Calculating STL Fade Margin

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Broadcast auxiliary facilities, and STL systems in particular, tend to get relegated to the back burner. Many times this equipment is glossed over in day-to-day operations, and usually winds up being one of the latter concerns in studio relocation. Understanding the mechanics of your RF program delivery system, and maintaining a keen grasp of the associated fade margin can go a long way to diagnosing the rare, but ultimately problematic impact path failure can have.

In a nutshell, the fade margin is the difference between the received signal level at the input to the receiver and the sensitivity of the receiver. Typically this quantity is expressed in decibels. The higher the number, the more reliable the path.

Creating a path

When analyzing or designing a path, the first step is to ensure the path is viable. Paths of extraordinary length are obviously problematic as are those with substantial terrain obstructions. Cases where obstructions enter the Fresnel zones, especially the first zone, or those where reflective paths exist can also chew away at your margin. Existing operational paths imply viability; however, keeping track of signal levels and margins still makes good sense.

For reference purposes the width of the nth Fresnel zone at an obstruction in meters is calculated by the following equation where d1 and d2 are the distances from the link end points in meters, D is the total link distance in meters and f is the frequency in megahertz. We will, however, neglect situations in this article where Fresnel zone incursion or reflections occur and continue the analysis with an ideal path.

Next it is crucial to know the length of the path. Assuming there are no reflection issues, etc., along the path, then the path attenuation in decibels between two isotropic antennas is approximated by the following where d is the path length in km and f is the frequency in megahertz.

System components

In addition to the free space attenuation, we need to look at all of the components between the output of the transmitter and the input of the receiver. This includes all antennas and transmission lines, as well as filters, combiners, surge protectors, etc., that may lie in the system. From the manufacturer's data for each component, a gain or an insertion loss can be assigned. In some cases, especially antennas, the manufacturer will assign a frequency-dependent range of gains. Pay special attention, however, to the way antenna gains are specified. This analysis is based on isotropic antennas, so if dipole gains are utilized, that is dBd instead of dBi, an addition of 2.15dB to the antenna gain will need to be made. If a range of gains is specified, considering each scenario may illuminate potential problems.

Adding the gains together and subtracting the insertion losses out results in a total system gain. Typically the sum of the antenna gains will be much larger than the total insertion losses, thus your resulting total system gain should be a positive number. This gain is then subtracted from the free space attenuation number derived in the second equation above. This resulting number is the total path attenuation, or net path loss.

Next the transmitter power output is identified and converted into dBm. Transmitter powers typically range from less than a watt up to several watts depending on the model and make. If the power is given in watts, convert to milliwatts by multiplying by 1,000. Then take the base-10 logarithm and multiply that result by 10 to get dBm. Outputs of 1W will result in +30dBm, while 10W will be +40dBm, and so forth.

--Continued on page 2

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