Lesson 10 - VSWR, Transmission Lines and Reverse Power Protection
What is VSWR?
VSWR (Voltage Standing Wave Ratio) is a measure of how well a load is matched to the circuit driving it.
You may have heard the expression "Impedance matched". In the old days, when audio amplifiers were made with vacuum tubes, it was important to match impedances with audio gear. Many tube type amplifiers had a jumper in the back to select the amplifier's output impedance to match the speaker's impedance. 4, 8 or 16 ohms were standard values (Note 1)
That's because tube amplifiers have a specific output impedance that must be matched by the speaker impedance for the amplifier to work well. It is the same thing with all RF equipment in general, including microwave tubes.
Impedance matching
Here is a simple circuit that will allow us to see what is happening when the load impedance is matched to the source impedance.
Because this simulation is done with DC voltages, I will now talk about resistance instead of impedance, because it is the convention at DC, but the principle is the same.
This circuit shows a voltage source of 10 V, a series 50 ohm resistor R2, simulating the output impedance of the voltage source, and a variable load.
Let's run a DC sweep analysis by sweeping the current load I1 from 0 to 0.2 A. 0.2 A is the maximum current the circuit can draw, it is obtained when the load is a short circuit and the 10 V source drives the 50 ohm resistor R2.
SPICE wants an initial value for the current through I1, even though it is not used for the sweep, so let's set it to 0 A for now.
<VSWR-2.png>>
The power delivered to the load I1 is the product of voltage and current. Since at the left of the plot, the current is zero, the power is also zero. At the right of the plot, the current is maximum, but the voltage is zero, so the power is zero too.
In between, the power is not zero. Let's use SPICE to compute and display the power for us.
Right-click on the plot and select "Add Trace". In the "Expression" window, enter
V(output) * I1and click "OK".
Then click on the "Run" button again.
<VSWR-3.png>>
We just demonstrated that the output power is maximum when the load resistance is equal to the source resistance.
Wiring versus Transmission Line
A difference between audio equipment and RF/microwave equipment is that audio signals are of low enough frequency that the wavelength (Ref 1) is typically much greater than the length of wires involved in the circuit processing or carrying the signal.
At RF/microwave frequencies, the wavelength is often shorter than the length of wires connecting parts of the circuit, particularly for the length of cable between the final amplifier stage of a transmitter and the antenna.
When the length of the cable carrying the signal is greater than approximately 1/10th of the wavelength, we no longer talk about wiring, but we use the term Transmission Line.
In a typical transmission line (when the length is greater than 1/10th of the wavelength,) the signal propagates like waves at the surface of the water, creating positive voltage peaks and negative voltage peaks along the way just like the wave propagates at the surface of the water, creating crests and throughs as it moves along the surface.
To continue the analogy with the audio frequencies of very long wavelength, we do not really observe waves in a water bucket, because the bucket is much smaller than the size of an average wave. So, when you fill a bucket, other than the occasional slushing around, the water comes up all around the bucket at the same speed.
Enough analogies, let's get back to VSWR.
VSWR is equal to the greater of the ratios of output impedance to load impedance. It is always equal to or greater than 1.0:1. For instance, in the RF/microwave world, circuits are designed around 50 ohm cables and transmission lines. So, amplifiers are designed to drive 50 ohm loads on their output, cables are designed to have 50 ohm "characteristic impedance" (we will come back to this term later) and components that are receivers of RF energy (such as an antenna for instance) are designed to have 50 ohm input impedance.
When everything is matched perfectly, the VSWR is 1.0:1, the ideal value for optimum transfer of energy.
In extreme cases (load impedance being zero, such as a short circuit, or infinite, such as an open circuit), the VSWR will be infinite.
If an amplifier with 50 ohm output impedance drives a circuit with 25 ohm input impedance, the VSWR will be 2.0:1. The VSWR will also be 2.0:1 if the receiving circuit has 100 ohm input impedance.
Either case is bad because in either case, a significant portion of the output power from the amplifier will not make it into the receiving device and will be reflected back towards the source, i.e. the output of the amplifier.
It may sound paradoxical, but most amplifiers are not capable of taking back all the power they can produce, and when they drive a load that is not well matched (a load with high VSWR), the amplifiers will either simply malfunction or worse fail.
Imagine a boy playing with a water hose. He is very happy to sprinkle water everywhere around himself, but he would not want to direct the hose to a wall and get all that water back on himself :-) The wall would be equivalent to a high VSWR load and would reflect a lot of the water.
An ideal VSWR is 1.0:1, indicating that all the transmitted power is absorbed by the load.
VSWR is a useful number for evaluating the actual voltages and currents present along a transmission line. However, sometimes it is more interesting to know how much of the power is being reflected back to the transmitter. In those cases, we use the term "reflection coefficient". The Voltage Reflection Coefficient is the fraction of the incident voltage that is reflected. A low reflection coefficient is good, as it indicates that little voltage is reflected back. When dealing with power levels, we use the term "Return Loss". The Return Loss is the fraction of the signal power that is absorbed (lost) into the load. An infinite Return Loss is good, indicating no power is reflected back towards the transmitter.
The table below gives you the correlation between Return Loss, VSWR and Reflection
Coefficient (click on it to print a clean copy).
| Return loss (dB) | VSWR | Voltage Reflection Coefficient |
| 1 | 17.39:1 | 0.891 |
| 2 | 8.72:1 | 0.794 |
| 3 | 5.85:1 | 0.708 |
| 4 | 4.42:1 | 0.631 |
| 5 | 3.57:1 | 0.562 |
| 6 | 3.01:1 | 0.501 |
| 7 | 2.62:1 | 0.447 |
| 8 | 2.32:1 | 0.398 |
| 9 | 2.10:1 | 0.355 |
| 10 | 1.93:1 | 0.316 |
| 11 | 1.79:1 | 0.282 |
| 12 | 1.67:1 | 0.251 |
| 13 | 1.58:1 | 0.224 |
| 14 | 1.50:1 | 0.200 |
| 15 | 1.43:1 | 0.178 |
| 16 | 1.38:1 | 0.158 |
| 17 | 1.33:1 | 0.141 |
| 18 | 1.29:1 | 0.126 |
| 19 | 1.25:1 | 0.112 |
| 20 | 1.22:1 | 0.100 |
| 21 | 1.20:1 | 0.089 |
| 22 | 1.17:1 | 0.079 |
| 23 | 1.15:1 | 0.071 |
| 24 | 1.14:1 | 0.063 |
| 25 | 1.12:1 | 0.056 |
| 26 | 1.11:1 | 0.050 |
| 27 | 1.09:1 | 0.045 |
| 28 | 1.08:1 | 0.040 |
| 29 | 1.07:1 | 0.035 |
| 30 | 1.07:1 | 0.032 |
| infinite | 1.00:1 | 0.000 |
Typical transmission lines
The most common type of transmission line is the Coaxial Cable (Ref 2). A good example is the cable used for TV signal transmission, either from an outside antenna to the TV, or from the cable distribution system. Coaxial cable is composed of a center conductor, an insulating layer and a surrounding outside conductor, covered by a protective jacket.
Coaxial cable is characterized by its Characteristic Impedance. To maintain a stable characteristic impedance, coaxial cable must be manufactured precisely using stable materials. Therefore, quality coaxial cable is usually expensive.
Coaxial cable looks very similar to Shielded Cable, but while shielded cable also has a center conductor, an insulating layer, an outside conductor, or shield, and an external protective jacket, shielded cable does not have a specified characteristic impedance. It is not defined and not controlled or guaranteed by the manufacturer. Shielded cable is designed for low frequency signals, such as audio signals and would be very lossy at RF/microwave frequencies. So, do not confuse shielded cable and coaxial cable.
Another type of transmission line is called Twin Lead (Ref 3). It is used on indoor TV antennas and also sometime for indoor FM antennas.
Twin lead, as its name implies, is made of two identical conductors running side by side. It is intended to be symmetrical.
Reverse or Reflected Power Protection
We have seen that excessive power reflected back from the load into the transmitter could damage it. The best way to avoid damage is to have a good match of course. In doing so, not only we reduce the risk of damage, but we also optimize the power transfer and minimize power loss.
It is not always possible to guaranty a good match under all operating conditions. It could be that the load VSWR is not well controlled, or that the system has to operate over a broad frequency range, because it is more difficult to ensure a good match across a broad frequency range than over a narrow frequency range.
An example of condition which could cause an otherwise well matched antenna to show high VSWR would be the presence of an obstacle in front of the antenna, reflecting some of the signal back to the transmitter. Other potentially damaging conditions include damage to the transmission line between the transmitter and the antenna, such as a coaxial cable cut or pinched, or water intrusion inside the cable.
For these reasons, a reverse or reflected power protection (both terms are interchangeable) is recommended on all high power transmitters.
Such protection is often refereed to as "VSWR Protection", because it will protect against a high VSWR. However, what can damage the transmitter is high reflected power, a high VSWR is only a problem when the transmitter operates at full power. In many cases, a high VSWR at lower power levels is not as much of a problem.
It turns out that in many cases it is easier to detect high reflected power than high VSWR. To compute the VSWR, you need to know the forward power and the reflected power. A Reflected Power protection only needs to worry about the amount of reflected power.
So, a transmitter that has Reverse Power Protection (or Reflected Power Protection, which is equivalent) may be able to operate into a high VSWR load when operated at low power, but will trip if the power level is increased.
Notes: