Transmission line theory is transmission line theory. It doesn't matter what the frequency is it all works the same. Power line transmission engineers worry about the same things in power transmission as do RF engineers. Only the wavelength is different.
> IR drops in feed lines is much less than a factor than the LC/dielectric > type losses. Again frequency shows this. 100 W at 10 MHz and 1000 MHz > will have radically different losses yet both have the same I and V and R. > A feed line at 10 MHz has a totally different R loss than the same feed line used at 1000 MHz. It does NOT have the same R at different frequencies. It has the same "Z" (surge or characteristic impedance) at all frequencies but not the same series resistance R. The resistance increases because of skin effect the higher the frequency is. This is where loss comes from. 73 Gary K4FMX > -----Original Message----- > From: Repeater-Builder@yahoogroups.com [mailto:Repeater- > [EMAIL PROTECTED] On Behalf Of Ron Wright > Sent: Wednesday, August 29, 2007 9:32 AM > To: Repeater-Builder@yahoogroups.com > Subject: Re: Re: Re: Re: RE: [Repeater-Builder] Re: Duplexers > > Ralph, > > Transmission line theory for RF and AC power is totally different. In AC > power lines little is paid attention to as for transmission except for R > losses and power factor. Yes up the voltage/lower the current and the IR > loss goes down. > > For RF this is totally different for the RF propergatesdown the line, not > just passes as voltage and currents. This is why feedlines have specific > impedances and loads used. > > One can have any impedance of coax or twin feeds one wants...that is if > you have the material and space for it. One can get off the shelf 75 Ohm > twin lead. Using 50 or 75 Ohm has more to do with stability especially at > RF. > > IR drops in feedlines is much less than a factor than the LC/dielectric > type losses. Again frequency shows this. 100 W at 10 MHz and 1000 MHz > will have radically different losses yet both have the same I and V and R. > > 73, ron, n9ee/r > > > > > >From: Ralph Mowery <[EMAIL PROTECTED]> > >Date: 2007/08/27 Mon AM 09:20:07 CDT > >To: Repeater-Builder@yahoogroups.com > >Subject: Re: Re: Re: RE: [Repeater-Builder] Re: Duplexers > > > > > > >--- Ron Wright <[EMAIL PROTECTED]> 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&sid=396545433 > > > > > Ron Wright, N9EE > 727-376-6575 > MICRO COMPUTER CONCEPTS > Owner 146.64 repeater Tampa Bay, FL > No tone, all are welcome. > > > > > > > > Yahoo! Groups Links > > >
