Re: [Phys-l] physics of dissipation

[Brian Whatcott <betwys1@sbcglobal.net>]

There's something that sounds not quite right about Curtis's description.
Any coax line  that's not matched at both ends becomes length sensitive
and likely to be resonant at some frequency. The idea that you cannot 
correctly terminate a coax that's fed with any kind of signal, chirped 
or not, is frankly mistaken.

A useful didactic approach to demonstrating coax cables - and one that 
can provide a fair indication of the frequencies at which the usual run 
of coaxes gets more and more lossy  (around 600 MHz)  is to send a 
square edge pulse up an unmatched coax.
20 meters on a roll will provide a useful delay time.
The generator is unpretentious: it can be a 6 volt dry cell connected 
via a push button or a relay contact to a 50 ohm series resistor 
connected to the coax center electrode. The return side of the dry cell 
connects to the coax shield.

If an oscilloscope is connected to this center electrode too, and set to 
an appropriate[fast]  sweep speed,  it will show an immediate rise to 3 
volts when the button or relay contact closes.   Supposing the coax far 
end is unterminated, the incident leading edge reflects at 6 volts, 
where it is easily seen after the two way transit time at the local 
speed of light (0.6 to 0.8 c).  It is convenient to set a relay to 
switch on and off at a slow rate say 1 or 2 Hz, for this purpose.

It is in the detail of the returning edge, that a wide bandwidth 
oscilloscope can provide useful hints about cable attenuation.    The 
slope of the returning  edge is less steep than the edge as launched. 
This rise time is an inverse function of the highest frequency  which is 
not greatly attenuated.      As an estimator, you could take the rise 
time to be a quarter to half wave of the highest frequency that is 
relatively unattenuated.

Brian W

curtis osterhoudt wrote:
> That's all fine, John, but RG-174 and, to a lesser extent, RG-223 and -59 can provide massive resonances at 5 MHz and above. Especially when one is using chirped signals in those frequency ranges (and thus cannot definitively impedance-match at both ends), the cable resonances show up so strongly as to make delicate little signals have real problems (we're talking at least orders of magnitudes in the transmitted amplitudes, let alone the requisite phase shifts). 
>     In fact, the resonances I observed ended up giving me a good measure of group velocities in the cables, as I was able to swap out some different lengths and see the resulting resonance peaks squish closer together and farther apart. 
>  /snip/
> flutzpah@yahoo.com
> ************************************/
>
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