Ok let's compare open wire line to coax. Assuming the same resistive loss in open wire line as that of a coax cable (same size center conductor as open wire line) we have the following:
200 watts into a 50 ohm line impedance will have a current of 2 amps. I squared R = 2x2=4x50 = 200 watts. Now if we put that same 200 watts into a 600 ohm open wire line we have a current of only .577 amps. .577x.577=.333x600 = 199.75 rounded to 200 watts. Notice how much less the current is. If our lines have a resistance of say 10 ohms the 50 ohm line at 2 amps will have a loss of 2x2=4x10 or 40 watts. (I just picked 10 ohms as an arbitrary loss resistance) With the open wire line with a current of .577 amps we will have a loss of .577x.577=.333x10 = 3.33 watts. Quite a difference in loss by just changing the impedance of the line! All because of less current! You can go to quite a bit of a smaller conductor in an open wire line than in coax and still be ahead of the losses. The ARRL handbook has a graph showing the relative losses of different coax cables compared to open wire line. The major factor in coax line impedance is the ratio of the center conductor size to that of the shield. The dielectric material does play a part but not near as much as the diameter ratios. Skin effect is the major contributor to line loss up to UHF. Skin effect is the cause of the resistance in the conductors in the line. It has nothing to do with the surge impedance of the line. Dielectric loss contributes very little to loss below UHF. Above that if does become a major factor. As frequency goes up so does skin effect loss. The surface of the conductor becomes thinner as far as RF goes with increased frequency as the RF does not penetrate as deeply. 73 Gary K4FMX _____ From: Repeater-Builder@yahoogroups.com [mailto:[EMAIL PROTECTED] On Behalf Of Jesse Lloyd Sent: Monday, August 27, 2007 11:21 AM To: Repeater-Builder@yahoogroups.com Subject: Re: Re: Re: RE: [Repeater-Builder] Re: Duplexers The whole problem here is the comparison between high impedance twin lead and low impedance coax. Is the difference of impedance really coax attenuation? If you kept I equal between twin lead and coax who would win? Also impedance shouldn't change because of frequency (with the exception of extreme situations). Its not C which defines the impedance of a cable its RLC and Shunt conductance. Skin effect has attenuation which increases with the square root of the frequency, so at higher frequencies it becomes less. Dielectric loss is directly proportional to frequency as you go up in frequency at some point it becomes the major factor of coax loss. Dielectric loss is because of the capacitor in the cable, not the capacitance. All capacitors have loss, this is the dielectric loss. It has to do with the dielectric material in the cable, air being one of the best. Jesse On 8/27/07, Ralph Mowery <[EMAIL PROTECTED]> wrote: --- Ron Wright <[EMAIL PROTECTED] <mailto:mccrpt%40verizon.net> > wrote: > 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. > > 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. > > Skin affect is a factor, but a small one compared to > the LC factor. > > 73, ron, n9ee/r > It is not open wire or coax that determins the power loss. It is the impedance of the line and the size of the conductors for frequencies up to 1000 Mhz or so. To transfer 1000 watts of power , the voltage will be higher and the current lower in most prectical open wire lines. That is because the impedance will be around 300 to 600 ohms. Coax is usually 50 or 70 ohms. To get 1000 watts of power through that impedance line it requires less voltage and more current. This is the principal of regular 60 hz power line transmission. Up the voltage to a few hundred thousand volts and the current will go down. This lowers the losses. I don't care to take time to do the calculations, but if you take some small guage wire (say # 20) and space it close to make about 200 ohms line and figuer the loss, it may be higher than some 1 inch or even 1/2 inch hardline. At frequencies below around 1000 Mhz the major loss is the IsqR loss in all lines. Radiation is a very small part. In coax there is a point in which the current on the shield is not flowing but jumping from point to point where the shield wires cross. This causes some resistance. That is partly why the foil shielded coax and hardline is lower in loss than regular coax. __________________________________________________________ Be a better Heartthrob. Get better relationship answers from someone who knows. Yahoo! Answers - Check it out. http://answers.yahoo.com/dir/?link=list <http://answers.yahoo.com/dir/?link=list&sid=396545433> &sid=396545433
