What is VSWR?
Voltage standing wave ratio (VSWR) is the most commonly used parameter in RF technology, which is used to measure whether the matching between components is good. When amateur radio enthusiasts 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. We often hear such questions: but what happens if we can’t reach 1? How small is the standing wave ratio before the antenna is qualified? Why is there no standing wave meter on old military radio stations such as size 81?
VSWR and nominal impedance
The matching condition between the transmitter and the antenna is that the resistance components of the two impedances are the same and the inductive reactance parts cancel each other. If the impedance of the transmitter is different, the impedance of the antenna is also required to be different. In the era of electronic tube, on the one hand, the output impedance of electronic tube is high, on the other hand, the coaxial cable with low impedance has not been popularized. The popular parallel feeder with a characteristic impedance of hundreds of ohms, so the output impedance of the transmitter is mostly hundreds of ohms. The nominal impedance of the antenna of modern commercial solid-state radio communication machines is mostly 50 ohms, so the commercial VSWR meter is also designed and scaled according to 50 ohms.
If you have an old radio station with an output impedance of 600 ohms, you don’t have to work hard to repair your antenna with a 50 ohm VSWR meter, because that will help. Just try to maximize your antenna current.
When the value of VSWR is not 1, it is meaningless to compare the value of VSWR
Because the value of VSWR other than 1 is not worth so accurate identification (unless there is special need), most VSWR meters are not calibrated as carefully as voltmeters and ohmmeters, and even few VSWR give its error level data. Due to the phase frequency characteristics of RF coupling elements in the meter and the influence of diode nonlinearity, the errors of most VSWR meters at different frequencies and different powers are not uniform.
VSWR = 1 does not mean that all antennas are good
The most important factor affecting the effect of the antenna: resonance
Let’s illustrate it with the strings of stringed instruments. Whether it is a violin or a zither, each string of it will have 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 middle swing is the largest, but the vibration tension is the most relaxed. This is equivalent to an antenna with a total length of 1 / 2 wavelength of free resonance. There is no current (current trough) at both ends, but the voltage amplitude is the largest (voltage antinode), the middle current is the largest (current antinode), and the voltage at two adjacent points is the smallest (voltage trough).
We want to make this string produce the strongest sound. First, the sound we want can only be the natural frequency of the string. Second, the ratio of the tension of the driving point to the swing should be appropriate, that is, the driving source should match the impedance of the driving point on the string. The specific performance is that the bow pulling the string or the plucked finger should be selected at the appropriate position of the string. In practice, it is not difficult for us to find that the wrong position of pulling the bow or plucking the string will affect the sound intensity of the string, but a little improper will not affect too much. It is very difficult to make a sound different from the natural frequency of the string. At this time, the vibration state of each point on the string is very complex and chaotic. Even if it vibrates, the promotion of each point to the air is not concerted, and the sound efficiency is very low.
The antenna is also the same. To make the electromagnetic field emitted by the antenna the strongest, first, the transmission frequency must be the same as the natural frequency of the antenna, and second, the driving point should be selected at the 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, in the two points that antenna matching needs to do, resonance is the most critical factor.
In the early transmitter, such as the 71 type telephone introduced, the antenna circuit only uses the method of series inductance and capacitance to obtain the strict resonance with the working frequency, and the further impedance matching is determined by the fixed coupling between the coils. It may not really achieve the strict matching of impedance at different frequencies, but the actual effect shows that as long as the resonance is enough to work well.
Therefore, when VSWR is absolutely 1, the most important adjustment of an amateur radio antenna is to make the whole antenna circuit resonate with the working frequency.
Standing wave ratio of antenna and 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 suspended in the air. We can only measure the VSWR at the lower end of the antenna cable. In this way, the VSWR of the whole antenna system including the cable is measured. WeChat official account: the filter is correct when the impedance of the antenna itself is 50 ohm pure resistance and the characteristic impedance of the cable is 50 ohms.
When the antenna impedance is not 50 ohms and the cable is 50 ohms, the measured VSWR value will be seriously affected by the antenna length. 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 at the lower end of the cable is exactly the same as that of the antenna. However, even if the cable length is the whole wavelength, but the cable has loss. For example, if the cable is thin and the electrical length of the cable reaches more than dozens of times of 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 cable.
Asymmetric antenna
We know that the electrical length of each arm of the dipole antenna should be 1 / 4 wavelength. So if the two arms are different in length, how to calculate their resonant wavelength? Will there be two resonance points?
If you think clearly of the above string examples, the answer is clear. A dipole antenna with a total length of less than 3 / 4 of the wavelength of the system (or a single-arm antenna with the earth and ground grid as the mirror image) 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 to obtain the lowest impedance. The length of the two arms is different, which is equivalent to pulling the bow closer to the string. The force is different, and 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 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 will become an end fed antenna, which is called the Zeppelin antenna used in motorboats in the early stage of radio development and the modern 1 / 2 wavelength r7000 vertical antenna. Of course, at this time, the necessary matching circuit must be added to connect to the 50-ohm low impedance transmitter.
When the two arms of a dipole antenna are asymmetric, or the influence of conductive objects around the two arms is asymmetric, the impedance at resonance will become higher. However, as long as the total electrical length maintains 1 / 2 wavelength and 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 deteriorate significantly.
Qrper does not have to be demanding on VSWR
When the VSWR is too high, mainly when the antenna system is not resonant, so there is a large reactance component in the impedance, the last stage device of the transmitter may need to bear a large instantaneous overvoltage. When the early technology is not very mature, high VSWR is easy to causes damage to RF final power devices. Therefore, it is necessary to control VSWR within a lower value, such as 3.
Some devices now have relatively high VSWR protection. When the VSWR is too high, the power of WeChat will be reduced automatically. The official account of WeChat is: filter, so the danger of burning the last stage is much lower than that of 20 years ago. But don’t be careless.
However, for QRP players, the power of the last stage is sometimes so small that there is almost no possibility of burning the last stage. In mobile applications, the portable temporary antenna should be adjusted to VSWR = 1, but it needs to rack its brains because of the changes of the environment. Don’t be too depressed at this time. From 1988 to 1989, the author tested 4W CW / QRP for by1pk. Using the curtain iron wire on the third floor with a length of less than 1.5m and the plastic wire with a length of about 1.5m as the feeder, the series-parallel capacitor was used to adjust the maximum antenna current. The measured VSWR was infinite, but it was also connected to JA, VK, u9, oh and other electric stations. Later, a small sky modulation was made and the VSWR was adjusted to 1. However, in the comparison test, the remote friend station reported that the great change of VSWR did not bring any improvement to the signal. It seems that the signal has become weaker, and the originally weak signal may be eaten up by the loss of sky modulation.
