
should not exceed around 500 meter. If the cells are larger, an overall
down tilt, for every cell, will reduce the overall coverage, but not have a
significant impact on the overall C/I levels at the cell-boarders. This is
due to that downtilt will lower the signal strength at the cell-boarders in
almost the same extent as it will lower the signal-strength further away
from the site where the cell is causing interference. Hence, the
conclusion is that a general down tilting strategy, down tilting all cells
more than the angle that corresponds to a 3dB loss at the horizon,
should only be applied in areas where the cells are small, with a range of
around 500 meter or less. This corresponds to a site/site distance of
around 700-800 meter.
3.2 SIGNAL STRENGTH MEASUREMENTS IN SIDE DIRECTION
For these graphs, only the measurement samples with a horizontal
angle of +/- 50° – 70° has been used. This angle has been chosen in
order to represent the signal strength at the cellborder towards the co-sited
17 (6) 1999-09-29
cells.
The results were consistent for every site, therefore one graphs, with a
typical result, are presented here. The graph for the forward direction is
included as a reference.

Side direction (+/- 50-70 degrees)
18 (6) 1999-09-29
20
10
0
-10
-20
-30
-40
-50
-60
-70
-80
-90
-100
-110
0 0.
5
1 1.
5
2 2.
5
3 3.
5
4 4.
5
5 5.
5
6 6.
5
7 7.
5
8 8.
5
dB
SS_0
SS_8
SS_14
delta_8
delta_14
20
10
0
-10
-20
-30
-40
-50
-60
-70
-80
-90
-100
-110
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 6.5 7
dB
SS_0
SS_8
SS_14
delta_8
delta_14
Evalutaion of Signals strength measurments in side direction
All measurements where done with mechanical down tilt. The result
shows that the down tilt has a similar effect on the signal strength at an
angle corresponding to the cell border to the co-sited cells, but at a lower
degree. The signal strength becomes stronger close to the site, and
weaker further away, but not as much as in the forward direction. This
result is in-line with what can be expected from a theoretical point of
view.
In practise, this result tells us that if the cell diameter is larger than
approximately 500 meter, the horizontal beam width of the cell becomes
wider. The cells become “shorter and wider”. This will have a negative
impact on frequency planning and the C/I relations between the cells. It
may for example make be impossible to plan a 4/12 plan with sufficient
C/I levels.

3.3 SIGNAL STRENGTH MEASUREMENTS IN BACKWARD DIRECTION
For the graph below, only the measurement samples that had a
horizontal angle of 140° - 220° was used. This corresponds to the back-lobe
19 (6) 1999-09-29
+/- 40°.
Cell 5B: 15.5dBi antenna, Sub-urban environment
50 meter antenna heigth
Backward direction (180 +/- 40 degrees)
20
10
0
-10
-20
-30
-40
-50
-60
-70
-80
-90
-100
-110
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5
dB
SS_0
SS_8
SS_14
SS_8-SS_0
SS_14-SS_0
Evalutaion of Signals strength measurments in backward direction
The measurements show that in the backward direction, the antenna
down tilt did not have any significant effect on the signal strength behind
the cell. Even at cells with large nearby buildings, where reflections are
expected, down tilting did not have an effect on the signal strength in the
backward direction.
The “risk” of being served by the back-lobe is however effected by tilting
since the signal strength in the forward direction changes. The risk of
being served by a back-lobe is reduced close to the site since the signal
strength in the forward direction (from the cell that should be serving) is
stronger. Further away from the site, the effect is the opposite since the
signal strength in the forward direction becomes weaker when the
antenna is down tilted. However, far away from the site, back-lobe
problems are not very common. The conclusion is therefore that tilting
does generally not make back-lobe problems more critical.

4 RECOMMENDATIONS
4.1 GENERAL RECOMMENDATIONS
· One general recommendation is not to apply a large down tilt for all
cells in an area. The reason for this is that:
1. It becomes difficult to to fix specific problems, e.g. interference
problems or cells that are too large. If a cell that already has a large
donwntilt applied, needs to be further down tilted due to for example
a local interference problem, this cell would need to have a very
large tilt angle. The effects from very large downtilts are difficult to
predict, and may lead to a significant loss of coverage in the area.
2. If mechanical downtilt is used, the horizontal beam-width of the
antenna becomes wider (see chapter 2.2). This effect is difficult to
consider in the frequency planning, and the wider antenna diagram
probably creates an overall worse C/I distribution in the network.
3. If the cellplan is not very tight (around 700 meter site-to-site distance
or more depending on antenna heights and type), antenna down
tilting will reduce the overall coverage in the network. This is of
course not good for the quality of the network, especially for indoor
areas with weak coverage.
· One good strategy is to have a few default tilt values that are
implemented on every site. The default value can be different
depending on the area, the size of the cell, antenna height and which
type of antenna that is used. The general recommendation is
however to keep it simple, and not do to many theoretical
calculations for every site. It is better to start with a low tilt for every
cell (see further the recommendations in chapter 4.2.1, 4.2.2), and
study the actual coverage and interference situation. On a case by
case bases, apply further down-tilt can be applied (and verified
through drive-tests and analysing statistics!).
· There is no point in tilting an antenna less than the angle which gives
a 3 dB loss at the horizon. This corresponds to around 7° tilt for a 15
dBi antenna, and around 3.5° tilt for an 18 dBi antenna. A smaller tilt
gives a limited impact and is hardly worth the effort.
· Study the antenna diagram carefully before selecting the tilt-angle.
Most of the tilting effect happens between the angle that corresponds
to the 3dB point towards the horizon, and the angle that corresponds
to tilting the first null towards the horizon. It is sort of like the “ketchup
effect”. For example 8° tilt gives far more than twice the effect
compared to 4°.
20 (6) 1999-09-29

· Avoid down tilting more than the angle that correspond to having the
first null towards the horizon. Further down tilting can be done in
extreme cases, but if there is a need for further reduction of
interference or cell-size, a reduction of output power, or possibly
lowering of the antenna height, should also be considered. Very
large down tilts (beyond the first null towards the horizon) should be
carefully verified since the effect of such large tilts is difficult to
predict.
· Define, for every antenna type, four or five tilt-angles, and do only
use these tilts. This makes it easier to work in a structured way, and
to have better control over all the down tilts in the network. An
example of such fixed tilt-values could be:
Default Tilt-angles
(exact values depends on
the antenna diagram)
Theoretical gain
21 (6) 1999-09-29
at horizon
(relative max.
Gain)
15 dBi
antenna
18 dBi
antenna
0 dB 0 0
3 dB 7 3.5
6 dB 9.5 5
10 dB 11.5 6
> 15 dB 14 7
· Document all antenna down tilts! It is important not only to know how
much each cell is down tilted, but also WHY the down tilt was
performed. If an antenna tilt was performed in order to solve a local
interference problem due to for example a bad co-channel, this tilt
should possibly be removed when a new frequency plan (without this
co-channel) is implemented. Another example is a cell that has been
down tilted because of congestion. If the cell is expanded with
additional transceivers, it might be possible to reduce the down tilt.
· A new site effects the coverage area of all cells that are neighbours
to the new site. The downtilt angles in these sites should be revised.
Additional downtilt should be considered in neighbouring cells that
gets a reduced coverage area.
· Verify all the effects after having performed a down tilt of more than
4° (18 dBi antennas) or 8° (15 dBi antennas). Remember that it is
just as important to check the coverage and quality in the down tilted
cell, as the area where the down tilt is expected to reduce

interference. Even if one problem is solved, a new problem might
have occurred.
· It is better to put a lot of effort tilting and verifying the result on a few
cells compared of doing a “quick and dirty” job tilting a larger number
of cells.
· Consider the nearby environment. Use common sense. If there for
example is a close by building, which is of almost equal height as the
antenna, a down tilt will make the coverage beyond that building to
almost disappear.
4.2 RECOMMENDED TILT-VALUES
4.2.1 Areas with large cells (approximately 800 meter site-site distance
or more):
· Around 3.5° for an 18 dBi antenna, and 7° for 15 dBi antenna could
be used as default tilting values. Compared to having no tilt at all,
this may give a possible minor positive impact on the C/I levels,
without any significant loss of coverage. The effect of such small tilt
is however minimal. If the cells in an area currently have no down tilt
at all, it might be better to leave them that way and to put the effort
and resources that it takes to apply downtilt on cells that are more
important.
· Cells that are very large and cause congestion can be further down
tilted. A cell with a very large number of handovers creates problems
with frequency planning, and is a sign that a cell may cause
interference problems. Down tilt the cells in pre-defined steps, e.g. in
steps of 2° or 3° depending on antenna type.
4.2.2 Areas with small cells (approximately 700 meter site-site distance
or less):
· With smaller cells, there is a better chance to get an overall
improvement of the interference situation in the network by adapting
a tilt strategy with a general tilt on all cells. A slightly worse coverage
in certain areas is also not as critical with a dense cellplan.
Recommended default-values is a tilt that corresponds to around 5
dB loss at the horizon. This means around 4° for an 18 dBi antenna,
and 8° for a 15 dBi antenna.
· With very small cells, with a range of 300 meter or less, the antennas
should definitely be downtilted, or the first null in the antenna
diagram might create poor coverage at the cell boarder. This may
lead to interference problems in the cell, and the quality will definitely
benefit from antenna down tilt.
22 (6) 1999-09-29

· In areas where interference is a large problem, and the cells are very
small (often the same area), additional down tilt can be applied.
Additional tilt should be decided on a case-by-case basis, and the
result should always be verified.
· Consider using 6° Electronic Down Tilt (EDT) antennas (18 dBi, 6°
EDT is available in Ericsson’s product package for 1800 MHz). The
6° EDT antenna may result in a overall loss of coverage if used in
every cell in an area. As a default setting, the EDT antennas can
therefore be mechanically up-tilted maybe 2°. This corresponds to
around 4° down tilt from a conventional non-EDT antenna. If a larger
tilt-angle is desired, the EDT antenna can be down tilted. When
additional mechanical tilt is applied, this mechanical tilt angle is
small, and will not effect the horizontal antenna diagram to a great
extent. In the forward direction, for example a 2° downtilt on a 6°
EDT antenna will give approximately the same effect as a 6° + 2 ° =
8° mechanical downtilt. An 8° mechanical downtilt does however
have a distorted horizontal antenna diagram (see chapter 2.2, Figure
5), while the 2° additional mechanical tilt on the EDT antenna will
only have a minor impact on the horizontal antenna diagram.
23 (6) 1999-09-29
5 CONCLUSION
Antenna down tilt can be a good tool in order to keep interference levels
under control in a network. Antenna down tilt does have most effect with
high gain, narrow vertical beam-width antennas. Best result is achieved
in areas with small cells, and/or high antenna positions. With large cells,
antenna down tilt can still be useful in order to solve local interference
problems, or to reduce the cell-size. This is however at the cost of
reduced coverage. The result of an antenna down tilt, if not very minor,
should always be verified. It is especially important to verify the effect
that the down tilt has on the coverage and quality in the area close to the
down tilted cell itself.

24 (6) 1999-09-29
Appendix
Picture from a typical “Urban” site. Some of the other Urban cells had
more nearby high-rise buildings, in some cases partly blocking the cell.

Antenna tiltguide

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