Easy Ways to Simulate the Strongest Microwave Links

Having recently been involved with several microwave link installations and servicing, I have gathered several best practices to installing digital IP microwaves and obtaining the best PtP (point-to-point) links from them. A PtP link is simply a directional link between microwave antennas with a clear line of sight. During installation, several prior calculations have to be made to ensure the best possible link as well as mitigate the chances of link failure. With the recent development of online geographical maps, these calculations have been even more simplified and in this post we will dive into using these tools.

In my field of work we apply microwave links in providing distribution links and contribution links. Distribution links are mainly the links between a studio and a transmitting site, such a link is mentioned here. Contribution links are the links between an OB (outside broadcast) site and their studio.

Path Profiles

A path profile refers to a straight line cross-section of two points on the earth’s surface. This can be used in obtaining the clearance characteristics of two points. From secondary school geography, for those who can remember, this was obtained by drawing a straight line between two points on a topographical map (scale of 1:50000 for example), obtaining the altitudes of all the points on the line then drawing an altitude versus distance graph. Joining the points on the graph smoothly gives you the path profile required for your point to point connection.

Beyond this an earth bulge calculation has to be performed on points on your profile close to the straight line of sight. This is to compensate for atmospheric refraction which causes variations of the atmospheric refractive index caused by the effective earth radius $k$ , these variations lead to bending of microwave rays. A $k$ value of 4/3 is usually taken to denote the change in atmospheric index with height due to the earth’s radius which may cause the microwave ray to bend downwards. Other changes in atmospheric refraction may cause the value of $k$ to drop to about 2/3 which may cause an upward bending of the ray. The earth bulge constraint helps you better plan the height to set your antennas with the calculation as:

$EB = \frac {d_1 d_2} {12.8k}$

where $d_1$ and $d_2$ are distances between the particular point on the path and the each end on the path in kilometers.

After this we obtain the radius of the first Fresnel Zone which to put in technically is the locus of all points surrounding a radio beam from which reflected rays would have a path length one half-wavelength greater than the direct ray but may be simply understood as the region where a direct line of sight is achieved with the strongest signal link. The first Fresnel zone may be calculated as follows:

$R_{FZ} =17.3 \sqrt {\frac {d_1 d_2}{F(d_1+d_2)}}$

where where $d_1$ and $d_2$ are the distances to each end of the path in kilometers, and F is the frequency in gigahertz.

A minimum 60% clearance criterion is also required. This is normally a clearance of 60% (0.6) over obstacles of the first Fresnel zone calculated above with $k$ equal to 2/3 . With this, we can now use the manufacturer’s data of our radios and antennas to determine the final parameters in our planning. These include:

Antenna gain
Branching losses at both ends of the link
Feeder equipment losses

Alternatively…

With all these in place you should be fine to gather your equipment and start right away. However, the process discussed may be too lengthy and prone to errors depending on your math skills. At the time of writing this post technology has made more complicated areas in life such as dating as simple as swiping right, so why not this too? Thankfully, a lot of manufacturers offer proprietary simulators to determine a clear PtP or even PtMP (point-to-multipoint) link from the comfort of your computer. One such manufacturer is Ubiquiti with their popular link simulator.

Using Ubiquiti’s tool is fairly simple. I did a simulation of two points, Figure 1, between Chiromo area in Nairobi area and Limuru where transmitter sites for broadcast are usually found.

**Figure 1:** A full screen image of Ubiquiti’s link simulator

To start off, obtain the coordinates of the two points. During site surveys I find it very useful using My GPS Coordinates which is a simple, free Android app which gives you your current coordinates. Having the coordinates of your access point (AP) and station (STA) we can now simulate link of the two sites. Here I have -1.275106,36.807504 as my access point and -1.127295,36.635714 as my station. In a previous installation, we used the Rocket M5 radios operating at 5GHz frequency using RocketDish antennas with a gain of 34dBi. Antennas with gains of 30dBi would also have sufficed but the higher gain antennas gave us better signal strengths. This information is filled on the section in Figure 2 together with the channel width.

**Figure 2:** Section in link simulator where you enter your location details, antenna gain and channel width

At this point you leave it up to the simulator to perform the calculations that give you details on the signal strength. If one has not been achieved between the two points the simulator will state that the link is obstructed. This can be rectified by adjusting the antenna height (to as high as is reasonable) or changing the station area by moving the antenna position to get a clear LoS. If this is still not achievable, consider setting up repeater stations to navigate around the obstructions.

With a clear line of sight the next step is to check if the signal strengths are satisfactory. The higher the better. According to my simulation, Figure 3, -90dBm is weak while -60dBm is good. Aim for the strongest link possible to counter effects of fog, precipitation and changes in atmospheric gases. The same steps defined can be used for PtMP links.

**Figure 3:** An established link on the simulator indicating good signal strengths and total capacity that can be sustained

Conclusion

This is a helpful skill I gathered from setting up microwave links and troubleshooting when failure occurs. To ensure the highest levels of availability in a year, the strongest possible links should be set at your client site. Understanding the important parameters helps you in prior planning before installation and the performance to expect from it.