  Measurement of the brightness temperature of extended radio emission demands
knowledge of the gain (or aperture efficiency) of the telescope and measurement
of the polarized component of the emission requires correction for the
conversion of unpolarized emission from sky and ground to apparently polarized
signal. Radiation properties of the John A. Galt Telescope at the Dominion
Radio Astrophysical Observatory were studied through analysis and measurement
in order to provide absolute calibration of a survey of polarized emission from
the entire northern sky from 1280 to 1750 MHz, and to understand the
polarization performance of the telescope. Electromagnetic simulators CST and
GRASP-10 were used to compute radiation patterns of the telescope in all Stokes
parameters, and aperture efficiency. Aperture efficiency was also evaluated
using geometrical optics and was measured using Cyg A. Measured aperture
efficiency varied smoothly with frequency between values of 0.49 and 0.54;
GRASP-10 yielded values 6.5% higher but with closely similar variation with
frequency. Overall error across the frequency band is 3%, but values at any two
frequencies are relatively correct to ~1%. Dominant influences on aperture
efficiency are illumination taper of the feed radiation pattern and shadowing
by the feed-support struts. A model of ground emission was developed based on
measurements and on empirical data from remote sensing of the Earth from
satellite-borne telescopes. This model was convolved with the computed antenna
response to estimate conversion of ground emission into spurious polarized
signal. The computed spurious signal is comparable to measured values, but is
not accurate enough to be used to correct observations. A simpler model, in
which the ground is considered as an unpolarized emitter with a brightness
temperature of ~240 K, is shown to have useful accuracy when compared to
measurements.
