Antenna Symmetry
Antenna Symmetry
Current data
Background
The inclinometry data channels known as TY and RY have always seemed
well correlated (as is expected because of their servo linkage).
A typical set of data below illustrates the raw Y- channels
of data :
RY and TY are clearly correlated, and
the phase difference between LY and the other two is clearly seen.
When the LY data is moved in
phase by 180 degrees and flipped in sign, as shown below, the correlation
between all Y-channels is apparent.
This is surely the expected state of things, but also implies that
at times the interaction between the A-frames
and the track has been independent of which A-frame is involved.
Particularly bad phase-adjusted Y-data from 1998 are shown below:
LY is now noticeably different in nature from TY & RY.
This presumably means that the interaction is now A-frame dependent, and
that inclinometry measures a dynamic phenomenon, viz.
the interaction of the antenna and track.
It would be expected that
the antenna is sufficiently accurately balanced and weighted,
and/or that the track is sufficiently robust under loading, that
the track's 'response' to the left and right A-frames should be
indistinguishable. While that scenario has existed in the past,
it is apparently not always the case.
The correlations betwen TY & RY and LY & RY may be quantified
by the fitting of lines and the calculation of scatters about these
lines. Throughout the past two years TY and RY have been well correlated
with rms scatters about the best fit line of about 0.7". On the other
hand, LY has at times been almost equally well correlated with
RY (1.0" rms on 980411) but has also been badly correlated (4.4" rms on
980808). The correlations on these two occasions are compared below :
A 4-year history of these correlations is shown below, together with the
dates of central bearing load adjustments and track welding work :
A correlation between the change in the LY-RY agreement and c.b.
adjustments during the last 2 years is highly suggested, while track
welding seems to have no impact.
May 1999 update
The cause of this asymmetry is now thought to have been the under-loading
of the front wheels, cured, we hope, by the addition of 3t of weight.
See Justin Greenhalgh's summary of this
theory and the photos (
1,
2,
3,
4),
of the weights added during the
central bearing replacement during May
1999.
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