Standing waves measured by moving the platform with the tiedowns.


    Standing waves occur between the horn and the dish. They cause ripples in the spectrum with a period of about 1 Mhz. This corresponds to a delay of 1 usec. Position switching at AO is used to cancel these standing waves (since they mostly repeat in the on and the off).

    On 04jan05 data was taken to measure these standing waves. The measurements started at 15:42.

The setup and processing:

    The data processing was:        This processing was done for platform moving up, down, polA, and polB separately.

   The bandpass correction was taken from the tiedown starting position and was used for the entire 20 minutes.  Any ripples present in the spectra near this tiedown reference position should be canceled by the bandpass correction (assuming they are stable in time). When we move away from the reference position the ripples should shift in phase and no longer cancel.

    The acf will have a spike if the same radiation takes two paths into the horn. The lag for the time delay of  the path difference will show increased correlation because the same signal is in the immediate and delayed elements of the multiplier. For a peak to move from 1 acf channel to the next, the signal path must increase by c*acfShiftLen. For150 Mhz bandwidth this is about 1 meter. If you have a complex acf, then the phase at each lag will shift through 2pi radians when the extra path length changes by 1 wavelength (21 cm).

Images of ripple strength versus tiedown offset

    The images show the magnitude of the acf versus the offsets from the tiedown starting position.     The ripple at 1.04 usecs is strongest. Weaker ripples spaced in steps of about .1 usec  about 1.04 usecs are also present. These weaker ripples are coming from the motion of the platform since they appear and disappear as we go farther/closer to the reference position. The things close to .1 usec are probably trouble with the bandpass correction since they are always there.
    The platform moved down and then back up. The bottom plot shows that when the platform returned to the starting position (20 minutes later) the original bandpass correction was still canceling the ripples. This shows that the ripples are relatively stable in time.

Plotting the amplitude and change of phase:

    To increase the signal to noise acfs were averaged together. 100 seconds of acf magnitude  were averaged when the tiedowns were farthest from the reference position (when the signal was strong) and .30 seconds of data were averaged close to the  starting position (to use as a comparison reference).
The plots show the strength of the ripples and the change in phase with tiedown offset (.ps) (.pdf).

Some distances:

    German cortes provided the following distances. The distance in usecs is the round trip time for the distance.
path distance
horn to tertiary 4.06 .027
tertiary to secondary 16.8 .112
secondary to primary 135.85 .906
primary to bottom  of dome 121.27 .808
horn to edge of tertiary (shortest) 2.94 .020
horn to edge of tertiary (longest) 5.82 .039
diameter of ray dome 25.30 .169
horn to dish 156.66 1.044

    These distance show that the 1.04 usec peak is between the horn and the reflector.
german's plot of the distances (.pdf):


processing: x101/050104/