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Model 415E
Section III
Paragraphs 3-25 to 3-32
3-25. LOW SWR. Standing wave ratio between 1.0 and
1.24 can be read quite accurately on the EXPAND scales
of the meter when the EXPAND switch is set to any
position other than NORM.
3-26. MODERATE SWR, HIGH RESOLUTION.
The
EXPAND and -DB scale can be used together with the
EXPAND switch to read any SWR with high resolution
in DB.
Figure 3-5 and 3-6 are used to convert DB to
SWR. The reference level (full scale meter deflection
at a voltage maximum) can be used with the EXPAND
switch at NORM (since 0 db NORM and 0 db EXPAND
correspond) but greater accuracy is obtained by setting
the reference level with the EXPAND switch to 0.
short- circuiting the transmission line are easy to lo¬
cate accurately). Compute the SWR from the following
formula:
SWR = Yir
(Xg/Ax).
3-29. TEN-TIMES-MINIMUM POWER METHOD. An¬
other convenient "level above minimum method"
to
use for computing SWR is a level 10 db above minimum.
The separation (AX) between these positions should be
put in the following formula:
A
,
SWR = M^Ax)
For standing wave ratios as low as 15 to 1, the accuracy
of this method is within 1%.
Figure 3-6. Converting Decibels to SWR
3-27. HIGH SWR.
High standing wave ratios (greater
than 30, or sometimes 10) present problems because of
excessive probe penetration (to lift the minimum above
the noise level) and departure of detector behaviour
from square-law. Both problems are lessened or elim¬
inated by measuring only the standing wave pattern near
the voltage minimum, where probe loading effects are
least disturbing.
3-28. TWICE-MINIMUM POWER METHOD. The basis-
for this method (and the TEN-TIMES-MINIMUM POWER
METHOD) is the fact that for a high SWR, the standing
wave pattern approximates aparabola in the vicinity of
a voltage minimum. The slotted line carriage must have
a good scale or dial indicator.
Measure the distance
(AX) between positions on the standing wave pattern
where the voltage is 3 db above the voltage at the min¬
imum. Also measure the transmission line wavelength
Ag (standing wave pattern minima are one-half wave¬
length apart and the sharp minima resulting from
3-30. SWR MEASUREMENT-SOURCES OF ERROR.
Several possibilities have already been mentioned:
excessive frequency modulation of source (smears out
sharp, deep nulls of high SWR pattern), harmonics of
signal frequency from source, departure of detector
from square-law behaviour, and excessive probe pen¬
etration.
Also, reflections in the transmission line
between the slotted line and device being measured
must be minimized.
3-31. ATTENUATION MEASUREMENT.
3-32. The 415E may be used for high resolution in¬
sertion loss measurements simply by inserting the
device to be measured between signal source and de¬
tector and noting the change in DB indication on the
415E. A typical measurement is shown in Figure 3-7.
The continuous coverage of the EXPAND scales allows
any attenuation measurement to be made on the EX¬
PAND scales.
For accurate results, both the signal
source and the detector should be well matched.
Im¬
pedance match of source and detector can be improved,
if necessary, with padding attenuators, isolators, or
tuners.
Figure 3-7. Attenuation Measurement Setup
02152-2
3-6
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