I take issue with the assertion that power is a scalar quantity
that can't have a direction. It is no more scalar than voltage or
current (they all have a sign associated with them, but none
of them are vectors). On the other hand, the poyting vector
"S" which is the cross product of the E and H field vectors
in a TEM transmission line has a direction which corresponds
to the direction of power flow in the transmission line. When
a TEM wave hits a physical discontinuity, the poynting vector
of the reflection created is flipped 180 degrees with respect to
the incident wave. The reflected power flows in the opposite
direction from the incident power.
73 de Mike, W4EF...............
----- Original Message -----
From: "Ian White, G3SEK" <G3SEK@ifwtech.co.uk>
Sent: Thursday, March 28, 2002 1:51 PM
Subject: [Amps] Re: [Amps] Re: [Amps] Bird® 43 Manual
> Gary Schafer wrote:
> > There is no 100 watts
> >of power coming out of the radio. The meter in this mode is not
> >functioning as a watt meter. There is no current flowing in the
> >line as there is no termination. No current no power. If there is no
> >power going out there is no power to be reflected back to the
> >transmitter. You are only seeing voltage that the watt meter is telling
> >you there is power. It is not telling you correctly.
> >Yes your final may be drawing current from the power supply when you
> >turn up the drive. Yes the final is dissipating power. Is it
> >dc power or rf power. Probably some of both as you will have some
> >circulating currents in the tank circuit due to the mismatch and
> >the efficiency will probably be way down. It is all happening internally.
> I agree with Gary - zero RF power is being generated in that idealized
> worst case of exactly zero or infinite impedance.
> It may be more helpful to get away from those zeros and infinities, and
> think what happens with a very severe (but not quite infinite) load
> mismatch. We may be pouring in lots of DC power, but the RF power being
> delivered to the load is almost zero. In other words, the more severe
> the load mismatch, the less RF power gets delivered to the load, and the
> closer the efficiency gets to zero... which is actually something we
> already knew.
> The question about the validity of "forward and reflected power" came up
> in the rec.radio.amateur.antenna discussion. As someone pointed out,
> power is a scalar quantity so it cannot have any direction. In other
> words, any statements about "forward" or "reflected" power can not be
> completely correct, and will not stand up to detailed examination.
> 73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
> 'In Practice' columnist for RadCom (RSGB)
> New e-mail: firstname.lastname@example.org
> New website: http://www.ifwtech.co.uk/g3sek
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