How to calculate the correct tilt for an AP

When planning and deploying a PMP Access Point it is important to make sure that the antenna is installed with the correct amount of mechanical tilt. The amount of tilt required depends on the effective height of the AP antenna relative to the subscribers and the type of antenna used.

Cambium now have different antennas some of which have no electrical tilt and others which have approximately 2 degrees of electrical tilt built into the antenna. The antennas with electrical tilt include the ePMP 90/120° antenna, the new PMP 450i Integrated antenna at both 3 and 5 GHz and the PMP 450m antenna.

If an antenna has no electrical tilt then the peak gain of the antenna is parallel to the ground with the antenna mounted with no mechanical tilt, as shown by the orange lobe in Figure 1. If it has electrical downtilt built into the antenna, then even when the antenna is mounted with no mechanical tilt, the peak gain of the antenna, will be pointing down below the horizon. The blue lobe in Figure 1 shows an exaggerated view of the tilt, as it is usually only a few degrees.


Figure 1: Illustration of the effect of electrical downtilt in an antenna.

When comparing different products and antennas during the planning stage it is important to take into account the electrical tilt on the antenna and adjust the antenna tilt as well as changing the antenna. The hover help over the Antenna Tilt setting will show the amount of electrical tilt for a given antenna. If the Electrical Tilt row is not shown then the antenna has no electrical tilt and the peak gain is at 0° elevation.

LINKPlanner has a function to help set the mechanical tilt correctly for a given group of subscribers connected to an Access Point. Once the subscribers have been connected to the AP, select the Calc Tilt button in the Antenna Configuration section of the Access Point Equipment menu, as shown on the right in Figure 2.


Figure 2: Antenna Configuration settings on the AP

This will pop up a new Calculate Tilt menu as shown in Figure 3. Set the minimum and maximum tilt to the values required for the range to be tested. These values will default to the range of mechanical tilt allowed for the antenna bracket. You can choose to either run an initial test over a wide range with the larger interval of 2° or take a look at the minimum and maximum AP Antenna Elevation values across the group of subscribers and use this to select a narrower window. In which case you can also then choose a narrower test interval, e.g. 0.5° as shown. Remember to take account of the electrical tilt when looking at the antenna elevation angles for each SM, for example a range of angles between 0 and 2° may mean you want to test between +1 and -3° without electrical tilt, but for an antenna with electrical tilt, the antenna may need to be mechanically uptilted and therefore you should test between +3 and -1°.

Once these values have been configured select Go. LINKPlanner will then test the different tilt options within the range for the group of subscribers, determine how many can be supported at the required performance and the aggregate throughput for the AP at each tilt angle.

Figure 3: Calc Tilt function menu

Once all the angles have been tested a table of results will appear in the bottom of the window. This table can be copied or saved to csv using the icons just above the table.

The optimum value is shown by the green tick and that row is automatically highlighted. Depending on the height of the AP and the topography of the subscribers you can get just one optimum angle or a range of angles all with the same throughput, in which case LINKPlanner will select the mid-range value.

LINKPlanner will select the row with the most subscribers and the highest throughput. It is possible to get higher throughput rates with fewer subscribers if low modulation mode subscribers drop out of coverage.

There may be other circumstances that are also influencing the tilt on the sector, therefore you can select a different row from the one automatically selected, if required.

Once the appropriate row is highlighted then select Apply Tilt and only then will the Access Point in the project be updated. If you simply close the window without selecting the Apply Tilt button, then there won’t be any change on the Access Point.

Note that throughout LINKPlanner a negative tilt angle refers to downtilt or an angle below the horizon.