My reply was a tongue in cheek reply to try and inspire some explanations rather than the "just because it is" kind.
You are correct about steady state DC. Just food for thought,,,, In order to get to that steady state at the far end of the cable you must first apply the DC at the opposite end. It takes time for that DC signal to reach the other end and that time will be the velocity factor of the cable. After that you have steady state DC. Upper frequency roll off of coax cable is mainly a function of the AC resistance (caused by skin effect) of the center conductor. Dielectric loss comes into play above VHF frequencies. Larger diameter coax has less loss because it has a larger center conductor with less AC resistance. During propagation of a signal down a coax line the energy is swapped between the magnetic and electric fields in the cable. I.e. The capacitor charges and discharges into the inductor and back again. Inductive and capacitive reactances have nothing to do with loss. 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 _____ From: [email protected] [mailto:[EMAIL PROTECTED] On Behalf Of Jeff Condit Sent: Saturday, September 01, 2007 10:24 PM To: [email protected] Subject: Re: RE: RE: RE: RE: RE: Re: Re: RE: [Repeater-Builder] Re: Duplexers Hi all! If a piece of coax is sitting at ground and you suddenly attach a battery (DC) across it, you're really talking about a step function change in voltage which carries a wide spectrum of high frequencies The 'change' propagates down the coax at near the speed of light as expected. True DC, on the other hand, means nothing is changing. Everything is constant forever. In this case speed of propagation is a moot point. Regarding the upper frequency rolloff its pretty easy to see how it comes about. Current flowing in a straight wire give rise to a magnetic field around it. Since it takes energy to create the field and whe the field collapses it returns the energy, we're talking about series inductance. Yes, the central conductor of a piece of coax exhibits a certain number of nH per inch. It also has parallel capacitance to the outer braid or cylinder in terms of pF per inch. As frequencies increase the series inductive impedance increases which tends to block the series flow. Simultaneously, as frequencies increase the parallel capacitive impedance decreases tending to shunt the flow to the shield. The combination of these two effects are what gives rise to the high frequency rolloff characteristics. Larger diameter coax has less capacitance per inch and so has less rolloff for a given frequency. There is one other effect that also causes rolloff at even higher frequencies, and that is increased dielectric loss. Hope this helps. ----- Original Message ----- From: Gary Schafer <mailto:[EMAIL PROTECTED]> To: [email protected] Sent: Saturday, September 01, 2007 6:58 PM Subject: RE: RE: RE: RE: RE: RE: Re: Re: RE: [Repeater-Builder] Re: Duplexers How do you know it is not 75 ohms at DC? How long do you think it will take for the DC signal to reach the other end of the coax if it is applied at one end? Will it be at the speed of light? 73 Gary K4FMX > -----Original Message----- > From: Repeater-Builder@ <mailto:[email protected]> yahoogroups.com [mailto:Repeater- > [EMAIL PROTECTED] <mailto:[EMAIL PROTECTED]> .com] On Behalf Of Ron Wright > Sent: Saturday, September 01, 2007 8:02 PM > To: Repeater-Builder@ <mailto:Repeater-Builder%40yahoogroups.com> yahoogroups.com > Subject: Re: RE: RE: RE: RE: RE: Re: Re: RE: [Repeater-Builder] Re: > Duplexers > > Jeff, > > 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. > > 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. I would think you would > agree one will not see RG59 being 75 Ohm at DC. The same can be said at 1 > Hz or 2 Hz or 5 Hz...etc. There is a point at which it starts to > propergate and does look like 75 Ohms. I think you might understand this. > > 73, ron, n9ee/r > > > > > >From: Jeff DePolo <[EMAIL PROTECTED] <mailto:jeff%40depolo.net> net> > >Date: 2007/09/01 Sat PM 01:18:35 CDT > >To: Repeater-Builder@ <mailto:Repeater-Builder%40yahoogroups.com> yahoogroups.com > >Subject: RE: RE: RE: RE: RE: Re: Re: RE: [Repeater-Builder] Re: Duplexers > > > > >> I don't know where the confusion is...all coax and feedline > >> has a upper and lower freq limit. Might try to learn > >> something about this. > > > >If what you say is true, can you tell me, using sound engineering and > math, > >why you can carry DC on coax if it has a 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 > > > _____ size=2 width="100%" align=center> No virus found in this incoming message. Checked by AVG. Version: 7.5.485 / Virus Database: 269.13.1/982 - Release Date: 8/31/2007 5:21 PM
