Imagine the difference when you are in a clothes packed walk-in closet… no echoes. The fabric around you absorbs sound, and the loss of the sound bounces is high high return loss. When we send RF down a coax or PCB trace, we want it to go into the antenna then radiate out into the world. We do not want it to bounce back towards us like sound in an empty room. Smaller return loss is bad, and means less energy is going into our antenna.
However, just remember better return loss is indicated by bigger return loss numbers, and that is better for your antenna. Here are some examples of the logarithmic scale, or loss in decibels:. The way in which the effect occurs can be demonstrated with a length of rope. If one end is left free and the other is moved up an down the wave motion can be seen to move down along the rope. However if one end is fixed a standing wave motion is set up, and points of minimum and maximum vibration can be seen.
When the load resistance is lower than the feeder impedance voltage and current magnitudes are set up. Here the total current at the load point is higher than that of the perfectly matched line, whereas the voltage is less. The values of current and voltage along the feeder vary along the feeder. For small values of reflected power the waveform is almost sinusoidal, but for larger values it becomes more like a full wave rectified sine wave.
This waveform consists of voltage and current from the forward power plus voltage and current from the reflected power. At a distance a quarter of a wavelength from the load the combined voltages reach a maximum value whilst the current is at a minimum. At a distance half a wavelength from the load the voltage and current are the same as at the load.
A similar situation occurs when the load resistance is greater than the feeder impedance however this time the total voltage at the load is higher than the value of the perfectly matched line. This loss is characteristic of the same, and defined in terms of dB per lenght unit - the longer the cable is, the greater is the loss. This attenuation also increases with increasing temperature and frequency. Unfortunately, these factors are not much scope of our control, since the frequency is already preset by the system we use, and the temperature will be exposed to climatic variations of where the cable has to pass.
The most we can do is try to use cable with less attenuation , ie, cableswith high quality materials used in its construction of the drivers internal and external and insulating dielectric. As a rule, the larger the diameter of the cable, the lower your attenuation. The choice of coaxial cable for the system is a process that requires a very comprehensive analysis, taking into account its characteristics is it softer, etc … and costs of several options of existing cables, necessary cable length - and the consequent loss that it will introduce, the loading of the tower or brackets where cables will be posted, among others.
But the other form of loss that we have in our system, and can be controlled a bit more is the loss by reflection, ie loss of the signal, which has just returned, lost by the end where it was injected. For this reason we call the Return Loss. So, part of the signal which ideally should leave by the antenna, then returns reflected! Speaking in terms of the matching impedances, if the value of X, Y and Z are equal, we have the following.
If we consider an ideal transmission line, the VSWR would be , ie all the power to reach your destination, with no reflection nothing lost.
And the worst means of transmission in the world, we would have infinite VSWR, ie all the power would be reflected lost. So what are the problems that we can in a bad VSWR very high? VSWR is a function of the reflection coefficient, which describes the power reflected from the antenna.
If the reflection coefficient is given by , then the VSWR is defined by the following formula: The reflection coefficient is also known as s11 or return loss. See the vswr table below to see a numerical mapping between reflected power, s11 and VSWR.
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