Animated Reflectometer Calculator - HTML 5 Version

Animated Reflectometer

Reflection Coefficient:
Return Loss (dB):
Mismatch Loss (dB):
SWR:
ZL:
er:

Tip: Try entering a value between 0 and 1 into the Reflection Coefficient field, then press 'enter.' The other reflection parameters are automatically calculated for you, and the animation updates to show the relative sizes of the transmitted wave, the reflected wave, and the standing wave pattern.

Background

When a load is not matched to the characteristic impedance of a transmission line, part of the incident wave is reflected back from the interface between the transmission line and the load.

Reflection Coefficient

shows what fraction of an incident signal is reflected when a source drives a load.

Standing Wave Ratio (SWR)

the ratio of the maximum to minimum values of the "standing wave" pattern that is created when signals are reflected on a transmission line. This measurement can be taken using a "slotted line" apparatus that allows the user to measure the field strength in a transmission line at different distances along the line.

Return Loss

shows the level of the reflected signal with respect to the incident signal in dB. The negative sign is dropped from the return loss value, so a large value for return loss indicates a small reflected signal. Example: a return loss of 26 dB is roughly equivalent to a reflection coefficient of 0.05.

Mismatch Loss

the power lost relative to the incident power, expressed in dB. This is the power lost due to mismatch between the source and load impedances.

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Comments

Fixed problem with ZL input field

Submitted by rfrobenius on

I have fixed a problem with the sign of the reflection coefficient that was occuring with input from the ZL field. If you input zero for the load impedance, the animation will now properly draw a short circuit at the interface. The numeric results were reported correctly, but the animation was drawn incorrectly for inputs from the ZL field. 

Answer to question about voltage maxima and minima

Submitted by rfrobenius on

@NJKirchner: I watched the video and that's a pretty cool simulator you've got going. One question I understood from the video was (paraphrasing), "how can the voltage maxima of the standing wave be greater than the original voltage (or less than the original wave, depending on where you take the measurement) and obey conservation of energy?" One of the reasons we work with power at RF instead of voltage or current, is that it is more consistent in these circumstances. In the case of the standing wave, the locations where the maximum voltage swings occur will coincide with the minima of the current wave, so the power will be constant along the line, even though the voltage and current are experiencing maxima and minima.
These maxima are nonetheless physically important because in overload conditions they can show up as regularly repeating spots where the cable has melted due to high current!

Transmitted Wave in the presence of non-zero phase reflection

Submitted by NJKirchner on

I've been making my own simulator very similar to this in LabVIEW and have been trying to figure out how to best calculate and illustrate the transmitted wave when there is a phase delay in the reflected wave. I see that this simulator does not allow for that and was interested in discussing what that would take.

Currently if I presume a 0deg phase on the reflected signal, then the math for the transmitted wave is easy, Transmitted = Incident-Reflected. This is the scenario which is displayed above. Also, the transmitted wave above is shown at one of the anti-nodes thus making the waveform at it's smallest amplitude.

However if the reflected wave has a phase shift, the voltage at the point of load can become much larger than the transmitted voltage. Now we all understand conservation of energy and we can't get more power out than put in. So if that is the case, what is the best way to describe the transmitted wave, because it's very clear that it's not just a matter of voltage at a point on the line.

Put on top of that, when we evaluate the uncertainty due to VSWR, is the maximum the incident peak -reflected peak?

Perhaps a brief video might better describe the scenario
http://screencast.com/t/uwWJNwu5L