This could be correct in that say a 50 Ohm coax at 10 MHz would be say 120 Ohms at 100 kHz or 90 Ohms at 50 kHz...freq dependent. There is still L and C. However, this would have to be for a specific design or application.
It would affect wideband stuff like video and it does. I guess one could build different loads seperated by filters...not me, hi. As I said in the beginning coax has a upper and lower limit as far as characteristic impedance. 73, ron, n9ee/r >From: Gary Schafer <[EMAIL PROTECTED]> >Date: 2007/09/02 Sun PM 08:41:54 CDT >To: [email protected] >Subject: RE: RE: RE: RE: RE: RE: RE: Re: Re: RE: [Repeater-Builder] Re: >Duplexers > >I was wondering when someone was going to dredge that up from the Beldon >papers. Good going Jesse. >But that still doesn't mean or show that coax cable has a low frequency >cutoff or that it stops looking like or acting like a coax cable at low >frequencies. It tells us that other factors come into play at low >frequencies. > >73 >Gary K4FMX > >> -----Original Message----- >> From: [email protected] [mailto:Repeater- >> [EMAIL PROTECTED] On Behalf Of Jesse Lloyd >> Sent: Sunday, September 02, 2007 12:38 PM >> 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. >> > >> > >> > >> > >> >> >> >> >> >> Yahoo! Groups Links >> >> >> > > Ron Wright, N9EE 727-376-6575 MICRO COMPUTER CONCEPTS Owner 146.64 repeater Tampa Bay, FL No tone, all are welcome.
