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: Repeater-Builder@yahoogroups.com [mailto:Repeater-
[EMAIL PROTECTED] On Behalf Of Nigel Johnson
Sent: Saturday, September 20, 2008 7:26 AM
To: Repeater-Builder@yahoogroups.com
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 Repeater-Builder@yahoogroups.com, Bob Witte K0NR [EMAIL PROTECTED]
wrote:
--- In Repeater-Builder@yahoogroups.com, 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
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