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Cecil Moore · August 26th 06, 07:13 PM posted to rec.radio.amateur.antenna

wrote:
I say it's 100 microjoules.
200W forward - 100W reverse = 100W net forward power.

Sorry Dan, you are right about the power and wrong
about the energy. There are indeed 100 watts of *net*
power. But we are not talking about the net energy
delivered to the load. We are talking about the total
energy in the transmission line and there's no such
thing (to the best of my knowledge) as negative energy.
Forward traveling energy is positive energy. Reverse
traveling energy is positive energy. The energy rejected
by the load is NOT negative energy. Forward traveling
energy and reverse traveling energy add, not subtract.

Hint: Two energy components cannot superpose to a zero
scalar value. The result is always a scalar sum.

If we have 200 microjoules in the forward wave and we
have 100 microjoules in the reflected wave, the total
energy in the transmission line is 300 microjoules. If
the standing wave model differs from that amount, it
is wrong.

You
can see this from the Poynting vector which is proportional to the
difference of the squares of the electric field amplitudes of the
forward and reflected waves.

True for net watts, not true for joules. In the standing wave
model, there's 100 watts of net power containing 100 microjoules.
The other 200 microjoules are stored in the (virtual) reactances.
If you calculate the energy necessarily stored in the L and C of
the line, you will find the other 200 microjoules. I would have
to hit the books to refresh my memory on that calculation but
any other result would violate the conservation of energy principle.

If you can't bump Ef up by using an impedance matching network, the net
power flux is REDUCED by the reflected wave, and as such, the stored
energy in the fields in the line is ALSO reduced.

That applies to the watts. It doesn't apply to the vars. The actual
voltages and currents are increased by the standing waves while the
phase angle goes non-zero. Vars require real energy. That real energy
can be calculated by knowing the current through a perfect inductor
and/or the voltage across a real capacitor.

Not 300 microjoules. 100 microjoules. The energy per unit length in
the line is proportional to the Poynting vector.

The energy per unit length is not proportional to the net Poynting
vector which is (Pz+ - Pz-) (using Ramo/Whinnery conventions). The
energy per unit length is actually (Pz+ + Pz-). Why that has to be
true is contained in the conservation of energy principle and is
the source of confusion for many posters on this newsgroup.

Hint: Has anyone ever seen a quart of negative water?
--
73, Cecil http://www.qsl.net/w5dxp