Ok. Coax doesn't have an impedance at DC it has a resistance. Coax impedance is found by: Zo = sqrt [ (R +j 2 pi f L ) / (G + j 2 pi f c) ]
where: f is frequency L is inductance C is capacitance R is the resistance G is shunt conductance in mhos caused by the dielectric j is of course the imaginary number At extreamly low frequencies 2 pi f L and 2 pi F c are small compared to R and G, So you can now rewight as: Zo= sqrt (R/G) once f gets large enough, R and G can be neglected so the equation then is: Zo= sqrt [j 2pi f L / j 2pi f L) or Zo = sqrt (L/C) So as you can see the equation for transmission lines involves f, therefor f does have an effect on imedance... Ron's right. Jesse On 9/2/07, Ron Wright <[EMAIL PROTECTED]> wrote: > > > > > > > Jeff, > > Impedance refers to both R and X, resistance and reactance. Impedance > affects all current flow, DC and AC. X affects AC only. > > Yes DC is steady state. Guess you can get the simple stuff. > > No a coax will not function the same at 5 Hz as it does at 2 meters. > > Evidently you have not had the previledge of working with equipment or > engineers that allows one to look at some of these issues. > > Oh well. > > 73, ron, n9ee/r > > >From: Jeff DePolo <[EMAIL PROTECTED]> > >Date: 2007/09/02 Sun AM 09:01:03 CDT > >To: [email protected] > >Subject: RE: RE: RE: RE: RE: RE: Re: Re: RE: [Repeater-Builder] Re: > Duplexers > > > > > >> > >> The question is way off base. No one said one cannot carry > >> DC or any other signal on coax. The question was what was > >> the impedance of a coax at given frequencies. > > > >You said coax has a low-frequency cutoff. I'm asking about that > >specifically. I didn't ask about about impedance. > > > >> At DC I can guarantee you RG59 is not 75 Ohms unless you got > >> enough to get enough R and this is totally another > >> discussion. > > > >Under steady-state conditions, yes, you'd be right. > > > >> At DC, I would think you would agree one will not see > >> RG59 being 75 Ohm at DC. > > > >At steady-state DC, there's no such thing as impedance, there's only > >resistance. By definition, impedance is the opposition to a varying > >electric current, i.e. it only applies when we're talking about AC. > > > >> The same can be said at 1 Hz or 2 > >> Hz or 5 Hz...etc. > > > >No, it can't. If you had a piece of cable long enough, it would behave the > >same way at 5 Hz as would a 100 foot piece of cable on 2m. > > > >> There is a point at which it starts to > >> propergate and does look like 75 Ohms. I think you might > >> understand this. > > > >I'm not trying to rake you over the coals Ron, but I *am* trying to prove a > >point: there is no low-frequency cutoff for coaxial cable, period. You may > >experience (or even measure) behavior at very low frequencies when the cable > >is a small fraction of an electrical wavelength that might make you want to > >think otherwise, but it's not due to transmission line theory, math, or > >physics breaking down at some low-frequency cutoff. > > > > --- Jeff > > > > > > Ron Wright, N9EE > 727-376-6575 > MICRO COMPUTER CONCEPTS > Owner 146.64 repeater Tampa Bay, FL > No tone, all are welcome. > > > >
