Bird Technologies 4308 - Manual (Page 22)

The Model 4308 is supplied with a special broadband 50 W element with a frequency
range of 440-960 MHz. This element is capable of operating at lower power levels and
up to 50 watts equally as well. At lower power levels, the range switch should be turned
to a lower setting for better resolution. Do not select a range lower than necessary or
damage to the meter may result.
Forward power (i.e., the forward direction of power flow) is indicated when the element’s
arrow is pointing towards the load. Reflected power can be indicated by rotating the
element in the line section so that the arrow points in the direction of the reflected power
flow, i.e., towards the source of power. Refer to figure 4. When the reflected power is
decisively lower, switching the range switch to a lower level, if possible, will give better
resolution.
Where appreciable power is reflected, as with an antenna, it is necessary to subtract the
reflected from the forward power to get load power. Power delivered to, and dissipated
in a load is given by:
W
l
= Watts into load = W
f
- W
r
This correction is negligible (less than one percent) if the load is such as to have a
VSWR of 1.2 or less. Good load resistors, such as Bird Termaline Load Resistors, will
produce a negligible or unreadable reflected power.
VSWR Calculations
After obtaining the forward and reverse power, the VSWR may be found by calculation,
as described in Theory of Operations - Chapter 2 , or by using the VSWR Nomographs
in figures 5 and 6 to simplify finding VSWR. Simply apply the obtained forward and
reflected power readings to the chart, following the cross lines to their intersection and
taking the VSWR from the nearest sloping line.
Bird Model 4308 Thruline Wattmeter