Yes standing waves can exist with a pure resistive load on a line but the mismatch of the load with the line impedance creates a reactance (depending on line length).
A transmission line must be long enough (wavelength wise) for standing waves to exist. A short line (wavelength wise) like an audio cable, will not exhibit standing waves because it is too short for them to exist. Even a short (wavelength wise) RF cable will not exhibit standing waves. Keep in mind that the typical SWR and power meters that we use to measure SWR with are NOT really measuring SWR. They are measuring impedance mismatch of the internal impedance that the meter is set for, with a scale on the meter that converts the impedance ratio to "a would be" SWR. A very long power transmission line can have standing waves on it if the power factor problems are not kept in check. So power factor could be thought of like SWR but only on long distribution lines. Keep in mind that short transmission lines whether it be power, audio or RF do not have standing waves on them when they are very short wavelength wise. 73 Gary K4FMX > -----Original Message----- > From: [email protected] [mailto:Repeater- > [EMAIL PROTECTED] On Behalf Of Nigel Johnson > Sent: Saturday, September 20, 2008 7:26 AM > To: [email protected] > Subject: [Repeater-Builder] Re: Does anyone else think of Power Factor > like SWR? > > Very interesting theory. I am teaching SWR at present to my third > year college students. Could be a good discussion point since they > have already studied power factor. However, SWR can exist with a > purely resistive mismatched load, so it needs a bit of modification to > take all into account. > > 73 > Nigel > ve3id > > > > --- In [email protected], "Bob Witte K0NR" <[EMAIL PROTECTED]> > wrote: > > > > --- In [email protected], "kb9bpf" <kb9bpf@> wrote: > > > > > > Since I'm way more into RF than industrial power distribution, I've > > > always been able to think of "power factor" on the electrical power > > > grid in terms similar to antenna system reflections, which are > > > commonly measured in terms of SWR. After all, both are AC systems > > > where the voltage and current bear a phase relationship to each > > > other. > > > > > > When they are perfectly in phase the power factor is 1.0, and a 60-Hz > > > SWR meter would measure 1:1. When they are out of phase (power factor > > > <1) that SWR meter would read greater than 1:1. I suspect, though I > > > haven't done the math or looked up the specific matahematical > > > definition of power factor, that it would be direcly proportional to > > > the reciprocal of the power factor. And as we know, when that happens > > > the power generating end has more difficulty delivering power > > > efficiently to the load. > > > > > > > A while back I was doing some analysis of power factor to understand > > it better and I found that it has a lot in common with SWR. Both are > > focused on the issue of power transfer, so I guess we shouldn't be > > surprised. The thing they really have in common is for max power with > > AC signals, the voltage and current need to be in phase (phase angle > > of zero). > > > > For linear systems with nice sine waves, PF = cos (phase angle) > > where phase angle = the angle between voltage and current sinusoids > > > > Wikipedia has a good explanation of PF at > > http://en.wikipedia.org/wiki/Power_factor > > > > Play around with some typical circuits and you'll find that an SWR of > > 1 also has voltage and current in phase. Again, not a surprise since > > it represents the best power transfer. > > > > This is from memory, so the usual disclaimers apply :-) > > > > 73, Bob K0NR > > > > > > ------------------------------------ > > > > Yahoo! Groups Links > > >
