What do you know about Standing Wave and How to Measure it?

[ptricks]

One of the best videos I ever saw on standing waves was done with water, a laser line level , and a tank.
The laser line level was aimed so that it projected a line on the wall reflected off the water surface just under the water level. The wall then showed a waveform about like an oscilloscope display with the water modulating the laser line. For people trying to understand standing waves this made it clear almost instantly where hours of lectures failed.
Waves generated at one side would propagate across the surface and as you kept generating the same wave on the left , after about 5-10 seconds, you would suddenly see a waveform appear going several times as fast as the original but in the opposite direction of the first, it would stand still for a second a little off center then would suddenly be overtaken by a new wave from the left until again the reflected waves would return, causing the standing wave to disappear.


I realize this doesn't address the needs of the OP, but if anyone is trying to understand transmission lines and SWR maybe thinking of the above example and how a wave travels might help.
 

[Mechatrommer]

One of the best videos I ever saw on standing waves was done with water, a laser line level , and a tank...
I realize this doesn't address the needs of the OP, but if anyone is trying to understand transmission lines and SWR maybe thinking of the above example and how a wave travels might help.

why do you think is that? or is my english really broken? this is exactly what i asked for from the title. duh! can anybody suggest whats the title should be? can you provide the link to the water laser video ptrick?

ps: maybe i have mistake in what "transmission line" is. which does not include a loose cooper wire, hence i edited the OP to include that.

[ptricks]

One of the best videos I ever saw on standing waves was done with water, a laser line level , and a tank...
I realize this doesn't address the needs of the OP, but if anyone is trying to understand transmission lines and SWR maybe thinking of the above example and how a wave travels might help.

why do you think is that? or is my english really broken? this is exactly what i asked for from the title. duh! can anybody suggest whats the title should be? can you provide the link to the water laser video ptrick?

ps: maybe i have mistake in what "transmission line" is. which does not include a loose cooper wire, hence i edited the OP to include that.

No , your english is fine. I meant that I don't address how to do the measurement in electronics.  I don't have a link for the laser video, but there are good videos on youtube that shows the same sort of thing.





For general wave information

[Mechatrommer]

thanks for the link... i've made a video about this issue, i thought it will be easier for people to understand albeit my broken spoken and written english. i hope someone can give some comment if my method is flawed...

as usual... if you like it, press the "thumb up" button, if not then no need to

[PA0PBZ]

After looking at your video I'm even more confused... What exactly is your question?

[Mechatrommer]

hehe what i tried to demonstrate....

opamp1 output is benchmark signal.
opamp2 output should = opamp1 output if no significant loading.
my question... why we got bigger signal when probing after the yellow wire? compared to opamp1 (benchmark signal). i've proved that probing before the yellow wire ie at opamp2 output is equal to opamp1 output, not bigger.

so this should rule out my measurement setup (i think) and the suspect is in the yellow wire. what i tried to demonstrate is as Janne's statement said that i have to diagnose each stage, each output step by step to pinpoint where the problem is. since i cannot see any woobling respond in any output sma 1,2 and 3... so it must be in the yellow wire (sma 4). with this test setup, i am certain that the problem is in the wire, hence i'm asking standing wave question, not impedance control question. and my mind also buggered with why "1/10X wavelength rule" didnt work. ie if the cable length is less than 1/10X of the frequency wavelength, then we can neglect reflection? but not in my case here.

maybe you still dont get the point of the circuit that i've built. it a "highly evolved highly thought" "scientific instrumentation" test , kinda like impedance analyzer, bode plotter circuit etc, except its simple, diy, and my own idea and way. this picture may help you to get to the idea. i beg you to give it some thought.

it is much like the internal components (i guess, conceptually) of the device that you have used to test the green aligator clips... the Agilent Technology "Stimulus Respond - Two Port Insertion Loss" test, except my circuit is way way cheap. you connect the aligator clips to the "Two Port" of your device... but for my device below, i need to solder the DUT to the "Two Port" as you can see at the bottom most of the pcb.

[Mechatrommer]

or maybe to explain it more simpler and more reliably....
how can you explain and why the woobling (jumping up and down) effect occured in your own picture?

it is actually the same question i keep asking from the beginning.

[Mechatrommer]

from my video, the most important point i want to demonstrate is at 11:15 - 11:39. please note the difference the oscilloscope showing when i change from DUT signal to the opamp2 (input signal) output. the rest of video are just to verify my "measurement setup".

[PA0PBZ]

I see what you mean now. I was confused by the word "problem", as if something unexpected happens.
What you see is not SWR but group and phase delay. Don't ask me to explain exactly, but you can have a look here: http://en.wikipedia.org/wiki/Group_delay_and_phase_delay

I made a model in LTSpice so you can see the effect and play with it yourself, just save this as an .asc file:

Code: [Select]

Version 4
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WIRE 96 80 -112 80
WIRE 800 80 656 80
WIRE -112 112 -112 80
WIRE 96 144 96 80
WIRE 176 144 176 80
WIRE 256 144 256 80
WIRE 336 144 336 80
WIRE 416 144 416 80
WIRE 496 144 496 80
WIRE 576 144 576 80
WIRE 656 144 656 80
WIRE -112 208 -112 192
WIRE 800 208 800 160
FLAG -112 208 0
FLAG 96 208 0
FLAG 176 208 0
FLAG 256 208 0
FLAG 336 208 0
FLAG 416 208 0
FLAG 800 80 Vout
FLAG -112 80 Vin
FLAG 496 208 0
FLAG 576 208 0
FLAG 656 208 0
FLAG 800 208 0
SYMBOL voltage -112 96 R0
WINDOW 39 0 0 Left 2
SYMATTR InstName V
SYMATTR Value ""
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SYMATTR Value 2µ
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SYMATTR Value 1000K
TEXT -112 24 Left 2 !.ac dec 100 10000000 100000000

See the picture...