Bird Technologies 4412A - Manual (Page 20)

10
Power Range Coverage
Each element is marked with its maximum power range capability, 10 W, 100 W,
1000 W, or 10000 W. Also stamped on the element nameplate is a factor num-
ber. The power range switch on the Series 4410 Wattmeter determines the full
scale power range to be used. The available switch settings are 0.1, 0.3, 1, 3, 10,
30, 100, BATT, and OFF. This switch, when set on a numbered position and multi-
plied by the element factor number, gives the full scale power value. For exam-
ple, if the element factor is 10 and a switch setting of 30 is opposite the ARROW
on the front face of the unit, 30 multiplied by 10 gives you 300 W full scale. In
this case, the lower scale (0-3) on the meter face will be used. If the number
opposite the ARROW is 10 and the element factor is 10, then 10 x 10 = 100, and
100 W will be the full scale reading and the upper scale (0-1) will be used.
For reflected power readings the element is rotated 180° in the element socket
and the same system is used, however, a much lower switch setting
may be
advantageous for better resolution.
Load Power
Where appreciable power is reflected, as with an antenna, it is necessary to sub-
tract the reflected from forward power to obtain load power. Power delivered
to
and dissipated in an antenna is given by:
This correction is negligible (less than 1 percent) if the load has a VSWR of 1.2 or
less.
The Series 4410 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 Series 4410 Thruline Wattmeter is not designed to provide direct VSWR
readings. However, VSWR readings can be determined very easily by using the
provided graphs:
Following the vertical and horizontal grid, determine intersection of forward and
reverse power values. Slanted lines passing closest to this point indicate VSWR.
1.
Determine the forward and reflected power as described above.
2. Refer to the appropriate graph, figure 7 or 8, and convert the forward and
reflected power readings to VSWR. Note that the graphs convert the read-
ings directly to VSWR without any intermediate computations.