Phys 728, Spring 2003
Homework Problem Set #5
5.1. A paper claims to detect spikes from a solar flare, based on
the plot shown below. Note the scale of solar flux units on the left,
and the bar indicating the range corresponding to 1900 K in antenna temperature.
Show that these two scales are not consistent by calculating how many solar
flux units would correspond to a 1900 K antenna temperature, given the
following information: Trx = 2000 K, antenna diameter
1.5 m, zenith sky opacity t = 0.58, elevation
angle 48 degrees, antenna efficiency h
= 0.6. What is the total atmospheric extinction at this elevation?
What is the total flux density corresponding to 1900 K before correction
for atmospheric extinction? After correction for extinction?
By what factor is the range of SFU in the figure too large?
5.2. Calculate the receiver temperature for the OVSA system when
using the lossy circular feed, as described in the lecture, by assuming
a loss factor LdB = 3.4 dB, rather than the 0.4 dB used
in the example in the lecture. Using equation (6) of the lecture,
what is the sensitivity on a weak cosmic source (Ta <<
Tsys)
for the circular feed, assuming a 50 MHz bandwidth and a 1 s integration
(assume Tsys = Trx)? Express
your answer in K and in Jy. Note that the sensitivity is linear in
Tsys.
By what factor is the circular feed worse than the linear feed (with LdB
= 0.4 dB)? What is the sensitivity on the Sun, where Ta
~ 5000 K? Express your answer in K. How bad is the circular
feed on the Sun compared to a perfect feed (Trx = 0)?