>From: Gary Schafer <[EMAIL PROTECTED]> >Date: 2007/09/02 Sun PM 09:07:18 CDT >To: [email protected] >Subject: RE: RE: RE: RE: RE: RE: RE: RE: Re: Re: RE: [Repeater-Builder] Re: >Duplexers
> > >Isnât it interesting to note thatthe impedance goes UP at low frequencies >but not by leaps and bounds. It goes up for the impedance is now more related to R and series L with the parallel C having little affect. The cable now is in series with the load. This is why we use transmission lines so as not to be part of the load, but to transfer power. > >However you didnât say if the âRâresistance in the equations is DC >resistance or AC resistance? The R is both AC and DC although skin effect can produce more R at higher frequency, but not much at 100 kHz. The beauty of R...same all the time unless one gets too much I then the R changes and sometimes get smoke. > >If you also look in those Beldon papersyou will see that the âcharacteristic >impedanceâ of coax is not aspecific number but rather an average number. The >impedance swings all over theplace with change in frequency. There are many >high and low swings in impedanceat specific frequencies. If you got characteristic impedance or just impedance swinging all over the place you got a problem. One does see slight bumps in cables like the big water pipe with flange connections due to they are most often air with donut type insulator. These insulators form a defferent dielectric and give some slight difference in characteristic impedance. When running 10s of kW they become a factor so applications like TV stations order specific lengths of this coax to null out these bumps. A continous length of hardline will have consistant impedance. I guess we were talking about wheather coax has a lower freq limit. > >At low frequencies (or most any frequency)a coax cable does not start to >exhibit coax cable (transmission line) propertiesuntil the length of the cable >approaches 1/10 wavelength. Yes this means thatwith most common lengths of >cable at audio frequencies for example, a piece ofcoax cable only looks like a >piece of shielded cable with capacitance acrossit. But lengthen that same >cable with the same frequency to 1/10 wave length ormore and the cable now >looks like a transmission line. >This same thing happens with powerdistribution lines. The long lines are >transmission lines (appropriately named)and suffer from the same problems as >any other transmission line includingstanding waves. > >73 >Gary K4FMX > > >From: [email protected] [mailto:[EMAIL PROTECTED] On Behalf Of >Jesse Lloyd >Sent: Sunday, September 02, 20074:10 PM >To: [email protected] >Subject: Re: RE: RE: RE: RE: RE:RE: RE: Re: Re: RE: [Repeater-Builder] Re: >Duplexers > >So to plug some numbersin: > >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, RonWright <[EMAIL PROTECTED]>wrote: >Jeff, > >I have plenty of text books here, oh well. All refer to impedance as Z >andZ=R+jX or Z = magnitude and phase angle. A 500 Ohm resistor has an >impedance of500 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 >highfreq differences in coax. Maybe not. > >Oh well. Good discussion. > >73, ron, n9ee/r > >>From: Jeff DePolo <[EMAIL PROTECTED]> >>Date: 2007/09/02 Sun PM 12:12:51 CDT >>To: [email protected] >>Subject: RE: RE: RE: RE: RE: RE: RE: Re: Re: RE: [Repeater-Builder] >>Re:Duplexers > >> >>> Impedance refers to both R and X, resistance andreactance. Impedance >>affects all current flow, DC and AC. X affects AC only. >> >>Impedance is specific to AC. There's no such thing asimpedance at DC, only >>resistance. Look up in the definition of impedance in anyengineering text >>and you'll find that it only applies to AC. >> >>A cable's characteristic impedance is determined by the ratioof E to I when >>there are no reflections on the line. Reflections can onlyexist when the >>current being carried is varying, i.e. an AC waveform. >> >>A coaxial cable that has a 75 ohm characteristic impedancewill conduct >>steady-state DC at any E to I ratio, and will do so withoutreflection. The >>cable does not perform any transformation regardless of theload, unlike the >>AC case. >> >>> No a coax will not function the same at 5 Hz as it doesat 2 meters. >> >>Why not? >> >>> Evidently you have not had the previledge of workingwith >>> equipment or engineers that allows one to look at someof >>> 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. > > Ron Wright, N9EE 727-376-6575 MICRO COMPUTER CONCEPTS Owner 146.64 repeater Tampa Bay, FL No tone, all are welcome.
