September 22, 2016
When designing and building 4G infrastructure, one goal of engineers is to maximize the RF signal level throughout the coverage area. Assuming the noise level remains constant, higher signal levels result in faster data rates and fewer dropped calls, ultimately resulting in happier customers and greater revenue for operators.
Operators often specify that the worst case reflections – a.k.a. return loss – over the operating frequency range of the system must be 18 dB to maintain the desired signal levels. Another way of looking at this is that the reflected signal must be less than 1/64th the magnitude of the signal transmitted into the system. As feed systems become shorter and antenna systems are required to operate over broader frequency ranges, however, achieving an 18 dB return loss may not be practical for today’s wireless networks.
Return Loss Impact on Power
One thing to remember is that additional dBs of improvement in return loss have less impact on power lost, thereby debunking the belief that “a change of 3 dB means the power is doubled or cut in half.” It is a common mistake to misapply this rule and falsely believe that improving return loss from 15 dB to 18 dB, for example, will double the coverage area of a site. What actually occurs is that the power lost is reduced by half (relative to its starting value) when the return loss changes by 3 dB.
To help understand the coverage impact associated with changes in return loss, we will assume a typical site configuration and use the propagation loss equation shown below to solve for the distance “d” associated with a desired signal level.
Propagation loss (LP)= –27.56 + 20*Log10(f) +n*10*Log10(d) Where:
f = Frequency (MHz)
d = Distance (m)
n = 3.0 for urban morphology
In this example, we will assume the following site configuration:
Power transmitted by the radio (PT) = 20W = 43 dBm Frequencies of interest = 700 MHz, 850 MHz, 1900 MHz, 2100 MHz Feed system loss due to absorption (LA) = 2.5 dB
Feed system loss due to reflections (LR) = Equation 6
Antenna gain (G) = 17 dBi
Desired power level arriving at the mobile (PR) = –75 dBm
Looking at specific examples, the change in coverage distance can be evaluated for a return loss improvement from 15 dB to 18 dB and from 17 dB to 18 dB as shown in Table 1 and Table 2.
As can be seen in figure 1, the coverage distance is improved, but only slightly.
If the operator determines that the non-compliant return loss specification is unacceptable and instructs the crew to perform repairs to achieve the desired value, it is important to use the proper methods. Here are two “good” changes that improve return loss without negatively impacting another variable that might degrade overall site performance.
- Disconnect and reverse an existing jumper cable – By doing so, you can change the phase relationship of reflections in the system. Sometimes, this change can cause a “failing” system to pass.
- Distance-to-Fault (DTF) measurement – This proven test can determine the RF connection with the highest individual return loss contribution. Opening this connection and improving workmanship may cause a “failing” system to now pass.
Conversely, here are a few approaches that are not recommended when trying to improve return loss.
- Loosening a RF connector – Adjusting the phase relationship of reflections in the system by loosening the connector and then taping over it is not acceptable. Loose connectors degrade the mechanical reliability of the feed system, as well as generate passive intermodulation (PIM).
- Increasing jumper cable lengths – If the antenna is the highest source of reflection in a system (which is often the case), adding 0.5 dB of cable loss will result in 1.0 dB improvement in return loss measured at the system input. While this might seem like a good idea, the added insertion loss has a negative impact on site coverage. Improving the system return loss from 17 dB to 18 dB by adding 0.5 dB of cable loss will result in a significant reduction in site coverage. Adding loss is never a good way to improve site performance!
Anritsu offers a complete line of cable and antenna analysis solutions and automation tools to help wireless professionals measure return loss and manage cell sites. To learn more about this subject and the solutions, download a white paper entitled The Impact of Return Loss on Base Station Coverage in Mobile Networks.