Understanding Standing Wave Ratio: SWR & VSWR #SWR #VSWR

VSWR or voltage standing wave ratio is a phenomenon that occurs on radio frequency feeders.

VSWR, voltage standing wave ratio is an important parameter in many areas, and especially where transmitters are used.
When looking at VSWR it is necessary to look at feeder and load impedances.

It is found that to transfer the maximum amount of power from a feeder to a load, the two must have the same impedance.

Coaxial feeder, twin-line feeder, waveguide and strip-line feeder all have a characteristic impedance. If the load has a different impedance then not all the power can be transferred and the remaining power is reflected back along the feeder.

The voltages and currents from the forward and reverse power combine together and set up standing waves along the feeder. It is found that peaks and troughs occur as a result of the standing waves. These peaks and troughs can rise to twice the voltage of current for a complete mismatch, i.e. infinity to 1 VSWR.

For lesser values of VSWR, the plot of voltage against current along the feeder is less marked.
Accordingly it is possible to defined the voltage standing wave ratio, VSWR as the maximum voltage divided by the minimum voltage.
The reflection coefficient is also a very useful quantity when looking at VSWR. This is the voltage of the forward waveform divided by the voltage of the reflected waveform. It is also possible to obtain the VSWR from a knowledge of the forward and reflected powers. These are all described in the video.

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[jsmith]

Great job and a simple and quick explanation. Thank you

[gianfrancomior]

Very good explanations! Quick & clear!

[Hino_]

ELI & ICE

Current lags Voltage by 90 degrees in an ac circuit with a pure inductive reactance. In a circuit with inductive reactance and resistance, the current lags the voltage by less than 90 degrees.


Current leads Voltage by 90 degrees in an ac circuit with a pure capacitive reactance. In a circuit with capacitive reactance and resistance, the current leads the voltage by less than 90 degrees.

[AbhishekKr]

Excellent video, I do not hail from RF background but this clearly helped me in the understanding!

[robetereo]

Thank you so much! Excellent explanation and video!

[blakely]

Huge thank you! it is my gold i found while preparing for an upcoming exam

[BenjaminGSlade]

Good video, but he doesn’t say that, for a short circuit, the reflected voltage is the inverse of the forward voltage. Also the voltage source has an orange sine wave in it, but the forward voltage wave in the graph is gray and the reflected voltage wave is orange. Very confusing for someone who’s trying to figure this out for the first time

[mateoestevez]

Nice animations and video, but for the plots mentioned in 3:40 and 4:01, shouldn't the standing waves look like a sinusoidal wave with 4V peak to peak? Just as the ones showed earlier in the video?

[advanceddesignsystemadsrfi]

3:39 this is the most important part of the video one must understand
Aren't those plots of the absolute values of the standing wave V and I ??

[sharonjuniorchess]

Surely the wavelength is measured as the gap between peak-to-peak. So instead of labelling your intervals as λ/4 they should actually be labelled as λ/2? see 4:10.

[backpackerthrulife]

3:39 Aren't those plots of the absolute values of the standing wave V and I ??

[UweWong]

For the short circuit, if there was a ground/earth connected, wouldn't the signal just go to ground just like any low-pass filters??

[OxTongue]

Wonderful explanation thanks for the video

[jackalakalaka]

It seems like the minimum voltage would always be zero, making the VSWR ∞. Is there just typically a DC offset?

[LiHao]

1:23 is it all the power or half of the power?

[oldcowbb]

you need a ground symbol for the short circuit, confused me for a sec

[DumbledoreMcCracken]

What happens when the feed isn't a multiple of λ/4?

[ancientbutstillhere]

Secret of the real wizz. Troubleshoot this problem with AM manual dial radio. Set at 1000, IF no radio near xmits there. I walk up n down cables. Listen to sound on radio. Signals of any RF swaves picked up. Almost always by damaged cables or bad connectors. Kids $3 am radio with headphones keeps hands free. Works on radar too. If its 100w xmitted stay 2 wave lengths away but you still hear it. Data from computers do the same. When found note distance. Tdr line. Or oscope. Boss will be impressed. Done dozens of times. Check coax. Ig rg-6 mixed with 11 could be issue.

[yoksel]

Not very clear explanation. Need to explain that the gray and red waveform at the short circuit end are opposite voltages to get zero in sum. Whereas at the open end, they are same value which gives double voltage in sum. So in both cases the unmatched end generates that red waveform going back. And still, not very clear why all that that happens at the end, especially for why the doubling happens at the open circuit case. Would be much nicer to start from the explanation of how the "step" voltage travels via transmission line and how it reflects off the unmatched end. That step explanation is very visual and easy to comprehend. Then, after that reflection phenomenon is clear, switch to sine waves.