Isn't it interesting to note that the impedance goes UP at low frequencies but not by leaps and bounds.
However you didn't say if the "R" resistance in the equations is DC resistance or AC resistance? If you also look in those Beldon papers you will see that the "characteristic impedance" of coax is not a specific number but rather an average number. The impedance swings all over the place with change in frequency. There are many high and low swings in impedance at specific frequencies. At low frequencies (or most any frequency) a coax cable does not start to exhibit coax cable (transmission line) properties until the length of the cable approaches 1/10 wavelength. Yes this means that with most common lengths of cable at audio frequencies for example, a piece of coax cable only looks like a piece of shielded cable with capacitance across it. But lengthen that same cable with the same frequency to 1/10 wave length or more and the cable now looks like a transmission line. This same thing happens with power distribution lines. The long lines are transmission lines (appropriately named) and suffer from the same problems as any other transmission line including standing waves. 73 Gary K4FMX _____ From: [email protected] [mailto:[EMAIL PROTECTED] On Behalf Of Jesse Lloyd Sent: Sunday, September 02, 2007 4:10 PM To: [email protected] Subject: Re: RE: RE: RE: RE: RE: RE: RE: Re: Re: RE: [Repeater-Builder] Re: Duplexers So to plug some numbers in: Say you have a cable with the following specs (50 ohm cable) Capacitance of 100.3 pF/m Inducatance of 251 nH/m Resistane of 0.164 ohms/m Shunt conductance of 12.8 mS/m Zo = sqrt [ (R + j 2 pi f L ) / (G + j 2 pi f C ) ] at 100 Hz= 113 ohms at 1 Khz= 111 ohms at 10 Khz= 97 ohms at 100 Khz= 65 ohms at 1 Mhz= 52 ohms at 100 Mhz= 50 ohms at 1 Ghz= 50 ohms Proved.com Jesse On 9/2/07, Ron Wright <[EMAIL PROTECTED]> wrote: Jeff, I have plenty of text books here, oh well. All refer to impedance as Z and Z=R+jX or Z = magnitude and phase angle. A 500 Ohm resistor has an impedance of 500 Ohms or 500+j0 or 500 0 deg phase. I think in Jesse's and my last posting you might see about the low and high freq differences in coax. Maybe not. Oh well. Good discussion. 73, ron, n9ee/r >From: Jeff DePolo <[EMAIL PROTECTED] <mailto:jeff%40depolo.net> > >Date: 2007/09/02 Sun PM 12:12:51 CDT >To: [email protected] <mailto:Repeater-Builder%40yahoogroups.com> >Subject: RE: RE: RE: RE: RE: RE: RE: Re: Re: RE: [Repeater-Builder] Re: Duplexers > >> Impedance refers to both R and X, resistance and reactance. Impedance >affects all current flow, DC and AC. X affects AC only. > >Impedance is specific to AC. There's no such thing as impedance at DC, only >resistance. Look up in the definition of impedance in any engineering text >and you'll find that it only applies to AC. > >A cable's characteristic impedance is determined by the ratio of E to I when >there are no reflections on the line. Reflections can only exist when the >current being carried is varying, i.e. an AC waveform. > >A coaxial cable that has a 75 ohm characteristic impedance will conduct >steady-state DC at any E to I ratio, and will do so without reflection. The >cable does not perform any transformation regardless of the load, unlike the >AC case. > >> No a coax will not function the same at 5 Hz as it does at 2 meters. > >Why not? > >> Evidently you have not had the previledge of working with >> equipment or engineers that allows one to look at some of >> these issues. > >Oh, I think have... > --- Jeff > > Ron Wright, N9EE 727-376-6575 MICRO COMPUTER CONCEPTS Owner 146.64 repeater Tampa Bay, FL No tone, all are welcome.
