The antenna match will not vary with power unless you use non-linear materials or components. For most antennas and components the vswr does not depend on the input power.

If you put in so much power that the antenna starts sparking then the vswr will change. Sparking is a non-linear process.

Dear Pascal Hervé , I agree with Malcolm White and Aparna Sathya Murthy. All radio devices have a limited linear dynamic range (LDR). If the signal has more power than the LDP then the VSWR will decrease depending on the non-linearities. At lower input power, the reflections will be limited, but at high power, the reflection levels can also be more powerful.

No, it should not unless some non-linearity caused by e.g. saturation in ferrite core transformer. Resonant magnetic loop antenna may get sparking in the capacitor.

I agree with Malcolm and also Nicolay..if there are potentially nonlinear elements like ferrite transformers then it may become nonlinear (apart from sparking and corona (in space for satellites ;such nonlinear antenna issues are treated in detail at the MULCOPIM (multipactor , corona passive intermodulation) conferences

Probably most of the nonlinearity in the process of VSWR measurement is created by the measurement tool. Before measuring it is necessary to check the equipment limits.

For high power levels, note the temperature of the parts of the antenna that may heat up due to losses and thus affect dimensional variations in practical antennas causing variations in VSWR.

Kevin, if the antenna changes properties under a certain level of power it means that it is unusable at this level and measuring VSWR is not necessary.

If a microwave circuits consisting non linear elements, then it's VSWR will vary with input power variation.

Hello,

Voltage standing wave ratio (VSWR) is defined as the ratio between transmitted and reflected voltage standing waves in a radio frequency (RF) electrical transmission system. It is a measure of how efficiently RF power is transmitted from the power source, through a transmission line, and into the load. So, as antenna is perfectly matched then there is no reflection. without matching, reflection occurs. it is depending but indirectly because this is going because of material which is being used.

thanks,

Smrity Dwivedi, You described a naked theory whereas in practice there is a bit different picture. Theoretically, there should not be reflections due to mismatch when matching is ideal. However, there are more processes, not only transmitting and reflection. The third process is the indirect measurement of VSWR, which is actually the measurement of two voltages and then calculating the VSWR. Voltages are measured broadband, not with high selectivity of frequencies, therefore any voltage that presents to the port will be measured. The nonlinearity creates some intermodulation voltages that change the measured results. The nonlinearity is double-sided – the nonlinearity of the device under test (DUT) and the nonlinearity of the measurement circuit itself. The latter may be more nonlinear than the transmission line due to diodes in the rectification circuit. If possible, the usage of a good bandpass filter (BPF) can help to improve the measurements. I often use BPF with measurements of nonlinear devices and circuits.

I agree with @ sam belkin.

The input power can affect the VSWR if the power level is high enough to cause damage to the components in the system or to cause non-linearities in the transmission line. In these cases, the VSWR will indicate that there is a mismatch, even if the impedance match was previously good. Additionally, high power levels can cause heating in the transmission line and components, which can change the impedance of the system, leading to a change in the VSWR.