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.
