What is VSWR?

What is VSWR?

After the read, you will learn about what is VSWR, what is good VSWR.

When dealing with antennas, microwave radio frequency engineers have to deal with standing wave ratio. What is the voltage standing wave ratio and how is this parameter calculated?

In any radio system that includes a transmission line, the transmitter of the radio signal, the transmission line that transmits the signal, and the load of the radio signal have a characteristic impedance.

The transmitting and receiving systems usually used for amateur radio signals mostly have a characteristic impedance of 50 ohms, while the characteristic impedance in a cable television system is mostly 75 ohms.

When the characteristic impedances of all devices in this system are the same, the problem is not big. But if the characteristic impedance of the transmitter and the transmission line are both 50 ohms and the load is not 50 ohms, some problems will arise.

We can simply think that when the characteristic impedance of the load is inconsistent with the transmission line and the transmitter, the power emitted by the transmitter cannot be completely received by the load. There must be some power going back to the transmitter along the transmission line. 

At this time, the voltage and current of the forward wave and the reverse wave in the transmission line will be superimposed and added and subtracted at different points according to the phase relationship, thereby generating a standing wave on the transmission line.

VSWR is a definition that measures the standing wave condition on a transmission line. It is defined as the ratio of the voltage at the maximum point to the minimum point on a lossless transmission line. Although the current on the transmission line also satisfies the same relationship, measuring voltage is always easier than measuring current. 

If the impedance matching of the antenna matches the impedance of the transmitter and the feeder, the measured VSWR should be 1:1 or 1. When the impedance cannot be completely matched, the VSWR may be 2:1 or even more than 5:1.

Why do we usually say that the standing wave ratio will affect the antenna feed system? The main factors are as follows.

1.   The power amplifier part of the transmitter may be damaged.

Due to the mismatch of the antenna feed system, the power returned to the transmitter may be unbearable by the final stage transistors of the power amplifier. Forcibly launching may burn the machine, most of the machines now have automatic protection, but still cannot be taken lightly. In addition, the machine automatically reduces the power when protecting, so that the effect of the launch is greatly reduced.

2.   The high voltage and current at the stagnation point may damage the feeder.

Although this is an extremely rare situation, if the transmitter is not protected and still emits radio waves at high power, the wrapping material of the feeder may be damaged.

3.   The reflected wave may cause distortion.

If the transceiver is a full-duplex machine, its own signal reflected back through a longer feeder may interfere with the receiver.

Then, we usually adjust the antenna according to the standing wave ratio, what does it have to do with the transmission efficiency?

If you recognize the essence of the standing wave ratio, you will find that the standing wave ratio actually describes the impedance matching relationship between the antenna system and the transmitter system, and depending on the antenna structure, not all excellent antennas the characteristic impedance is 50 ohms. 

The impedance of the Yagi antenna can be 28 ohms, while the impedance of some other antennas can be several hundred ohms. If it is directly connected to the transmitter, its VSWR will be very high, but you cannot call it a low-efficiency antenna, but should use an appropriate impedance matching method to properly match it. Similarly, a 50-ohm non-inductive resistor connected to the end of the feeder can achieve a perfect match of 1:1, but its radiation efficiency is 0.

The voltage standing wave ratio (VSWR) is the most commonly used parameter in radio frequency technology to measure whether the matching between components is good. When radio amateurs make contact, of course, they will first think of measuring whether the standing wave ratio of the antenna system is close to 1:1. If it is close to 1:1, of course, it is good. I often hear the question: But what if it can’t reach 1? How small is the standing wave ratio for the antenna to be qualified?

The condition for the matching of the transmitter and the antenna is that the resistance component of the impedance of the two is the same, and the inductance part cancels each other. If the impedance of the transmitter is different, the impedance of the antenna is required to be different. 

In the electronic tube era, on the one hand, the output impedance of the electronic tube is high, and on the other hand, the low-impedance coaxial cable has not been promoted. The parallel feeder with a characteristic impedance of several hundred ohms is popular, so the output impedance of the transmitter is mostly several hundred ohms. 

However, the nominal impedance of the antenna of modern commercial solid-state radio communication devices is mostly 50 ohms, so the commercial VSWR meter is also designed according to 50 ohms.

If you have an old radio with an output impedance of 600 ohms, you don’t have to bother to repair your antenna with a 50-ohm VSWR meter, because that would be a waste of help. Just try to adjust to the maximum current of your antenna.

When VSWR is not 1, it is meaningless to compare the value of VSWR

Just because the value of VSWR other than 1 is not worthy of being determined so accurately (unless there are special needs), most VSWR meters are not calibrated as carefully as voltmeters and resistance meters, and very few VSWR even give its error level data. Due to the influence of the phase-frequency characteristics of the RF coupling components in the meter and the nonlinearity of the diode, most VSWR meters have uneven errors at different frequencies and different powers.

VSWR = 1 does not mean that all antennas are good

The most important factor affecting antenna effect: resonance

To make the electromagnetic field emitted by the antenna the strongest, one is that the emission frequency must be the same as the natural frequency of the antenna, and the other is that the driving point must be selected at an appropriate position of the antenna.

If the driving point is inappropriate and the antenna resonates with the signal frequency, the effect will be slightly affected, but if the antenna does not resonate with the signal frequency, the transmission efficiency will be greatly reduced.

Therefore, resonance is the most critical factor among the two points that need to be achieved for antenna matching.

When there is no condition to achieve a VSWR of absolutely 1, the most important adjustment of the amateur radio antenna is to make the entire antenna circuit resonate with the operating frequency.

The standing wave ratio of the antenna and the standing wave ratio of the antenna system

The VSWR of the antenna needs to be measured at the feed end of the antenna. But the antenna feed point is often high in the air. We can only measure the VSWR at the lower end of the antenna cable so that the VSWR of the entire antenna system including the cable is measured. When the impedance of the antenna itself is indeed a pure resistance of 50 ohms, and the characteristic impedance of the cable is indeed 50 ohms, the measured result is correct.

When the antenna impedance is not 50 ohms and the cable is 50 ohms, the measured VSWR value will be seriously affected by the length of the antenna. Only when the electrical length of the cable is exactly an integral multiple of the wavelength and the cable loss can be ignored, the lower end of the cable. The impedance presented is exactly the same as the impedance of the antenna. 

But even if the length of the cable is a multiple of the wavelength, but the cable has lost, for example, the cable is thin and the electrical length of the cable is more than tens of times the wavelength, the VSWR measured at the lower end of the cable will still be lower than the actual VSWR of the antenna.

Therefore, when measuring VSWR, especially in the frequency band above UHF, do not ignore the influence of the cable.

Asymmetric antenna

We know that the electrical length of each arm of the dipole antenna should be 1/4 wavelength. if the lengths of the two arms are different, how to calculate its resonance wavelength? Will there be two resonance points?

A dipole antenna with a total system length of less than 3/4 wavelength (or a single-arm antenna that mirrors the earth and ground network) has only one resonant frequency, which depends on the total length of the two arms. The two arms are symmetrical, which is equivalent to driving at the lowest point of impedance, and the lowest impedance is obtained. The length of the two arms is different, which is determined by the total length of the two arms. 

If it goes to the extreme, one arm is lengthened to 1/2 wavelength and the other arm is shortened to 0, and the driving point impedance increases to almost infinity, it becomes a terminal-fed antenna, which is called the Zeppelin antenna and the modern 1/2-wavelength R7000 vertical antenna, of course, must add the necessary matching circuit to connect to the 50-ohm low-impedance transmitter.

The two arms of a dipole antenna are asymmetric, or the influence of conductive objects around the two arms is asymmetric, which will increase the impedance at resonance. But as long as the total electrical length maintains 1/2 wavelength, the asymmetry is not very serious. Although the characteristic impedance will become higher and affect VSWR to a certain extent, the actual emission effect will not be significantly worsened.

QRPer does not have to demand VSWR

When the VSWR is too high, it is mainly when the antenna system is not resonant, so that there is a large reactance component in the impedance, the final device of the transmitter may need to withstand a large instantaneous overvoltage. When the early technology was not very mature, high VSWR easily caused damage to the RF final power device. Therefore, it is necessary to control the VSWR to a low value, such as 3 or less.

Some devices now have a relatively complete high VSWR protection. When the VSWR measured online is too high, the drive power will be automatically reduced.

The matching condition between the antenna system and the transmitter with an output impedance of 50 ohms is that the antenna system impedance is a pure resistance of 50 ohms. To meet this condition, two things need to be done:

First, the antenna circuit resonates with the operating frequency (otherwise the antenna impedance is not pure resistance);

Second, choose an appropriate feed point.

Some magazine articles often give VSWR curves when introducing antennas. Sometimes there will be an illusion, as long as VSWR=1, it will always be a good antenna.

In fact, VSWR=1 can only indicate that the energy of the transmitter can be effectively transmitted to the antenna system. But whether these energies can be effectively radiated into space is another question. A dipole antenna made according to the theoretical length and a shortened antenna with a length of only 1/20, as long as appropriate measures are taken, they can achieve VSWR=1, but the transmission effects are definitely very different and cannot be compared.

As an extreme example, a 50-ohm resistor, its VSWR is ideally equal to 1, but its emission efficiency is zero.

And if VSWR is not equal to 1, for example, it is equal to 4, then there are many possibilities: the antenna is detuned inductively, the antenna is capacitively detuned, the antenna is resonant but the feeding point is wrong, and so on.

On the impedance circle chart, each VSWR value is a circle with infinite points. when the VSWR value is the same, there are many possibilities for the state of the antenna system. Therefore, it is not too strict to use the VSWR value for a simple comparison between two antennas.

Antenna VSWR=1 indicates that the antenna system and the transmitter meet the matching conditions, and the energy of the transmitter can be most effectively transmitted to the antenna. There is only this kind of matching situation.

The standing wave ratio is called the voltage standing wave ratio, also known as VSWR and SWR, which is short for English Voltage Standing Wave Ratio.

Refers to the ratio of the standing wave antinode voltage to the valley voltage amplitude, also known as the standing wave coefficient and standing wave ratio. When the standing wave ratio is equal to 1, it means that the impedance of the feeder and the antenna is completely matched.

At this time, all high-frequency energy is radiated by the antenna without energy reflection loss; when the standing wave ratio is infinite, it means total reflection, and no energy is radiated.

The standing wave ratio is a numerical value used to indicate whether the antenna and the radio wave transmitting station match. If the value of SWR is equal to 1, it means that the radio waves transmitted to the antenna have no reflections and all are transmitted. This is the ideal situation. 

If the SWR value is greater than 1, it means that a part of the electric wave is reflected back and eventually turns into heat, causing the feeder to heat up. The reflected radio waves can also generate a relatively high voltage at the output port of the launching station, which may damage the launching station.

The meaning of the antenna standing wave ratio indicates the matching degree of the antenna feeder and the base station (transceiver).

Definition of standing wave ratio:

VSWR=Umax/Umin

Umax——The antinode voltage of the feeder;

Umin——The valley voltage on the feeder.

The standing wave ratio is caused by the fact that the incident wave energy is transmitted to the input terminal B of the antenna without being completely absorbed (radiated), generating reflected waves, and superimposed.

The greater the VSWR, the greater the reflection, and the worse the match.

So, what are the disadvantages of a poor standing wave ratio? What is the acceptable standing wave ratio in engineering? An appropriate standing wave ratio index is a trade-off between the amount of energy lost and manufacturing cost.

⑴ VSWR>1, it means that part of the power input into the antenna is reflected back, thus reducing the radiated power of the antenna;

⑵ Increase the loss of the feeder. The 7/8" cable loss is 4dB/100m, which is measured under VSWR=1 (full matching); with reflected power, the energy loss is increased, thereby reducing the input power of the feeder to the antenna;

(3) At the feeder input terminal A, when the mismatch is severe, the output power of the transmitter T cannot reach the design rating. However, the output power of modern transmitters is allowed to reach the rated power under certain mismatch conditions such as (VSWR<1.7 or 2.0).

The relationship between standing wave ratio and reflected power is as follows.

It can be seen that it is not necessary to pursue a standing wave ratio below 1.1, generally, 1.5 is enough, and 96% of them are launched.

Standing wave ratio reflectivity

1.0 0.00%

1.1 0.23%

1.2 0.83%

1.3 1.70%

1.5 4.00%

1.7 6.72%

1.8 8.16%

2.0 11.11%

2.5 18.37%

3.0 25.00%

4.0 36.00%

5.0 44.44%

7.0 56.25%

10 66.94%

15 76.56%

20 81.86%

VSWR and a nominal impedance

The condition for the matching of the transmitter and the antenna is that the resistance component of the impedance of the two is the same, and the inductance part cancels each other. If the impedance of the transmitter is different, the impedance of the antenna is required to be different.

In the electronic tube era, on the one hand, the output impedance of the electronic tube is high, and on the other hand, the low-impedance coaxial cable has not been promoted. The parallel feeder with a characteristic impedance of several hundred ohms is popular, so the output impedance of the transmitter is mostly several hundred ohms. 

However, the nominal impedance of the antenna of modern commercial solid-state radio communication devices is mostly 50 ohms, so the commercial VSWR meter is also designed according to 50 ohms.

The standing wave ratio (VSWR) is used to detect the working status of antenna and feeder systems, radio frequency connectors, and all radio frequency equipment connected to the base station. Too high VSWR will lead to dropped calls and a high bit error rate, and the attenuation of the transmit/receive power introduced by this will cause the coverage radius of the cell to shrink.

Only when the load impedance is fully matched with the signal source impedance can the signal be transmitted from the signal source to the load to the maximum extent. For the base station system, the signal source is the transmitter, and the load is the antenna feeder subsystem.

The antenna feeder subsystem includes antennas, feeders, radio frequency connectors, lightning arresters, and other auxiliary equipment. If the load and the signal source cannot be completely matched, part of the signal will be reflected back to the signal source. This is what we do not want.

At this time, a forward wave and a reverse wave will be generated. These two signals are combined to form the standing wave. Standing wave ratio (VSWR) refers to the ratio of the maximum level to the minimum level of the standing wave, and its size ranges from 1:1 (completely matched) to ∞.

Acceptable VSWR range

Since we cannot achieve 100% impedance matching between the load and the signal source, part of the signal will inevitably be reflected back by the antenna, so we need to determine a VSWR range as a measure of acceptable VSWR. Usually, we use 1.13:1 – 1.38:1 as the VSWR measurement standard.

In addition, the return loss can also be used as a measure. The ratio of forwarding power to reverse power is the return loss. If the forward power of 40dBm and the reverse power of 20dBm are known, then we can calculate the return loss. It is 20dB. If the output power of the base station is known to be 20W and the return loss is 16dB, then we can calculate that the reflected power is 0.5W. Usually, we take 16-24dB as a measure of return loss.

When a serious failure occurs in the transmission path from the transmitter to the antenna, very low return loss will occur, such as loose radio frequency connectors, antenna failure, feeder damage, lightning arrester breakdown, and filter/coupler damage, etc. This serious VSWR failure will cause failures such as dropped calls, increased bit error rate, and smaller cell coverage radius.

Looking at the standing wave ratio from the perspective of power

From the viewpoint of power, the standing wave ratio can be expressed as SWR = (√Po + √Pr)/(√Po-√Pr)

Po: Power entering the antenna system

Pr: The power reflected from the antenna system

After calculation, the relationship between SWR and Pr/Po (reflected power percentage) is as follows: Pr/Po = [(SWR-1)/(SWR+1)]^2

In fact, the standing wave ratio tester is basically a power meter, it can measure input power and reflected power

What is a standing wave?

Two coherent waves with the same amplitude are superimposed on each other when they propagate in opposite directions on the same straight line, called standing waves.

Features:

The troughs and crests remain stationary, the waveform does not propagate forward, and only vibrates at the equilibrium position; the waveform does not move in space.

Generally, the electromagnetic waves on the transmission line are composed of traveling waves (waves transmitted forward) and reflected waves. The standing wave ratio reflects the state of the wave staying. For example, the larger the standing wave ratio, the more the wave stays in place. If the standing wave ratio is infinite, means that the wave stays in place. 

On the contrary, the reciprocal of the standing wave ratio can be defined as the traveling wave coefficient, which represents the state of the wave traveling. The larger the traveling wave coefficient, the more forward the wave is.

What is matching?

In the mobile communication system, auxiliary equipment such as transmitters, antennas, feeders, radio frequency connectors, and lightning arresters need to be connected with cables and connectors. Only correct connections can ensure the normal operation of the equipment. In general, people tend to pay more attention to the steps of antenna direction, elevation, polarization, frequency selection, etc. The connection between equipment is often ignored in the installation and debugging steps. These equipment components should be connected from beginning to end.

The principle of matching.

Matching, from the perspective of power, means the maximum output power, that is, in the power supply circuit, the load impedance is equal to the conjugate value of the internal impedance of the power supply (the resistance is equal, the reactance is equal, and the sign is opposite), which is called matching. 

The purpose of matching is to get the maximum output power. From the perspective of the transmission line means the lossless transmission, that is, when applied to the transmission line, the load impedance is equal to the characteristic impedance of the transmission line, which is called "matching". The purpose of matching is to eliminate reflections caused by the load, avoid standing waves, and enable the load to obtain maximum power.

In the mobile communication system, many places need to be matched. The receiving antenna must have the same polarization and the same rotation direction as the transmitting antenna to achieve polarization matching to receive all energy. All parts of the feeder connection and the connections between the components need to be matched. 

In the mobile communication system, as long as there is a mismatch, the signal will be reflected in the middle of the components and between the feeders, which will reduce the signal quality and increase the noise. 

Therefore, for installation and commissioning users, every step needs to be done well to ensure that the system matches.

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