What is VSWR?

Voltage Standing Wave Ratio (VSWR) is the most commonly used parameter in RF technology to measure how well matched components are to each other. When an amateur radio enthusiast makes a contact, of course the first thing that comes to mind is to measure whether the VSWR of the antenna system is close to 1:1.

If it's close to 1:1, good. The question is often heard: but what happens if it doesn't reach 1? How small is the VSWR before the antenna is qualified? Today, let's learn more about this parameter!

What is VSWR?

VSWR is translated as Voltage Standing Wave Ratio (VSWR), generally referred to as VSWR. Electromagnetic waves are transmitted from medium A to medium B. The difference in the medium will cause the energy to be transmitted to medium B. Due to the difference in the medium, some of the energy will be reflected. Thus, a "traveling wave" is formed in the A region. Voltage VSWR is the ratio of the peak voltage to the valley voltage of a traveling standing wave, unitless. This value can be calculated from the modulus of the reflection coefficient: VSWR=(1+modulus of the reflection coefficient)/(1- modulus of the reflection coefficient). And the ratio of incident wave energy to reflected wave energy is 1: (square of reflection coefficient mode) From the above, it can be seen that the larger the VSWR is, the higher the reflected power is and the lower the transmission efficiency is.

Related terms:

VSWR and Nominal Impedance

The condition for matching the transmitter and antenna is that the resistive component of both impedances is the same, and the inductive component offsets each other. If the impedance of the transmitter is different, the impedance of the antenna is also different. In the tube era, on the one hand, the output impedance of the tube is high, on the other hand, low-impedance coaxial cable has not been promoted, the popularity of the characteristic impedance of several hundred ohms of parallel feeder, so the output impedance of the transmitter is mostly several hundred ohms. The nominal antenna impedance of modern commercial solid-state radios is mostly 50 ohms, so the commercial VSWR meter is also designed to be scaled to 50 ohms.

If you own an old radio with an output impedance of 600 ohms, then you don't need to go through the trouble of repairing your antenna with a 50-ohm VSWR meter, because that would help. Just try to tune your antenna for maximum current.

No point in comparing VSWR values when VSWR is not 1

Because VSWR values other than 1 are not worth determining so precisely (unless there is a special need), most VSWR meters are not calibrated as carefully as voltmeters and resistance meters, and very few VSWRs even give data on their error levels. Due to the phase-frequency characteristics of the RF coupling element in the table and the effect of diode nonlinearity, most VSWR meters are not uniform in error at different frequencies and under different powers.

VSWR are = 1 does not mean all are good antennas

The most important factor affecting antenna effect: resonance

Let's use the strings of a stringed instrument to illustrate. Whether it's a violin or a koto, each string has its own intrinsic frequency at a specific length and tension. When a string vibrates at its intrinsic frequency, the ends are fixed and cannot move, but the tension is greatest in the direction of vibration. The middle oscillates the most, but has the loosest vibrational tension. This is equivalent to a freely resonant antenna with a total length of 1/2 wavelength, with no current at the ends (current trough) and maximum voltage amplitude (voltage wave belly), and maximum current in the middle (current wave belly) and minimum voltage at two adjacent points (voltage trough).

We want to make this string to produce the strongest sound, one is the desired sound can only be the intrinsic frequency of the string, and the second is the driving point of the tension and swing ratio should be appropriate, that is, the driving source to match the impedance of the driving point on the string. The specific performance is to pull the string bow or plucking finger to choose the appropriate position on the string.

Antenna is the same, to make the antenna emits the strongest electromagnetic field, one is the emission frequency must be the same as the intrinsic frequency of the antenna, and the second is the drive point should be selected in the antenna's appropriate position. If the drive 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 transmitting efficiency will be greatly reduced. Therefore, in the antenna matching need to do two points, resonance is the most critical factor.

In practice, it is not difficult to find, pull the bow or pluck the string position error will affect the string sound strength, but a little inappropriate is not too much impact, and to send out with the string intrinsic frequency sound is very difficult, at this time the vibration of the string on the various points of the vibration state is very complex, chaotic, even if the vibration up, the points of the air is not the promotion of unity and cohesion of force, the sound is very inefficient.

Antenna system and output impedance

Antenna system and output impedance of 50 ohm transmitter matching conditions is the antenna system impedance of 50 ohm pure resistance. To meet this condition, two things are needed: first, the antenna circuit is resonant with the operating frequency (otherwise the antenna impedance is not pure resistance); second, the proper feed point is chosen. Some foreign magazine articles often give the curve of VSWR when introducing antennas. Sometimes there is an illusion that 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 this energy can be effectively radiated into space is another question. A pair of dipole antenna made according to the theoretical length, and a pair of shortened antenna length of only 1/20, as long as the appropriate measures, they may do VSWR = 1, but the transmitter effect is certainly very different, can not be the same. As an extreme example, a 50 ohm resistor with a VSWR ideally equal to 1 will have a transmitting efficiency of zero.

If the VSWR is not equal to 1, say 4, there are many possibilities: the antenna is inductively detuned, the antenna is capacitively detuned, the antenna is resonant but the feed point is not right, etc. On an impedance garden diagram, each VSWR value is a garden with an infinite number of points. In other words, there are many possibilities for the state of the antenna system when the VSWR values are the same, so a simple comparison between two antennas using only the VSWR values does not make much strict sense.

Antenna VSWR = 1 indicates that the antenna system and the transmitter satisfy the matching condition, and the energy from the transmitter can be delivered to the antenna most efficiently, and there is only this one case of matching.