Direct analysis of the readings of the measuring device when tuning the antenna demonstrates a direct relationship between the level of reflected power and the value of the standing wave ratio. If your analyzer shows an increase in SWR above 1.5, this automatically means an increase in the reflection coefficient, which leads to overheating of the transceiver output stage and a decrease in the effective radiated power. Understanding the physical nature of this connection is necessary for correct diagnosis of the antenna-feeder path, since ignoring impedance mismatch can lead to failure of expensive transmitting equipment.
The relationship between these two parameters is not linear but quadratic, making even small deviations in matching critical at high power levels. In an ideal system, where the characteristic impedance of the transmission line completely matches the input impedance of the antenna, all the energy of the transmitter is radiated into space. However, in real conditions there is always some amount of energy that returns back to the generator, forming a standing wave in the feeder.
Consideration of the physical processes occurring in the transmission line makes it possible to identify the cause of the occurrence of standing waves. When an electromagnetic wave reaches the end of a line (antenna) and encounters a resistance different from the characteristic impedance of the cable, some of the energy is absorbed by the load and some is reflected. Reflectance (denoted as G or Gamma) shows exactly what fraction of the amplitude of the incident wave returns back. It is this reflected wave, interfering with the incident one, that creates a characteristic pattern of voltage maxima and minima along the cable.
The SWR value (or VSWR in English literature) is a derived value from the modulus of the reflection coefficient. The calculation formula is as follows: SWR = (1 + |G|) / (1 - |G|). From this relationship it is clear that with zero reflection (perfect matching) the SWR is equal to unity. Any increase in the modulus of the reflection coefficient leads to an exponential increase in the SWR. Antenna analyzers SWR is often shown, since this is a more visual indicator for the operator, although internally the device calculates precisely the complex reflection coefficient.
⚠️ Attention: Operating a transmitting antenna with an SWR above 3.0 can lead to instantaneous failure of the output transistors of the power amplifier due to exceeding the permissible level of reverse power.
Mathematical model and conversion formulas
For an accurate engineering calculation of the parameters of an antenna system, it is necessary to operate with precise mathematical expressions connecting the incident and reflected waves. Knowing one of the parameters, you can easily calculate the other, which is useful when working with tables or software calculators. The basis of all calculations is the complex load resistance and the characteristic resistance of the line.
The voltage standing wave ratio is defined as the ratio of the maximum voltage in the line to the minimum. This relationship directly follows from the amplitude of the reflected wave. If we denote the incident wave as $U_{inc}$, and the reflected wave as $U_{reject}$, then the modulus of the reflection coefficient will be equal to $|G| = |U_{neg}| / |U_{pad}|$. Substituting this into the SWR formula, we obtain a complete description of the process.
- 📉 At SWR = 1.0, the reflection coefficient is 0, which means 100% energy transfer.
- 📈 At SWR = 2.0, about 11% of the power is reflected, which is already considered a borderline value for many systems.
- 🔥 At SWR = 5.0 and above, more than 50% of the power is returned to the line, causing severe heating of the components.
It's important to note that feeder losses also influence the measured values. If the cable has high attenuation, it can mask the actual antenna mismatch by absorbing the reflected energy and turning it into heat. In this case, the SWR measured at the cable input will be lower than the actual SWR at the antenna input. For accurate diagnostics, it is necessary to take into account the cable attenuation at the operating frequency.
Formula for calculating power loss
Return Loss in decibels is calculated as -20 * log10(|G|). The larger the RL value, the better the agreement. For example, an RL of 20 dB corresponds to an SWR of about 1.22.
Impact of mismatch on transmitter performance
Modern radios and power amplifiers are equipped with protection systems that respond to high standing wave levels. However, the operating principle of these systems varies: some simply reduce the output power, while others completely disable the transmission. Understanding how reflection coefficient influences the final cascade, helps to avoid false alarms or, conversely, equipment damage.
At high SWR, the voltage at the antinodes of a standing wave can be many times higher than the transmitter supply voltage. This phenomenon is especially dangerous for transistor amplifiers operating at low supply voltages but with high currents. Insulation breakdown or thermal breakdown of a semiconductor crystal occurs precisely at moments of such peak overloads.
| VSWR | Coef. reflections (|G|) | Gained power (%) | Return loss (dB) |
|---|---|---|---|
| 1.0 | 0.00 | 0.0% | ∞ |
| 1.5 | 0.20 | 4.0% | 14.0 |
| 2.0 | 0.33 | 11.1% | 9.5 |
| 3.0 | 0.50 | 25.0% | 6.0 |
| 5.0 | 0.67 | 44.4% | 3.5 |
In addition, the presence of a reflected wave changes the effective output impedance of the generator, which can lead to frequency instability or the appearance of parasitic oscillations. Antenna tuner (matching device) is designed to eliminate this problem by transforming the antenna impedance to 50 ohms at the feeder input, but it does not eliminate the standing wave itself in the cable between the tuner and the antenna if the tuner is installed at the transmitter.
To minimize feedline losses at high SWR, use low-attenuation cable (such as Foam or Air-dielectric) and try to locate the antenna tuner as close to the antenna as possible, rather than at the transmitter.
Measurement methods and instruments used
To determine the relationship between SWR and reflection coefficient, specialized measuring instruments are used in practice. The most common tool is SWR meter (SWR meter), which is usually a directional coupler with two detectors. One detector registers the incident wave, the other detects the reflected wave.
A more advanced tool is an antenna analyzer, which measures complex impedance (active and reactive components) and calculates SWR mathematically. This allows you to see not only the magnitude of the mismatch, but also its nature (inductive or capacitive). Calibration of instruments before measurements is a mandatory procedure to obtain reliable data.
- 🔌 Connect the measuring device directly to the transmitter output or to the feeder gap at the antenna.
- 📡 Take measurements at several frequencies in the operating range to build a frequency response graph.
- 🔋 Make sure the transmitter outputs a stable carrier frequency without modulation during the test.
By using vector network analyzers (VNAs), a complete picture of the behavior of the antenna system can be obtained. Such instruments show the phase of the reflectivity, which is critical for tuning complex multi-band antennas. An error in determining the phase can lead to the fact that adding a compensating element will worsen the situation instead of improving it.
Factors influencing SWR changes
The parameters of the antenna system are not static and can change under the influence of external factors. Understanding these factors helps distinguish a real problem from a temporary phenomenon. Change often standing wave ratio associated with weather conditions or the proximity of foreign objects.
The dielectric constant of the environment plays a key role. Rain, snow or high humidity can change the electrical length of the antenna elements, shifting the resonant frequency. An antenna that is perfectly tuned in dry weather may show a high SWR during a rainstorm. This is a normal physical phenomenon associated with a change in the speed of wave propagation along the conductors.
The proximity of metal objects, building walls, or even the human body during measurements introduces significant distortions. The reactive component of the antenna impedance is very sensitive to nearby objects. Therefore, measurements should be carried out in free space, away from reflective surfaces.
⚠️ Attention: Carrying out SWR measurements in close proximity to metal structures (balconies, corrugated roofs) will give false results that do not reflect the actual operation of the antenna on the air.
Practical aspects of antenna matching
The antenna tuning process is aimed at minimizing the reactive component of the impedance and bringing the active part to the value of the characteristic impedance of the feeder (usually 50 or 75 Ohms). For this, various methods are used: changing the geometry of the emitter, using matching transformers or loops.
The method of successive approximations is often used. The current SWR is measured, a correction is made (for example, the vibrator is shortened), and the measurements are repeated.
☑️ Antenna tuning algorithm
The use of baluns (balanced transformers) allows you to switch from a symmetrical antenna to an unbalanced cable, suppressing currents on the outside of the braid. Cable sheath currents can distort the radiation pattern and contribute to SWR measurements, making them incorrect. A high-quality balun is the key to stable system parameters.
The main goal of tuning is not to achieve an ideal SWR 1.0 at any cost, but to ensure stable operation in the frequency band with an acceptable level of losses and radiation efficiency.
Typical errors and ways to resolve them
The most common mistake is trying to tune an antenna that has mechanical defects. Cracks in insulators, oxidized contacts or damaged cables cannot be compensated for by adjusting the length. Before you begin adjusting the SWR, it is necessary to conduct a visual inspection and check the integrity of all elements of the path.
Another mistake is neglecting the quality of detachable connections. A poor contact in a SO-239 or N-type connector introduces active resistance, which absorbs energy but does not radiate it. This can artificially lower the SWR readings, creating the illusion of a good match, while the actual efficiency of the antenna will be low.
- 🛠 Use quality connectors and crimp or solder them securely.
- 💧 Be sure to seal street connections with heat shrink or special tape.
- 📏 Check the electrical cable length in multiples of half wavelength if measurements are taken at the transmitter.
In conclusion, it should be noted that the relationship between SWR and reflection coefficient is the fundamental basis of radio engineering. Proper management of these parameters allows you to create effective communication systems, minimize losses and extend the service life of equipment. Regular monitoring of the parameters of the antenna-feeder path should become the norm for any radio amateur or professional.
What does SWR equal to 1 mean?
An SWR of 1 means perfect matching of the load with the feeder. In this case, the reflection coefficient is zero, all the supplied power is radiated by the antenna, and there is no standing wave in the line. In practice, such a value is unattainable; they strive for values of 1.0–1.2.
Can high SWR damage the receiver?
In receive mode, high SWR is not dangerous for the equipment, since signal levels are minimal. However, it degrades the signal-to-noise ratio and sensitivity of the system, since part of the received signal is reflected from the receiver input back to the antenna and lost.
Does SWR depend on cable length?
In an ideal lossless cable, the SWR is independent of length. In a real lossy cable, the SWR measured at the input of the long line will be lower than the actual SWR at the antenna because the cable absorbs some of the reflected energy. The longer and “worse” the cable, the stronger this effect.
How often should I check the SWR of my antenna?
It is recommended to check the SWR after installing the antenna, after severe thunderstorms, hurricanes or icing. Also, preventive measurements should be done once a season, especially if the antenna is used in harsh climatic conditions.