Bird Electronic 4314B, Operation Manual

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Operating Instructions
13
Load power
Where an appreciable amount of power is reflected, as with an
antenna, it is necessary to subtract the reflected from the forward
power to obtain the effective load power. Power delivered to and radi-
ated by an antenna is given by:
Where: W
1
= load power
A load closely matched to 50 ohms resulting in a VSWR of 1.2 or less,
will require less than a one percent correction. A good RF load resis-
tor, such as a Bird Termaline Load Resistor, will produce this sort of
negligible or unreadable reflected power.
The Model 4314B Thruline Wattmeter used with a Bird Termaline
Load Resistor of proper power rating forms a highly useful absorption
wattmeter. Since the reflected power will be negligible, it will be
unnecessary to rotate the element from the forward direction.
Determining VSWR
The Model 4314B Thruline Wattmeter is not designed to provide direct
VSWR readings. In most cases, operators find that the ratio of forward
to reflected power is of equal use. However, VSWR readings can be
determined very easily by the use of the provided graphs as follows:
1. Determine the forward and reflected power as described above.
2. Using the appropriate VSWR Conversion Nomograph, determine
the intersection of the forward and reflected power values.
Note:
The slanted line passing closest to this point indi-
cates the VSWR.
W
1
W
f
W
r
– =
W
f
ForwardPower =
W
r
ReflectedPower =