Bird Technologies 4527 - Manual (Page 20)

8
When the same element is used to measure both forward and reflected power,
meaningful readings are possible down to about
φ = 5% (ρ = 1.5). For accurate
measurement of very low levels of reflected power, i.e.
φ = 0.6% (ρ = 1.17), use a
second element rated at one tenth of the full scale power of the forward ele-
ment. This method should not be used with element ranges differing by more
than 10:1.
Example - Consider an 80 watt transmitter and a Bird 43
with 100 and 10 watt elements. Measure W
f
with the 100 W
element. Measure W
r
using the 10 W element (with the
arrow pointing towards the transmitter). W
r
can be mea-
sured down to at least 0.5 W, so that φ = 0.5 / 80 or about
0.6%, corresponding to ρ = 1.17.
Transmitter Monitoring
The Thruline Wattmeter can be used for the continuous monitoring of transmit-
ter output or reflected power, for instance in checking intermittent antenna or
line faults.
Component Testing
The Bird 43 is very helpful in component testing, and may be employed in sev-
eral ways:
•
VSWR or
φ may be measured by placing the component under test
between the wattmeter and a good load resistor.
•
Attenuation (power lost by heat in a line) as well as VSWR may be mea-
sured by inserting the unknown line between two Thruline wattmeters, or
between a Thruline wattmeter and a Termaline absorption wattmeter.
Note: Very small attenuations require allowance for normal instru-
ment errors. To correct for this without any calculations, simply con-
nect the wattmeters directly, with no line between them, and adjust
their zero settings until they are both zeroed.
•
Line loss using open circuit calibration: The high directivity of elements
can be exploited in line loss measurements, because of the equality of
forward and reflected power with the load connector open or short
circuited. In this state the forward and reflected waves have equal
power, so that
φ = 100% and ρ = ∞. Open circuit testing is preferred to
short circuit, because a high quality open circuit is easier to create than
a high quality short.