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Author Topic: Standing Waves - Visualized  (Read 1063 times)
AC6LA
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« on: November 30, 2017, 11:54:21 PM »

Just came across this old gem from AT&T:

https://www.youtube.com/watch?v=DovunOxlY1k

WB2WIK, you and other Bell Labs alums will get a kick out of this.

Dan, AC6LA
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KD6RF
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Posts: 577


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« Reply #1 on: December 01, 2017, 04:35:26 AM »

Very cool.  Thanks.  I hadn't seen a mechanical representation of a matched load before!
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VTenn Antennas
Bay Area Technical Equip Rental and Test Range
http://vtenn.com/Blog/
K0UA
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Posts: 1466




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« Reply #2 on: December 01, 2017, 09:46:01 AM »

Wow.  I know that man is dead but he sure had a way of getting complex ideas explained in a manner that was easy to understand .  These old videos from long ago always seem superior to the new methods. I am glad you posted that.
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SWMAN
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« Reply #3 on: December 01, 2017, 10:52:18 AM »

 Thanks for sending that, Very interesting video.
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AA5MT
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Posts: 130




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« Reply #4 on: December 02, 2017, 07:41:00 AM »

This is my favorite: Standing up for Standing Waves:

https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&uact=8&ved=0ahUKEwiWqNHg1OvXAhVC_4MKHdhbAFYQtwIIJzAA&url=https%3A%2F%2Fwww.youtube.com%2Fwatch%3Fv%3DoejsHzurzv4&usg=AOvVaw3aFB9c09DbhoSgKopVslCU

Tom
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VK6HP
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Posts: 186




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« Reply #5 on: December 03, 2017, 01:29:46 AM »

Dr Shive's demonstration might not be the original, but it's certainly one of the best, and his enthusiasm and good humour come through these many years later.  I notice it's rating only modestly on the site hit counter whereas it ought to be compulsory viewing.

Somewhat less hands on, but still effective, these days we can use movie (and other) visualizations from simulation tools to provoke thinking.   I always ask graduate students to produce and study visualization models of their antennas (for example), because (a) it forces a reconciliation with physical principles and (b) adds to a store of good illustration material.  It's fairly easy to do but you'd be surprised at how few people are inclined to bother.  It was all more difficult in Dr Shive's day and I raise my hat to him, and his cool machine.



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N4MU
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Posts: 133




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« Reply #6 on: December 03, 2017, 05:01:35 AM »

Great FILM. (Please note that I didn't say "video".) It was taught with a piece of chalk and a few home made mechanical devices...not a computer in the room. How is this possible?
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K9AXN
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« Reply #7 on: December 03, 2017, 07:32:43 PM »

I believe there's a minor error in the film.  The part explaining the electrical characteristics of shorted and open transmission lines are reversed.  A shorted end point reverses the polarity and the open end retains the polarity.  As I recall he later added a step in the impedance clip at the end showing the polarity inversion when using impedances above and below the characteristic values.

His film was in use in applied Physics and wave lecture classes in the late 50's and 60's along with the Newton's cradle device.

A remarkable man he was.

regards Jim     
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VK6HP
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Posts: 186




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« Reply #8 on: December 03, 2017, 08:43:01 PM »

Jim, I'm traveling and can't easily view the movie again at this time.  However, I think Dr Shive was regarding line current (not voltage) as his discussion point. It seemed to me that he was doing this to maintain the analogy with other wave propagation cases, such as mechanical or acoustic situations. Impedance is then the ratio of the "drop" to "flow" quantity.   In the transmission line case of course, the current reflection coefficient is indeed the negative of the voltage reflection coefficient.  Anyway, have a look with that context in mind and see if you think it's consistent.
73, Peter.
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K9AXN
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Posts: 351


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« Reply #9 on: December 07, 2017, 09:02:17 PM »

Jim, I'm traveling and can't easily view the movie again at this time.  However, I think Dr Shive was regarding line current (not voltage) as his discussion point. It seemed to me that he was doing this to maintain the analogy with other wave propagation cases, such as mechanical or acoustic situations. Impedance is then the ratio of the "drop" to "flow" quantity.   In the transmission line case of course, the current reflection coefficient is indeed the negative of the voltage reflection coefficient.  Anyway, have a look with that context in mind and see if you think it's consistent.
73, Peter.

Evening Pete,

Had a holiday house full until yesterday ---- now for some R&R.

Your point is correct regarding current however I believe it's easier to teach antenna internals using voltage as an anchor especially the end fed version. 

Good point.

A great day to you!

Jim
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VK6HP
Member

Posts: 186




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« Reply #10 on: December 08, 2017, 06:47:15 AM »

Jim,

I guess the multi-disciplinary Shive explanations clicked with me because I was well dosed with physics taught in that style before becoming an electrical engineer. There's always an exact voltage-current duality of course, although it needs insight to resolve it in some systems.  In practical terms it's usually easier to measure RF voltages directly or indirectly, and because of that it's frequently easier to use voltage in day-to-day practice or teaching demonstrations. But the concepts of charge storage, movement, acceleration, ... should never be lost sight of.  And there's a whole wealth of vexing quantum physics and information theory viewpoints to reconcile when we find ourselves getting too cocky.  No wonder our RF voltmeters and current transformers take us to a comforting and useful classical reality.

73, Peter.
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KV7W
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Posts: 148




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« Reply #11 on: December 08, 2017, 08:13:43 AM »

This is a common issue that's been around for a long time.

On one side you have people training electronic technicians who will be installing and repairing equipment efficiently as possible. The easiest way to get the job done is to give them tools like electron flow theory and algebra based physics. Tactile tools that involve voltage.

On the other side you have people training electronic engineers who design the equipment. These guys are taught current flow theory and calculus based physics, because they need to be able to design things based on current. They are taught the same subjects as technicians in a different way to increase productivity.

You can have a really good understanding of either side and just get blown away when shown a different way to approach something you know like the back of your hand. Oversimplified, but generally an engineer will look at a schematic from the source and work to ground, while a technician will start at ground and work to the source. They both travel on the same road going different ways. 
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K9AXN
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« Reply #12 on: December 08, 2017, 08:35:57 AM »

Peter,

Well taken.  It's easier to measure current in an antenna so it is incorrectly assumed that only current is responsible for EM radiation.

It's difficult to explain antenna internals for end fed antennas using the current notion, --- it's far easier and intuitive using the voltage notion of a traveling wave.  The use of the current notion has led to some misguided theories that an antenna cannot be fed at the end.

We started into the wave theory semester a bit over 50 years ago using the Newton's cradle device.  It was a good demo for surge impedance, wave disturbance, and several other behaviors.  Was great to distinguish the difference between electron flow and charge displacement but was limited to negative displacement.

A great day to you

Jim
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