On Sun, 6 Jun 2010 14:21:16 -0700 (PDT), Keith Dysart
wrote:

The 100W forward and 50W reflected have no relation to actual powers

From HP Journal V.7n.2:

If an incident wave is applied at
the left as shown in the diagram, the
wave passes down the main arm. In
the region where the lines are cou
pled, a wave 20 db below the inci
dent wave will be coupled to the
"Forward" terminal, while a second
wave 20 db below the incident wave
will be coupled to the resistive ter
mination in the "Reverse" arm.
Since the combined power in the
two split-off waves amounts to only
2% of the power in the main wave,
the main wave is essentially unalt
ered and continues to the right-hand
terminal.
A wave applied at the right end
of the coupler is coupled in an analo
gous manner. Waves 20 db below the
left-traveling wave will be coupled
to the "Reverse" terminal and to the
resistive termination in the "For
ward" arm, while the main part of
the wave continues to the left-hand
terminal.
The couplers thus provide equal
fractions of right-traveling and left traveling
waves at separate termin
als. The ratio of these waves will be
equal to p *, the magnitude of the
reflection coefficient of any device
connected to the output of the cou
pler. This ratio can be measured
(Fig. 4) by applying the outputs of
the "Forward" and "Reverse" ter
minals to the -hp- 416A Ratio Meter,
using suitable detectors to demodu
late the amplitude-modulated power
which must be used with the system.
In an ideal directional coupler, no
power from a forward wave would
be received at the reverse terminal
and no power from a reverse wave
would be received at the forward ter
minal. In practice some undesired
power is received at these terminals,
although it has been possible to de
sign the couplers so that this unde
sired power is at least 46 db below
the parent wave, i.e, at least 26 db
below the desired wave at the oppo
site terminal. In other words the di
rectivity of the couplers is at least
26 db (30 db in the lower frequency
couplers) over the complete fre
quency range [Fig. 3(a), (b)].
The coupling mechanism itself
consists of quarter-wavelength sec
tions of the conductors placed suit
ably near one another to achieve the
desired degree of coupling. The
combined effects of electrical and
magnetic coupling impart directiv
ity to the coupled wave. The unused
terminal of each of the auxiliary
arms is terminated in a special widerange
low-reflection resistor to ab
sorb any power coupled to that ter
minal.

73's
Richard Clark, KB7QHC