Re: [Phys-l] physics of dissipation

[curtis osterhoudt <flutzpah@yahoo.com>]

It's very much frequency dependent. For example, above about 5 MHz, the "50 Ohm" standard starts to break down, but the exact details are also dependent on the coax diameters, etc. Larger cables tend to stick closer to the 50 Ohm standard to higher frequencies. 

  Particular field work a few years ago was made practically laughable due to the need for some finely-tuned preamplifiers (tuned to nearly-oscillating). The combination of that plus dealing with thin coax cable resonances in the 4 - 10 MHz range made things nigh unto impossible. A combination of damping resistors at either cable end, plus inductive loading, made things workable, but it was a bad oversight to have neglected the cables' effects on the experiment.

   Stick any "50 Ohm" cable on a network analyzer and sweep through the frequencies. Cable manufacturers are well aware of this, and different cables are recommended when starting to push the boundaries of what's feasible. 

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Down with categorical imperative!
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________________________________
From: chuck britton <cvbritton@mac.com>
To: Forum for Physics Educators <phys-l@carnot.physics.buffalo.edu>
Sent: Wed, November 4, 2009 11:22:46 AM
Subject: Re: [Phys-l] physics of dissipation

JD writes:

> Nyquist's model can be boiled down to a single sentence:  An
> ohm is an ohm is an ohm, and a coaxial cable is 50 ohms.  That
> is, we model the energy being "dissipated" in terms of energy
> being _radiated_ into an infinitely-long piece of coax.
>
> This gives a perfect model of dissipation.

Yes, I've wondered about '50 ohm' coax and such.
Isn't this '50 Ohms' somewhat frequency dependent?

Or is it as simple and universal concept as you imply?
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