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What does the VSWR value Mean?
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, what happens? The standing wave ratio is small enough for the antenna to be qualified? Why is there no standing wave meter on old military radio stations such as size 81?
VSWR and 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 inductive reactance partly 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 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 commodity solid-state radio communication devices is mostly 50 ohms, so the commercial VSWR meter is also designed with a scale of 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
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, the errors of most VSWR meters at different frequencies and different powers are not uniform.
VSWR=1 does not mean that they are all good antennas
The most important factor affecting the antenna effect: resonance
Let us use the strings of a stringed instrument to illustrate. Whether it is a violin or a guzheng, each string of it has its own natural frequency under a specific length and tension. When the string vibrates at its natural frequency, both ends are fixed and cannot move, but the tension in the vibration direction is the largest. The swing is greatest in the middle, but the vibration tension is the most relaxed. This is equivalent to a free resonant antenna with a total length of 1/2 wavelength. There is no current at both ends (current trough) and the voltage amplitude is the largest (voltage antinode), the middle current is the largest (current antinode) and the voltage between two adjacent points is the smallest. (Voltage trough).
We want this string to make the strongest sound. First, the desired sound can only be the natural frequency of the string, and second, the ratio of the tension of the driving point to the swing amplitude must be appropriate, that is, the driving source must be the same as the impedance of the driving point on the string. match. The specific performance is that the bow of the string or the finger of plucking should be selected in the appropriate position of the string. In practice, it is not difficult to find that drawing the bow or wrong plucking position will affect the sound intensity of the string, but a little improperness will not affect too much, and it is very difficult to make a sound that is different from the natural frequency of the string. At this time, the vibration state of each point on the string is very complicated and chaotic. Even if it vibrates, the air is not pushed together by each point, and the sound efficiency is very low.
The same is true for antennas. 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 the appropriate location 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.
In the early transmitters, such as the 71-type walkie-talkie introduced in this issue, the antenna circuit only uses series inductors and capacitors to achieve strict resonance with the operating frequency, and the further impedance matching is determined by the fixed coupling between the coils. Yes, it may not really achieve strict impedance matching at different frequencies, but the actual effect proves that as long as the resonance is enough to work well.
Therefore, when there is no condition to achieve an absolute VSWR of 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. However, 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 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 loss, for example, if 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.
We know that the electrical length of each arm of the dipole antenna should be 1/4 wavelength. So if the lengths of the two arms are different, how to calculate its resonance wavelength? Will there be two resonance points?
If you think about the above example of strings, the answer will be clear. A dipole antenna with a total system length of less than 3/4 wavelength (or a single-arm antenna mirrored by 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 not equal, which is equivalent to putting the bow closer to the string of the horse. The effort required is different. The impedance of the driving point is higher, but the resonance frequency is still one, 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, the driving point impedance increases to almost infinity, and 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 asymmetrical, or the influence of conductive objects around the two arms is asymmetrical, 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 deteriorated.
QRPer does not have to demand VSWR
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 a pure resistance); Second, select an appropriate feed point. Some foreign 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 energy 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 said in the same day. As an extreme example, a 50 ohm resistor, its VSWR is ideally equal to 1, but its emission efficiency is 0.
And if VSWR is not equal to 1, for example, it is equal to 4, then there are many possibilities: antenna detuning inductively, capacitive detuning in antenna, resonant antenna but wrong feeding point, etc. On the impedance circle chart, each VSWR value is a circle with infinite points. That is to say, 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 make a simple comparison between the two antennas using the VSWR value.
Antenna VSWR=1 means that the antenna system and the transmitter meet the matching condition, and the energy of the transmitter can be most effectively transmitted to the antenna. This is the only matching situation. This article does not intend to repeat the theoretical descriptions of voltage standing wave ratio in many radio technology books, but just want to talk about some practical issues from the perceptual level.
When the VSWR is too high, it is mainly when the antenna system is not resonant, so 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 is not very mature, high VSWR is likely to cause damage to the RF final power device. Therefore, it is necessary to control the VSWR to a low value, such as 3 or less.
Now some devices have relatively complete high VSWR protection. When the VSWR measured online is too high, the drive power will be automatically reduced, so the risk of burning the final stage is much lower than 20 years ago. But still don't be careless.
But for QRP players, the power of the final stage is sometimes so small that there is almost no possibility of burning the final stage. For mobile applications, it is necessary to adjust the portable temporary antenna to VSWR=1, but it has to rack my brains because of the changing environment. Don't be too discouraged at this time. From 1988 to 1989, the author tested the 4W CW/QRP for BY1PK. The third floor curtain wire less than 1.5 meters in length and the plastic wire about 1.5 meters in length were used as the feeder, and the antenna current was adjusted to the maximum by the method of series-parallel capacitors. VSWR is infinite, but it is also connected to JA, VK, U9, OH and other radio stations. Later, a small adjustment was made and the VSWR was adjusted to 1. However, in the comparative test, the remote friend station reported that the great change in VSWR did not bring any improvement to the signal. It seems that the signal has become weaker, which may be weak at first. The signal is eaten up by the loss of the antenna.