Bird Technologies 4331 - Manual (Page 30)

Open circuit testing is somewhat to be preferred to short circuit, since the
reference short (used to check equality initially) must be good, and because
the initial equality is somewhat better on open than on short circuit.
Again, for quite low values of attenuation, it is advisable to note exact
readings, or difference, on the initial equality check, and to allow for this
difference.
Frequency Response
The Plug-In Elements have a very flat frequency response over a frequency
ratio of more than 2-1/2 to 1. This characteristic provides a practically flat
response within the assigned frequency ranges for all the elements. An
illustrative set of curves for three elements of one of these frequency bands
is shown in figure 6. Notice that on the low power element, the fall-off above
and below the assigned frequency band is more pronounced than it is for the
high power element. The degree of drop in response varies progressively less
for each power level from low to high. For example, at 40 MHz, the 10C
element will typically read about 4 dB low which is about 40 percent of the
true value or 4 watts. The 100C will read only about 1 dB low or about 80
percent of the true value (80 W) and the 500C should read within the normal
five percent of full scale tolerance. These curves may be assumed to be about
typical for all of the band types (H, A, B, C, D, etc.) at their respective stated
frequencies.
Harmonics, or subharmonics, may be known to exist in the measured circuit,
outside of the element frequency band. If so, a rough approximation of the
response of the element to these harmonics may be made by the use of these
curves. The frequency ordinate to be read on the graph will be obtained by
proportioning the frequency of the element used with that of the one
illustrated. Interpolation of the curve values will give an approximation of
the extent to which these harmonic signals are being measured by your
element.
Nevertheless, the use of an element for direct power measurements outside
of its stated frequency range is not recommended. The responses given are
only typical, and are by no means guaranteed.
Bird Model 43 Wattmeter
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