Transmission Line Demonstrators

A piece of rope works pretty well. It can demonstrate termination, reflection in both polarities, loss, dispersion, stuff like that, and you can *see* what's going on.

John

Chris,

You can build a very nice legacy device by making a coaxial air line.

this would be two tubes one centered within the other one. by selecting the id an od you can very close 50 ohm structure, a structure 60- 42 ohms would be great.

place a suitable rf connector on each to terminate the air line. the connector wail enable connecting sources or vna on one end, and loads, or test device on the other end.

In the middle of the air txline make a small hole 3/8 od for inserting voltage current or power flow probes.

I would make the air line 5-12 inches to make is useful an practical to handle mount etc.

Just measuring the tx lines impedance and loss tangent is an intersing exercise for graduate students.

undergrads can watch what happens for matched and mis matched conditions,,,, others may use it to set up a know field at the hole access pint and then use this field to calibrate field sensor that other graduate students can make to obtain various bandwidths, sensiivites, etc.

Lots of fun, I have done this myself.. Best regards,

marc Popek

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I built a Lecher line demo with a trivial 150MHz tube generator for school somewhat 25 years ago. It is very simple and illustrative. Still working in our days. BTW, doing that with modern RF transistors would be a lot more complicated.

Vladimir Vassilevsky DSP and Mixed Signal Consultant

John,

Hmmm...interesting idea. I can see how you'd show a pulse into a short (getting -A coming back with zero amplitude at the end) but how would you demonstrate the pulse into an open (getting +A coming back and 2A at the end)?

In the mean time, I'm off to Home Depot to get some rope.

Thanks.

Bob

Hang it.

I beg your pardon? Why don't you go...oh, I see.

Yeah, that's what I was thinking, too. Okay, now I'm really gonna go to Home Depot.

Bob

The best I've seen, though tedious to make, is a torsion system. There's a central rod, with connectors at each end, and cross pieces. The impedance depends on the moment of the cross pieces; the system I saw used uniform center rods, I believe. (It was a lonnnnggg time ago, in Bell Labs educational films.) It's especially impressive if you paint the ends of the cross pieces with fluorescent paint...

I'd say there were perhaps four cross pieces per foot, with central rods about three or four feet long. The system could be made from plastic rods or wooden dowels.

The advantage over rope is that you can demonstrate specific reflection properties, and you can make tapered lines to match impedances over a range of frequencies. The system in the film had a motor that could be used for sinusoidal excitation; it just needs to pull up and down on the end of a cross piece. It must also have had loads, though I don't remember them specifically.

The images were so vivid to me that I still remember them when thinking about transmission line problems.

If you decided to do it that way, you could perhaps enlist the help of some other students and make it a more interesting team effort.

Cheers, Tom

As JP says, you can hang it. But propagation is nicer with some tension on the rope, so "terminate" the end into a longish piece of fine string or fishing line. That's not a true open, but it is a transition to a much higher impedance line, so you will get an almost

Hey, you can even "capacitively" load a section of the line, lumped or distributed, and see the wave slow down in that section.

John

Like this one?

"You see, the behavior of radio waves has always been hard for me to keep straight in my mind; yet one must understand this subject thoroughly in order to have a clear knowledge of such things as resonance, impedance-matching, standing wave ratio, and antenna theory. I was talking to my high school science teacher back home about it recently, and he suggested that I build a wave machine as described by Dr. John N. Shive, Director of Education and Training of the Bell Labs, in his little book called Similarities in Wave Behavior. This booklet tells how to build the machine and describes several experiments that can be performed with it. Well, Carl built one, and that's what we want to show you tonight."

Robert

That is excellent. What a simple idea, and should be very effective at demonstrating the fundamentals. I've located Dr. Shive's book at a local bookstore and I'll pick it up tomorrow ($10 for the teacher's edition).

Thanks.

Bob

And also a much more uniform impedance. A hanging rope has an increasing tension as you go up toward the anchor, and the impedance is a function of the mass per length and tension at any point.

I was playing around with a bunch of repelling magnets in a horizontal clear plastic tube a while back, and they behaved a lot like a transmission line.

department

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I assume you are not referring to high voltage or "power transmission lines"? EE could be Electrical Engineering or Electronics Engineering.

If you have some basic electronics test gear and a roll of coax then you only need to apply the theory;

Or this

Is this the demonstrator you found?

HEre's another lab trainer

Buy some extra rope to hang the Donkey and the 'Morphmister', while you're at it.

But think of the hassle of disposing of the giant, smelly, bloated carcasses. The toxic waste fees would be outrageous.

John

Which brings up the interesting idea of demonstrating the wave behavior in a tapered line.

John

Crack that whip.