The Voltage Standing Wave Ratio (VSWR) is a widely used parameter in RF technology to evaluate the compatibility between components. When amateur radio enthusiasts communicate, the first thing they usually consider is whether the VSWR of the antenna system is close to 1:1. If it's close to 1:1, it’s considered good. Often, people ask: what if I can't reach 1:1? If the standing wave ratio is just a few, is the antenna acceptable? Why doesn’t the old-fashioned military radio station of size 81 have a standing wave monitor?
VSWR and Nominal Impedance
The condition for the transmitter and antenna to be matched is that the resistive components of their impedances are the same, while the reactive parts cancel each other out. In the era of vacuum tubes, on one hand, the output impedance of vacuum tubes was high, and on the other hand, low-impedance coaxial cables had not yet been popularized. The prevalent setup was a parallel feeder with a characteristic impedance of several hundred ohms. Hence, the output impedance of the transmitter was 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, there’s no need to fuss with the 50-ohm VSWR meter to adjust your antenna. Simply aim to maximize your antenna current.
When VSWR is not 1, comparing the values isn’t particularly meaningful.
Because the value of VSWR other than 1 is not so precisely determined (unless there is a specific need), most VSWR tables aren't as meticulously calibrated as voltmeters and ohmmeters, and few VSWRs provide 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 varies at different frequencies and powers.
VSWR = 1 Doesn't Equal Good Antennas
The most important factor affecting the antenna's effectiveness is resonance.
Let's explain this using the strings of stringed instruments. Whether it's a violin or a guzheng, each string has its own natural frequency based on its 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 greatest. The middle swing is the largest, but the tension is the most relaxed. This is akin 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 the string to produce the strongest sound. First, the desired sound can only be the natural frequency of the string. Second, the ratio of the driving point's tension to the swing amplitude should be appropriate, meaning the impedance of the driving source at the driving point on the string should match. Practically, it's not hard to notice that the incorrect position of the bow or the plucked string affects the vocal intensity of the string, but making a sound different from the string's natural frequency is very challenging. At this point, the vibration state of each point on the string is very complex and chaotic. Even if it vibrates, the push of each point to the air is not unified, and the sounding efficiency is very low.
The same applies to the antenna. The electromagnetic field emitted by the antenna must be the strongest. First, the transmission frequency must match the antenna's natural frequency. Second, the driving point should be appropriately positioned on 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, among the two factors that antenna matching needs to address, resonance is the most critical factor.
In 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. Exact impedance matching is not necessarily achieved at different frequencies, but the actual results prove that as long as the resonance is sufficient, it works well.
Thus, when there is no 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.
Standing Wave Ratio of Antenna and Standing Wave Ratio of Antenna System
The VSWR of the antenna needs to be measured at the feed end of the antenna. However, since the antenna feed point is often high in the air, we can only measure the VSWR at the lower end of the antenna cable, thus measuring the VSWR of the entire antenna system including the cable. When the antenna itself has an impedance of 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, will the lower end of the cable present the same impedance as 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 overlook the impact of the cable.
Asymmetric Antenna
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 is its resonant wavelength calculated? Will there be two resonance points?
If we refer back to the example 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 being symmetrical is equivalent to driving at the lowest point of the impedance, and the lowest impedance is obtained. The lengths of the two arms not being equal 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, determined by the total length of the two arms. If the extremes are 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 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.
QRPer Does Not Have to Demand VSWR
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 was not very mature, a high VSWR was likely to cause damage to the RF power device. Therefore, it was necessary to control the VSWR to a lower value, such as 3 or less.
Some devices now have 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 take it lightly.
However, for QRP players, the final power is sometimes small enough to have almost no burning level. When moving, using a portable temporary antenna with a VSWR=1 requires racking your brains due to environmental changes. Don't be too frustrated at this time. From 1988 to 1989, the author used the 4W CW/QRP of the BY1PK test. A third-floor curtain wire with a length of less than 1.5 meters and a plastic wire with a length of about 1.5 meters were used as the feeder. Serial current was used to adjust the antenna current to the maximum. The VSWR was infinite, but it was 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 reasoning. 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.
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