The equation is for characteristic impedance which means a line of infinite length or one that is terminated with a resistive load equalling the impedance of the transmission line. An interesting note, twisted pair telco lines are about 600 ohms at baseband levels but are 120 ohms at DSL frequencies. Cat 5 has a impedance of 100 ohms if I can remember right.
Impedance is given by belden as "nominal impedance" which means the impedance of the cable at frequencies where Zo doesn't change (much) ie frequencies above 5 Mhz-ish. You'll notice that the characteristic impedance flat lines at after a certain point if you plot the function on a graph. I cant see a reason why the characteristic impedance of the cable would change when you start getting into high frequencies. Skin effect maybe, but the increased resistance will still would be negligible compared to 2 pi F L. The real problem here is the definition of cutoff i suppose... it can mean either loss, impedance mismatch or both. Also cutoff is really a critical frequency rather than a gradual curve. Therefor the is no real low frequency cutoff, only a gradual curve that is impedance related (not loss related). For high frequencies there is a gradual curve and a cutoff frequency like Gary said. The gradual curve is cable loss. The "High frequency cutoff" is the point where the size of the cable reaches that of TE0.... (waveguide). The cutoff wavelength is: pi [ ( D + d ) / 2 ] This is for air dielectric. With something other than air, equations starts getting messy... what happens is that wavelength gets shorter thus frequency goes up... but there is still a cutoff frequency. Jesse On 9/2/07, Gary Schafer <[EMAIL PROTECTED]> wrote: > > Correction to below: Change the word "frequency" to wavelength. It > should read; > > > > There is no "high frequency cutoff" but as the spacing of the center > conductor and shield gets larger compared to WAVELENGTH a point is reached > where the propagation mode of the cable changes and other modes come into > play (where multiple propagation modes exist) and the multiple modes can > interfere with each other causing partial cancellations. This causes > additional losses. > > > > > > There is no "high frequency cutoff" but as the spacing of the center > conductor and shield gets larger compared to frequency a point is reached > where the propagation mode of the cable changes and other modes come into > play (where multiple propagation modes exist) and the multiple modes can > interfere with each other causing partial cancellations. This causes > additional losses. > > > > 73 > > Gary K4FMX > > >
