Bird Technologies 4331 - Manual (Page 23)

ence. Thruline Wattmeter measurements are also independent of their posi-
tion along the standing waves.
Standing Wave Ratio vs Reflected/Forward Power Ratio
As mentioned above, the Thruline Wattmeter technique uses the Travelling
Wave viewpoint to measure most of the outstanding facts about transmission
line operation. Another widely used and related viewpoint, is the Standing
Wave one, which is quite elaborately developed both mathematically and in
existing equipment. This technique can be traced to the early development
of slotted lines as tools of exploration.
The slotted line is a standing wave instrument and emphasizes this view-
point. However, the slotted line is too long, too expensive if good, not portable,
and slow in operation. These objections increase rapidly as the frequency
drops below 1000 MHz. Whereas the Thruline Wattmeter is surprisingly
quick, convenient, and accurate by comparison. With the exception of phase
angle reflection (distance, load to minimum), it tells everything a slotted line
will.
The relationship between the Travelling Wave and the Standing Wave
viewpoints is given in most high frequency textbooks.
Load Power
Power delivered to and dissipated in a load is given by:
W
l
= Watts into Load = W
f
- W
r
Where appreciable power is reflected, as with an antenna, it is necessary to
subtract the reflected power from the forward power to get the actual load
power. This correction is negligible (less than one percent) if the load has a
VSWR of 1.2 or less. Good load resistors, such as Bird Termaline loads, will
show negligible or unreadable reflected power.
VSWR scales and their attendant controls for setting the reference point have
been intentionally omitted from the Model 43 for two reasons.
Theory of Operation
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