Sect. 111 Page 6 Model 803A
4) The impedance seen by the bridge, point S, has 3-8 CORRECTION CHARTS
been transformed by the length of the transmission
line away from the known zero impedance, point 0, The Model 803A can be expected to giveimpedance
.420 wavelength around the chart. Therefore, the
indications within f 5 + -percent for
bridge reading for the unknown load, point A, must
{Freq. MC)
be transformed toward the load .420 wavelength impedance magnitude and within f 3 +
5oo
to obtain the actual impedance. For convenience
degrees for phase angle without further correction
we can assume that point S may be moved to the
except for the line length corrections discussed in
nearest half-wavelength point away from the load
paragraphs 3-5 and 3-6.
to read the actual short. This distance is .08
wavelength clockwise on the chart from point T to
point 0, since point 0 represents what the bridge When greater accuracy is desired the specific
sees at the load and at each half-wavelength away correction charts, supplied in the back of this
from the load (neglecting losses in the line). manual, should be used. These correction charts
can be used directly for three particular cases:
when the impedance phase angle is +90", 0", and
Transforming point B to the nearest half-wavelength
when it is -90". On both charts proportionate
point on the line is accomplished by moving around
corrections may be applied to obtain corrections
the chart with the same magnitude and direction as
when phase angles between those charted are ob-
arc TO; in this case .08 wavelength clockwise to
tained.
point C on the chart.
On the IMPEDANCE CORRECTION CHART for
5) Construct the actual load impedance for the
phase angles between 0" and 90" the correction
lossless case, point D, by drawing an arc with radius
factor may be interpolated in accordance with
KA through line KC. The intersection is labeled
a sine wave function. For example, if the phase
point D.
angle is 60", 0.866 of the difference between the
two correction factors is used. If the phase angle
6) Point E represents the actual load impedance
is 45", 0.707 of the difference between the two
and phase angle for the lossy line case. It is de-
correction factors is used.
termined from the following proportion:
On the PHASE ANGLE CORRECTION CHART the
KE/KD = KT/KS
correction factor is interpolated on a linear basis.
For example, if the phase angle is 45", one-half
In the above example the number of wavelengths of the difference between the correction factor for
rotation which point E undergoes to reach point A zero degrees and the factor for 90" is used.
(the measured impedance) is equivalent to the
number of wavelengths ih the line between the load
and the point of measurement. The number of Although the interpolation is not absolute in all
times which point E rotates, however, does not cases, it is usually possible to achieve a bridge
affect the method of solution described. accuracy of 1% using the above procedure. The
accuracy is even better for the three specific
cases given on the charts.
3-7 REMOTE MEASUREMENT 9 LINES
OTHER THAN 50 OHMS 3-9 LIMITATION REMOTE MEASUREMENT
The procedures in paragraphs 3-5 and 3-6also&ply In a short-circuited transmission line impedance
to loads connectedacross the bridge through trans- minima and maxima occur at quarter-wavelength
mission lines with a characteristic impedance other multiples down the line, and the magnitudes will
than 50 ohms. In such cases the impedance read- fall outside the measurement range of the bridge
ings are normalized around the actual line im- which is from 2 to 2000 ohms. If this condition
pedance by dividing all impedances of interest by occurs, the frequency of operation should be changed
the characteristic line impedance. Plotting is then slightly since a very small change in frequency
carried out on the normalized Z-8 chart. (Zo = 1) will shift the quarter-wavelength points enough
See paragraph 3-3. to make accurate measurements. |
ACEC
