You can not measure R with an ohmmeter either. R is the AC resistance of the wires in the line and must be measured at the operating frequency.
Note that AC resistance is a different thing than impedance. AC resistance is the result of skin effect losses in a wire. Skin effect is the result of eddy currents within a conductor causing cancellations in current flow below the surface of the conductor so less depth of the conductor is effective in carrying the current thus the thinner surface available to carry current. The thin surface gets thinner as frequency is increased. This thinner surface has higher resistance the higher the frequency applied to the conductor. Thus the term AC resistance. It is the measure of resistance to an AC signal. Much different than DC resistance of the same conductor. As far as bandwidth goes,,, where do you get this .5 MHz for rg59 cable as a lower limit? Open wire lines begin to radiate as frequency is increased to the point where the line spacing becomes an appreciable portion of a wave length due to the time it takes for propagation of fields between wires. 73 Gary K4FMX > -----Original Message----- > From: Repeater-Builder@yahoogroups.com [mailto:Repeater- > [EMAIL PROTECTED] On Behalf Of Ron Wright > Sent: Wednesday, August 29, 2007 9:37 AM > To: Repeater-Builder@yahoogroups.com > Subject: Re: RE: Re: Re: RE: [Repeater-Builder] Re: Duplexers > > The 300 Ohm or 50 Ohm is not part of the R in IR losses. The 300 and 50 > are the characteristic impedances of the line and not the R values. One > cannot measure the chartistic impedance using an Ohm meter...the R one can > and it will most often be very low. > > Also the characteristic impedance of a transmission line has a bandwidth. > Typical RG59 has a lower freq at being 75 Ohms of about 0.5 MHz. Larger > lines have lower high freq limits mainly because they start to look like > wave guide. Increasing the feedline size say at 100 GHz might actually > increase the losses. > > I think we are getting somewhat confused here,hi. > > 73, ron, n9ee/r > > > > >From: Jeff DePolo <[EMAIL PROTECTED]> > >Date: 2007/08/27 Mon AM 08:55:41 CDT > >To: Repeater-Builder@yahoogroups.com > >Subject: RE: Re: Re: RE: [Repeater-Builder] Re: Duplexers > > > > >> Jesse, > >> > >> Then why do twin feeders have much less loss than coax??? > >> Skin affect is even more of a factor there due to the > >> differences in the area of the outer shield in coax vs the > >> twin feeders wire. > > > >The current in a 50 ohm cable is higher as compared to a 300 ohm cable > for a > >given power (by a factor of the sqrt(300/50), or about 2.5) . Power lost > >due to I2R losses vary in proportion to the square of the current > >(obviously), so for a given effective resistance in the conductors, a 50 > ohm > >cable would have 6 times greater I2R losses than a 300 ohm cable. > > > >But like Ron said, the conductor sizes are typically smaller in a 300 ohm > >twin lead cable (as compared to, say, 7/8" Heliax), so at some point you > >start getting into comparing apples and oranges... > > > >> Maybe it is because of the larger C coupling in the coax due > >> to the larger surface area of the shield. Coax has a lower R > >> even with skin effect than twin line feeders. > > > >Again, it depends on the size of the conductors. It's not a valid > statement > >that "all 300 ohm balanced lines have lower loss than 50 ohm coax". But > if > >you want to compare the two cables at approximately the same size (say, > the > >diameter of the coax is equal to the width of the twin-lead), then the > >balanced line is probably going to be the winner in the loss department > at > >VHF. > > > >> Skin affect is a factor, but a small one compared to the LC factor. > > > >Please define "the LC factor". > > > > --- 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 > > >
