Bird Technologies 4314B - Manual (Page 25)

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 effec-
tive load power. Power delivered to and radiated 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 resistor, 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 inter-
section of the forward and reflected power values.
Note: The slanted line passing closest to this point indicates the
VSWR.
W
1
W
f
W
r
– =
W
f
ForwardPower =
W
r
ReflectedPower =