The formula used to calculate the required do assumes that the main beam is to be pointed the cell edge. Simple trigonometry is used to calculate the downtilt angle DT in degrees
DT = ArcTan
(
h 1+h 2−h3 d 1∗5280
Where h1, h2, h3 are all measured in feet and d1 is measured in miles.
1) What is the ground elevation (ASL) at the site ? (ft) 2) What is the antenna height above ground (AGL) at the site (ft) 3) What is the distance to the cell edge ? (miles) 4) What is the ground elevation (ASL) at the cell edge ? (ft) 5) What is the vertical beamwidth of the antenna ? (angle in degrees between upper & lower 3dB points)
30 118 2 30 8.3
(h1) (h2) (d1) (h3) (b1)
To point the antenna main beam at the cell edge, use the following downtilt (negative value indicates uptilt) To point the upper 3dB point at the cell edge, use the following downtilt (negative value indicates uptilt)
0.6 degrees 4.8 degrees
ula used to calculate the required downtilt that the main beam is to be pointed at
gonometry is used to calculate tilt angle DT in degrees
= ArcTan
(
h 1+h 2−h3 d 1∗5280
, h2, h3 are all measured in feet measured in miles.
)
Antenna Roof Clearance Calculation 1) Locate the furthest point on the roof within the sector coverage angle 2) Note the distance to this point (ft)
75
(d2)
3) What is the height of the obstruction e.g. parapet if applicable. (ft)
5.5
(h5)
4) What is the required downtilt in degrees (see DOWNTILT sheet)
2.2
5) What is the vertical 3 dB beamwidth of the antenna in degrees
5
6) Required clearance angle is the sum of downtilt + half the antenna vertical beamwidth plus 3 degrees to allow additional downtilt in the future if necessary
7.7
(a2)
15.6
(h4)
7) For sufficient clearance, base of antenna should be this high above the main roof level
Diversity Calculation The top diagram shows the ideal placement for diversity antennas In this diagram the antennas are not "skewed" (i.e. the skew angle is zero degrees) In order to achieve good diversity performance, the effective separation of the two antennas must be around 8 to 10 wavelengths. This is most critical at the sector boundary as shown in the top diagram In the bottom diagram the antennas are skewed by an angle alpha In this situation, the required antenna separation can be calculated using the formula shown below.
d=
8λ cos( 60+ α )
Enter the frequency in MHz Therefore wavelength in metres =
1930 0.1554404
Enter the skew angle in degrees
15
Therefore the distance between antennas in metres must be at least
4.8
Required Separation in feet =
15.8
Note : if the antennas are skewed 30 degrees the required separation is infinity !!
ersity antennas the skew angle is zero degrees)
e effective separation of the two is is most critical at the sector
