Bird Technologies APM-16 - Manual (Page 21)

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3. 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 f = 100%
and r = ¥. Open circuit testing is preferred to short circuit, because a high
quality open circuit is easier to create than a high quality short.
To measure insertion loss, use a high quality open circuit to check forward
and reverse power equality, then connect an open-circuited, unknown line
to the wattmeter. The measured f is the attenuation for two passes along
the line (down and back). The attenuation can then be compared with pub-
lished data for line type and length (remember to halve Ndb or double the
line length to account for the measurement technique).
This measurement should be supplemented by either time domain reflec-
tometry or DC continuity and leakage checks, since the attenuation mea-
surement alone cannot account for faults such as open or short circuits
partway down the line.
Note: Very small attenuations require allowance for normal instru-
ment errors. Make sure to note exact readings, or their difference,
on the initial equality check, and correct for this.
Frequency Response
Bird Plug-In Elements have a flat frequency response over their specified oper-
ating range. A sample set of curves is shown in Figure 5. Notice that for the low
power element, the rolloff outside its frequency band is more pronounced than
for the high power elements. For example, at 40 MHz the 10C element will have
a loss of 4 dB, giving a reading of about 40% of the true value For the 100C, the
loss will only be about 1 dB, for a reading at 80% of the true value, and the 500C
should be within the normal 5% of full scale tolerance.
Figure 5 Representative Frequency Response