Practical_Antenna_Handbook_0071639586

CHAPTER 22
Radio Astronomy Antennas
For centuries astronomers have scanned the heavens with optical telescopes. But
today, astronomers have many more tools in their bag, and one of them is radio
astronomy. The field of radio astronomy emerged in the 1930s and 1940s through
the work of Grote Reber and Carl Jansky. Even during World War II, progress was made
as many tens of thousands of operators were listening to frequencies from “dc to daylight”
(well, actually, the low end of the microwave bands). British radar operators
noted during the Battle of Britain that the distance at which they could detect German
aircraft dropped when the Milky Way was above the horizon.
Although there is a lot of amateur radio astronomy being done, most of it requires
microwave equipment with low-noise front ends and is beyond the scope of this book.
For example, most deep space (beyond our solar system) radio sources will require the
use of high-gain configurations—such as long baseline arrays—that are far beyond any
one individual’s ability to implement. However, there are several things that almost
anyone can do as an introduction to this “hobby within a hobby”.
Radio astronomy antennas can assume nearly all forms. It is common to see Yagis,
ring Yagis, cubical quads, and other antennas for lower-frequency use (18 to 1200 MHz).
Microwave gain antennas, such as parabolic reflectors, can be used for higher frequencies.
Indeed, many amateur radio astronomers employ TV receive-only (TVRO) satellite
dish antennas for astronomy work. In this chapter we limit our coverage to some
antennas that are not discussed in other chapters—at least, not in this present context.
General Considerations
The strongest extraterrestrial radio sources we can typically detect are from our own
sun. These emissions are very broadband and so our receivers typically detect only a
small portion of the total radiated spectrum at any one time. When we listen on a receiver,
using conventional heterodyne reception feeding a loudspeaker, what we hear is
noise that varies in amplitude—sometimes slowly, sometimes in bursts.
As a rule, we are restricted to monitoring the sun and other astronomical bodies at
frequencies higher than those blocked by earth’s ionosphere. The low end of the useable
frequency range varies with the sunspot cycle, time of day, etc., but frequencies
above 18 MHz, as mentioned, should work virtually all the time. Many radio astronomers,
both amateur and professional, concentrate on the region around 20.1 MHz, but
the author has heard the sun on 28 MHz many times with a simple three-element Yagi.
Of course, if these bands are open for HF skip propagation, or if many local groundwave
signals are present, other frequencies may contain fewer interfering terrestrial
signals.
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