Read the common sense and significance of standing wave ratio (VSWR) in a text

Voltage standing wave ratio (VSWR) is the most commonly used parameter in RF technology to measure the good match between components. When amateur radio enthusiasts contact, of course, first think about measuring whether the VSWR of the antenna system is close to 1:1. If it is close to 1:1, it is good. I often hear this question: But what if I can't reach 1,? If the standing wave ratio is as small as a few, is the antenna qualified? Why is there no standing wave watch on the old-fashioned military radio station of size 81?

The condition that the transmitter and the antenna are matched is that the resistance components of the impedances of the two are the same, and the inductive reactance portions cancel each other. If the impedance of the transmitter is different, the impedance of the antenna is required to be different. In the era of electron tube, on the one hand, the output impedance of the electron tube is high. On the other hand, the low-impedance coaxial cable has not been popularized. The popular one is a parallel feeder with a characteristic impedance of several hundred ohms. Therefore, the output impedance of the transmitter is several hundred ohms. . The nominal impedance of modern commercial solid-state radios is 50 ohms, so the commercial VSWR meter is also scaled to 50 ohms.

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

It is precisely because the value of VSWR other than 1 is not so accurately determined (unless there is special need), most VSWR tables are not as carefully calibrated as voltmeters and ohmmeters, and even few VSWRs give their error level data. Due to the phase frequency characteristics of the RF coupling components in the table and the nonlinearity of the diode, the error of most VSWR meters at different frequencies and different powers is not uniform.

VSWR = 1 does not equal to good antennas

Let us illustrate with the strings of stringed instruments. Whether it is a violin or a guzheng, each of its strings has its own natural frequency for a specific length and tension. When the string vibrates at the natural frequency, both ends are fixed and cannot move, but the tension in the vibration direction is the largest. The middle swing is the largest, but the vibration tension is the most slack. This is equivalent to a free-resonant antenna with a total length of 1/2 wavelength. There is no current (current valley) at both ends and the voltage amplitude is the largest (voltage antinode), the intermediate current is the largest (current antinode) and the voltage at the two adjacent points is the smallest. (Voltage trough).

We want to make the string produce the strongest sound. First, the desired sound can only be the natural frequency of the string. Second, the ratio of the tension of the driving point to the swing amplitude should be appropriate, that is, the impedance of the driving source to the driving point on the string. match. The specific performance is that the bow of the string or the finger of the plucking should be selected at the appropriate position of the string. In practice, it is not difficult to find that the wrong position of the bow or the plucking string will affect the vocal intensity of the string, but 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 push of each point to the air is not united, and the sounding efficiency is very low.

The same is true for the antenna. The electromagnetic field emitted by the antenna must be the strongest. First, the transmission frequency must be the same as the natural frequency of the antenna. Second, the driving point should be selected at the appropriate position of the antenna. If the driving point is not appropriate 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, in the two points that antenna matching needs to be done, resonance is the most critical factor.

In the early transmitters, such as the type 71 radios introduced, the antenna circuit only used series inductance and capacitance to achieve strict resonance with the operating frequency, and further impedance matching was determined by the fixed coupling between the coils. The exact match of impedance is not necessarily achieved at different frequencies, but the actual results prove that as long as the resonance is enough, it works well.

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

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 50 ohms pure resistance 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 one 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. However, even if the cable length is a multiple of the wavelength, the cable has a loss. For example, if the cable is thin and the electrical length of the cable is more than several times the wavelength, the VSWR measured at the lower end of the cable will be lower than the actual VSWR of the antenna.

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

We know that the electrical length of each arm of the dipole antenna should be 1/4 wavelength. So if the length of the two arms is different, how is its resonant wavelength calculated? Will there be two resonance points?

If you want to clear the above examples of the strings, the answer is 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) has only one resonant frequency, depending on the total length of the two arms. The two arms are symmetrical, which is equivalent to driving at the lowest point of the impedance, and the lowest impedance is obtained. The lengths of the two arms are not equal, which is equivalent to placing the bow close to the piano string. The force is different, the impedance of the driving point is higher, but the resonant frequency is still one, which is determined by the total length of the two arms. If the extreme is extreme, one arm is lengthened to 1/2 wavelength and the other arm is shortened to 0, and the driving point impedance is increased to almost infinity. It becomes a terminal-fed antenna, which is called the Zeppelin antenna used in the early development of the radio. Modern 1/2 wavelength R7000 vertical antennas, of course, must be added to the necessary matching circuit to connect to a 50 ohm low impedance transmitter.

The two arms of the dipole antenna are asymmetrical, or the influence of the conductive objects around the two arms is asymmetrical, which makes the impedance at resonance higher. However, as long as the total electrical length is kept at 1/2 wavelength, the asymmetry is not very serious, and although the characteristic impedance will become high, the VSWR will be affected to some extent, but the actual emission effect will not be significantly deteriorated.

When the VSWR is too high, mainly when the antenna system is not resonant, and thus the impedance has a large reactance component, the final device of the transmitter may need to withstand a large transient overvoltage. When the early technology is not very mature, the high VSWR is likely to cause damage to the RF power device. Therefore, it is necessary to control the VSWR to a lower 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 is automatically reduced, so the danger of burning the final stage is much lower than that of 20 years ago. But still don't care.

However, for QRP players, the final power is sometimes small enough to have almost no burning level. When moving, use the portable temporary antenna to VSWR=1, but you have to rack your brains because of the change of environment. Don't be too frustrated at this time. From 1988 to 1989, the author used the 4W CW/QRP of the BY1PK test. The third floor curtain wire with a length of less than 1.5 meters and the plastic wire with a length of about 1.5 meters were used as the feeder. The serial current was used to adjust the antenna current to the maximum. VSWR is infinite, but it is also connected to JA, VK, U9, OH and other radio stations. Later, I made a small day adjustment and adjusted the VSWR to 1. However, in the comparison test, Yuanyoutai reported that the great change of VSWR did not bring any improvement to the signal. It seems that the signal is weaker, and it may be weak. The signal is eaten up by the loss of the sky.

In short, VSWR is a lot of reason. Since you have an amateur radio station, you will always have to deal with VSWR. You may wish to observe, accumulate and exchange your own experiences.

The matching condition of the antenna system and the transmitter with an output impedance of 50 ohms is that the impedance of the antenna system is 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 resistor); second, select the appropriate feed point. Some foreign magazine articles often give a VSWR curve when introducing an antenna. Sometimes it creates an illusion that as long as VSWR=1, it will always be a good antenna. In fact, VSWR=1 only indicates that the transmitter's energy can be efficiently transmitted to the antenna system. But whether this energy can be effectively radiated into space is another problem. 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 may all achieve VSWR=1, but the launching effect is definitely quite different and cannot be said in the same breath. As an extreme example, a 50 ohm resistor has a VSWR that is ideally equal to 1, but its emission efficiency is zero. And if VSWR is not equal to 1, for example, equal to 4, then there are many possibilities: antenna inductive detuning, antenna capacitive detuning, antenna resonance but feed point is wrong, and so on. On the impedance map, each VSWR value is a garden with an infinite number of points. That is to say, when the VSWR values ​​are the same, there are many possibilities for the state of the antenna system. Therefore, it is not too strict to use the VSWR values ​​for simple comparison between the two antennas. The antenna VSWR=1 indicates that the antenna system and the transmitter satisfy the matching condition, and the energy of the transmitter can be most efficiently transmitted to the antenna, and the matching situation is only one. This article does not intend to repeat the theoretical narrative of voltage standing wave ratio in many radio technology books, but only wants to talk about practical problems from the perspective of perceptual knowledge.