RF Directional Thruline® Wattmeter Model APM–16
8
Note:
Very small attenuations require allowance for normal
instrument errors. To correct for this without any calculations, con-
nect the wattmeters directly, with no line between them, and
adjust their zero settings.
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
published 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
reflectometry or DC continuity and leakage checks, since the attenuation
measurement alone cannot account for faults such as open or short circuits
partway down the line.
Note:
Very small attenuations require allowance for normal
instrument errors. Make sure to note exact readings, or their differ-
ence, on the initial equality check, and correct for this.
