Jesse, You got it, well said.
If you take a simple 100 ft piece of 1/4" superflex a typical value for its C=2400pf, L=6 uH and R=570 Ohm. At 5 Hz the Ls and Cs mean little compared to the R. At 10 MHz Ls and Cs mean a lot compared to the R. One can see there becomes a point where the coax will not look like coax at low frequencies or atleast have a characteristic impedance of something other than it normal value. I did this about 30 years ago for RG59, but cannot remember the numbers for some reason, hi. 73, ron, n9ee/r From: Jesse Lloyd <[EMAIL PROTECTED]> >Date: 2007/09/02 Sun PM 12:38:28 CDT >To: [email protected] >Subject: Re: RE: RE: RE: RE: RE: RE: Re: Re: RE: [Repeater-Builder] Re: >Duplexers > >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. >> >> >> >> > Ron Wright, N9EE 727-376-6575 MICRO COMPUTER CONCEPTS Owner 146.64 repeater Tampa Bay, FL No tone, all are welcome.
