# Transmission line query

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The velocity of propagation for a transmission line can be expressed as Vp = 1/sqrt(LC) where L and C are the per unit length inductance and capacitance.

Obviously, Vp cannot be > c, which is 3E8m/s.

What _physically_ stops me from from making a coax, say, with 2.5nF/m and 2.5nH/m giving a calculated Vp of 4E8m/s?

Cheers

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Clive```
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The universe doesn't let you do it. Space has limiting values of capacitance and inductance, which compute to exactly c.

As you make a free-space wire skinnier, the surface capacitance goes down but the inductance goes up, nailing the prop velocity at c. You can add capacitance or inductance to make it slower, but there's no way to make it faster.

Pity.

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John Larkin         Highland Technology, Inc

lunatic fringe electronics```
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Pretty much what John said: the electromagnetic properties of free space plus the geometry of the line will dictate that the LC product is too big to break the light barrier.

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Tim Wescott
Wescott Design Services ```
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ITYM too small?

Thanks, I thought it must be something like that, but what about, say, taking a coax with eg Vp = 2.5E8m/s then use a ferrite dielectric to increase the inductance?

Cheers

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Clive```
• posted

Sorry, too big it is.

Cheers

```--
Clive```
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Sure, you can make an electrical transmission line as slow as you like. It is difficult to slow it down without trashing risetime. Delay lines make terrible memories.

There are some big tough-guy principles lurking in the shadows - c, conservation of energy, things like that - that pop out and bash us when we try to come up with clever schemes. Remembering those can be a shortcut to simplify things and embarrass enemies.

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John Larkin         Highland Technology, Inc
picosecond timing   precision measurement  ```
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My mistake, had it upside down in my head. Increasing the inductance will decrease Vp. Now I understand, thanks.

Cheers

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Clive```
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ITYM?

At any rate, increasing the inductance slows things down.

There were (and still are, I think) specialized coax resonators made by plating a dielectric with something conductive. The result can be much smaller than a regular transmission line resonator of similar electrical length. That's about all I know on this subject, although if someone knows they can expand.

Dunno why not ferrite, unless it's too lossy to be useful.

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Tim Wescott
Wescott Design Services ```
• posted

Coaxial ceramic resonators. They have Er's and equivalent Qs in the thousands, and tempcos in the single-digit PPMs.

They really behave and TDR like low-Z (10 ohms or so) shorted transmission lines.

That one is the resonator in a 600 MHz oscillator.

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John Larkin         Highland Technology, Inc
picosecond timing   precision measurement  ```
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Fifty years ago, one electronics read/write package could cycle a few thousand bits through a transmission line, and when transistors were a dollar each, it was a wonderful memory scheme. That was what put programmable calculators on the desktop.

There's ways (plated wire, various acoustic schemes) to make good-quality risetimes, but a generation of lumped-constant "delay line" approximations were marketed for memory timing, and THOSE were terrible approximations of a real delay line. They were smaller than a 'real' transmission line with equivalent delay, though.

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sand

it was a

on

Mercury delay line memory (as used in ENIAC and the early Univacs) was much better than nothing, but it couldn't compete for speed and random access e ven with Williams tube (CRT) memory, let alone magnetic cores.

IIRC all the Univac delay line computers were tube-based, as well.

Cheers

Phil Hobbs

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Magnetic drum memory computers were interesting to program. You had to wrap instructions all around the drum to optimize speed.

People still sell potted LC delay lines, sips and dips and bigger.

There are also CMOS digital delay lines, a long string of gates. They are terrible.

There are a few ECL lines like that, too, expensive power hogs with terrible TCs.

I guess you can store a lot of data in a glass fiber, but access time is bad.

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John Larkin         Highland Technology, Inc
picosecond timing   precision measurement  ```
• posted

If you want to play with coax transmission lines that have resistive or inductive, or both as a part of the structure, look at the varieties available for use in cars as spark plug wiring.

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...or bounce you signal off the moon.

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...

That's important if you need random access, but can be wonderful (again, think fifty years ago style) if you only have a one-bit arithmetic logic unit: the bits just arrive when you need 'em, no need to pump addresses out. All the algorithms depended on serial bit access (free from the delay line, had to clock a shift register to use a non-memory operand).

All the old CORDIC programming was a marvel of compact and powerful bit twiddling.

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The British Elliott 803 computer (in the 1960's) used nickel wire spiral delay lines as registers with weird delay amounts (28 bit times,

46 bit times etc). Most of the logic in the computer was made of square-loop ferrite rings (like those in memories).
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-TV```
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Lovely machine :) Saw one working in a museum a few months ago. When the attendant heard I'd used one, he got out the original circuit diagrams and we poured over them until closing time.

That's /my/ kind of museum.

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Sorry to say, our Elliott has been scrapped, only some parts are in the local Museum of Technology.

I have been babysitting Elliott 803's and 503's for well over

1000 hours in the 1960's.
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-TV```
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It's a pity to pour anything over such a beautiful museum piece. There is nothing against poring over it, however.

Jeroen Belleman (In language nit-picking mode. Sorry)

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