Exploring Power Patterns of Parabolic Antennas: Implications for Water
Vapor Radiometers

Y.-S. Jerry Shiao, Leslie W. Looney and Edmund C. Sutton
University of Illinois at Urbana-Champaign

Water vapor radiometers (WVRs) are designed to compensate atmospheric
phase fluctuations at millimeter and submillimeter wavelengths for radio
interferometers on short time-scales. Water vapor clouds have been
considered the major components that affect the phase fluctuation.
Monitoring water vapor along the lines of sight of antennas and deriving
path delay differences for eac h baseline should allow us to implement a
phase correction scheme. However, ground-based antennas are surrounded by
water vapor at various heights. The mechanism for detecting the water
vapor columns of the antennas may be very diff erent from the astronomical
sources. To quantify the impact of the water vapor locatio n on the WVR
phase correction scheme, we explore the power patterns of antennas with a
series of numerical simulations. In this memo, we define the water vapor
detection efficiency as antenna temperature to water vapor brightness
temperature ratio.  With the simulations, we demonstrate that the
detection efficiency substantially depends on the sizes and the heights of
water vapor clouds. We suggest that this effect may play a key role in the
WVR phase correction scheme and explain that the well-known scale factor
of the 22 GHz water line varies in many WVR experiments.