The following plot shows the maximum and minimum sky temperatures that would be observed with beam sizes of 1, 5, 20 and 40 degrees. Note the logarithmic scales, with temperatures up to about a million degrees at 30 MHz. At frequences of about 1 GHz and higher the 2.7 K cosmic background radiation sets the limiting temperature.
The strongest source in the radio sky is often - but not always - our Sun. The Sun is extremely variable, on timescales from seconds to decades. The graph below shows the antenna temperature expected from the Sun using a simple antenna of gain 10 dBi - this could be a 5-element Yagi. The lower, solid lines in blue bracket the range of intensities expected from the quiet Sun but at the solar sunspot minimum and maximum. The upper, dashed curve shows the strength of a typical solar outburst.
