# ATTENUATION/SRL RATIO

When designing a transmission line, it is necessary to carefully choose the cable to be used, based on the frequency and the distance between the transmitter and the antenna. We assume that the impedance matching between the various components has been treated with the utmost diligence. Everyone knows how important is to buy a REALLY low-loss cable, but not everyone reminds that -3dB = ½ the power available. It is also important to verify that the difference between the value of SRL and attenuation should be as wide as possible. In fact, as seen in the picture, it is inevitable that the two curves will cross each other. With increasing frequency, the attenuation curve (A) is approaching more and more to that the reflected waves (B). Comes the point where the attenuation value in dB and that of SRL meet each other. **Starting from this frequency and beyond, the output signal will be ZERO**, regardless of the input power value. The example concerns a test on the cable **M&P-ULTRAFLEX 7**, (a 35 meters long coil). In these conditions the signal is reduced to zero at the frequency of 10 GHz (in transmission only). It is clearly inadvisable to use such a cable length at this frequency, but the chart clearly indicates that at all frequencies lower than 4.2 GHz, the transmission line works in an excellent manner. Increasing the cable length, inevitably increases the attenuation so that the intersection with the SRL curve, will happen before (at a lower frequency). Differently, shortening the cable length will assure a correct use at higher frequencies. In the following charts we can see how the SRL affects the power.** **The graph is showing a 50m long, perfectly tuned transmission line. The cable used is again **M&P-BROAD-PRO 50/C**. The red curve is the attenuation, the blue curve is the SRL. The three black curves, are 3 different input powers: 200, 500 and 1000 Watts. As previously said, regardless of the input power, when the SRL dB

values are equivalent to attenuation values, there is no more output signal. Please note that as soon as the SRL value increases, (for example due to an impedance mismatch), the output power quickly collapses. Although an optimal SRL (**S**tructural **R**eturn **L**oss, in simple words, attenuation on the reflected wave) is typically between -40 and -30 dB, we can say that until -18 dB there are no considerable losses. Increasing the SRL to higher values, the closer the SRL values are to 0, the more the effects evolve from troublesome to destructive. In the presence of strong SRL, (dB values close to zero), along the cable will occur overvoltage and overcurrent.

Given a power fed to the X value (any value expressed in Watts), the actual power output of the cable is shown in the table in the form of remaining percentage.

(for example, if we use a cable such as **M&P-BROAD-PRO 50/c**, entering 1000 Watts over a length of 35m, at a frequency of 144 MHz, there remains 70.7% of 1000). For maximum applicable power, see the Power Handling of the cable concerned. From these values, have already been deducted the SRL values, typical of each one of our models, for the respective frequencies.

**REMEMBER: Make sure to match the line accurately!**

Given a power fed to the X value (any value expressed in Watts), the actual power output of the cable is shown in the table in the form of remaining percentage.

(for example, if we use a cable such as **M&P-ULTRAFLEX 7**, entering 1000 Watts over a length of 35m, at a frequency of 144 MHz, there remains 57,3% of 1000). For maximum applicable power, see the Power Handling of the cable concerned. From these values, have already been deducted the SRL values, typical of each one of our models, for the respective frequencies.

**REMEMBER: Make sure to match the line accurately!**

Given a power fed to the X value (any value expressed in Watts), the actual power output of the cable is shown in the table in the form of remaining percentage.

(for example, if we use a cable such as **M&P-ULTRAFLEX 13**, entering 1000 Watts over a length of 35m, at a frequency of 144 MHz, there remains 75.3% of 1000). For maximum applicable power, see the Power Handling of the cable concerned. From these values, have already been deducted the SRL values, typical of each one of our models, for the respective frequencies.

**REMEMBER: Make sure to match the line accurately!**