Bird Model 43 Wattmeter
12
Low Reflection Measurements
φ = 10% (ρ = 2) is the typical limit of antenna match. Further effort is
frequently not worthwhile because below this level reflected power is
hard to measure, and W
l
can not be significantly increased. TV and
VHF transmitters are examples of systems requiring lower reflected
power but for reasons other than maximizing power transmission.
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 element. This method should not
be used with element ranges differing by more than 10:1.
For 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 measured down to at least 0.5 W, so that φ =
0.5 / 80 or about 0.6%, corresponding to
ρ = 1.17.
CAUTION
For low reflection measurements, do not rotate the reflected power
element to read forward power. Damage to the element or
wattmeter could result
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