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Amateur radio astronomy, by Jeffrey Lichtman, SARA
Introduction (I) Numerous
amateurs think that radioastronomy is reserved to engineers able to
understand subtilities of radio communications and experts in electronics.
However, this article written by a pioneer amateur in this field will convince
you of the contrary and that it is really an activity accessible to amateurs.
Only
one contrainst, to be interested in astronomy, in radio propagation and
communication equipment. You are ? Great ! We can work together.
Jeffrey M. Lichtman is a long-time amateur radio astronomer and he is the
founder of the Radio Astronomy Supplies company
(Nitehawk) and the Society of Amateur Radio Astronomers
(SARA) whom he served as president from 1981 to 1991. SARA is an
international organization of approximately 500 amateur radio astronomers
which regularly surveys each of its members regarding their interests in
the field of radio astronomy, as well as how SARA may address these interests.
Invariably, most new member asks the question : "How do I get started
?". It is to these people that this article is addressed. We will deal
with both general and specific information and recommendations. How
to start in radio astronomy ? Basically, amateur efforts in this discipline
fall into two general categories: -
Indirect method studies of solar phenomena, meteor infall and Jupiter noise storms.
This type work is usually done at the low radio frequencies, with relatively
narrow band receivers. It does not involve sharp imaging of the radio noise
source. This work is conducted mainly with communications-type receivers,
requiring only a minimal need for auxiliary equipment. The expansion equipment
usually takes the form of a strip chart recorder or an A/D (Analog to Digital)
computer as a readout instrument, and a suitable DC (Direct Current) amplifier
required to drive the readout (in some cases). This work, of course, does
require a quiet radio band in the spectrum of interest.
- Imaging radio astronomy. This work makes up the bulk of amateur radio astronomy efforts. It is, by its
very nature, best practiced in the VHF, UHF, and EHF radio spectra with
receiving equipment of relatively broadband design. The reason for the broadband
receivers is that all discrete radio objects radiate over a very broad spectrum,
and the bandwidth of the receiver equates to the energy received from the object.
The
Milky Way recorded at 408 MHz by John smith, All Sky Survey.
The two bright patches near the top of the imae are the
radisources Cygnus X (left) and Cygnus A (right). Units
are in Kelvins. Document BAS. Technology Discrete
radio sky objects are very weak emitters. A power flux unit has been adopted for
radio astronomy. It has to do with the tiny incremental power falling from the
sky upon one square meter of Earth surface, per cycle per second. This unit is
called the Jansky, after the original radio astronomy pioneer. By common
accord, one Jansky is defined as 10-26 watt/(meter2
Hertz),
a very small flux indeed. Upon
examination, one would think this infinitesimal amount of power impossible of
detection at all. Radio astronomy has indeed been described as the examination
of ripples riding upon waves, above an entire sea of noise. It is estimated that
all of the energy which has fallen upon Earth's radio telescopes would not equal
the energy in a single snowflake. Yet radio astronomers have refined the
sensitivity of their equipment such that these small powers are not only
detected, but also evaluated into information about the Universe which has been
both illuminating and exciting. This, despite the fact that the receivers used
to make these measurements typically generate as much as a million times the
noise signal as the energy from the desired object. How
is this accomplished ?
The assault on the problem is multi-directional and is
conducted in the following ways. One begins with as large an antenna as can be
achieved, in order to trap as much energy as is possible from the desired
object. This usually involves a radio-quiet location, but does not necessarily
require huge single antennas. The problem may be successfully addressed with
phased antenna arrays. The receiver is designed to be of low internal noise,
very high gain, and of wide bandwidth. The stability of such receivers
represents a continual challenge to the radio design engineer. Happily, the
design of low-noise radio equipment has been made easy with the arrival of very
low-noise receiving equipment using the latest low noise components or modules.
Input noise temperatures of GaAsFET antenna
amplifiers typically fall to 26 Kelvin at room temperature with 28 dB of
gain, and without any attempt at cryogenic cooling of the devices. The
noise temperature of the input amplifiers pretty well determines the sensitivity
of the total instrument. Mass production of these devices has brought their cost
down to well within the budget of the average radio astronomy amateur. Additionally,
the balance of the radio astronomy receiver is designed such that the internal
noise is canceled out. This is usually accomplished by converting all the
receiver noise, plus the desired signal, into a fluctuating DC voltage. A
counter voltage is then introduced such that the internal receiver noise is
canceled out. The residual desired signal is then amplified to a very high
level, in order that it may be measured by the readout device. In practice, the
cancellation of the receiver noise is accomplished in one of two ways : -
In so-called total power receivers, the full power of the instrument is
delivered to the DC amplifier, and the receiver noise is canceled out by the
introduction of a back-biasing voltage at this point. This permits the DC
amplifier to greatly amplify what is left, which is, of course, the desired
signal. This practice works quite well as long as there is no appreciable drift
of gain in the receiver. Long-term observations will inevitably show gain drift
of the receiver. In such cases where the zero reference line deviates, a known
calibration signal is introduced at the start, sometimes during, and at the end
of the observation. This permits quantitative evaluation of the received data. -
There is yet another type receiver which is designed to automatically cancel out
its own internal noise. In practice, this is accomplished by circuitry which
causes the receiver to alternately "look at" the signal plus the
noise, then at its own internal noise only. This is usually done with the
introduction of a square wave generator, which functions as an on-off switch. In
one instant of time, the receiver is connected to the antenna system; at another
instant the receiver input is terminated into a load resistor such that only the
internal noise is present at the receiver output. A phase-sensitive detector
circuit, driven by the same square wave generator, is then employed to deliver
the difference to the DC amplifier used to drive the readout instrumentation.
Here, again, this difference represents the desired signal. This so-called Dicke
switching method improves the receiver sensitivity by one to two orders
of magnitude. Because the receiver only looks at this difference, the effects of
gain drift are largely erased.
In consideration of all of the above, it becomes
obvious that the design of radio astronomy receivers has a great deal to do with
just what the observer is after in the data. It therefore follows that each
project must be begun with a firm idea of just what the observer has in mind as
a project.
The equipment is either acquired or built, and tailored to do the
job. The story of all modern science, regardless of the specific discipline,
proceeds as follows : -
Conceive the project. -
Build or otherwise acquire the instrumentation to do the work. -
Conduct the measuring of observations in a clear-cut and methodical way, giving
attention to all observing parameters. -
Analyze the data without the introduction of personal bias. -
Publish the results. Publications
of results and Q&A Is negative result observing data useful ? The answer is most assuredly yes; if for no other reason than to prevent other observers from duplicating effort which is unlikely to bear fruit. The purpose of the Society of Amateur Radio Astronomers is to provide sufficient technical information to enable amateurs to do this kind of work, commensurate with the antenna aperture which may be acquired. This involves the free circulation within the society of technical information. Such information is regularly published in SARA's monthly journal, "RADIO ASTRONOMY". Additional specific information is also available from SARA's technicaly knowledgable mentors, many of whom are electronic engineers, software specialists, and long time electronic enthusiasts (not to be scared off by those titles). These technical mentors regularly field questions from beginners. For the benefit of those who are still trying to define a receiving/observational projects which fit the individual's span of expertise, the balance of this publication is devoted. We invite you to survey the potential of each radio band, and to evaluate your own technical potential. Specific design information may then be secured from the SARA Journal office, or from any of SARA's many technical mentors. Please use the address at the end of this article for obtaining more information on SARA. The tabled information that will be review on the next page is taken from the "The Radio Astronomy Handbook" (1986/1993) by Robert M. Sickels. The main questions are : - Which Band ? - Which Receiver ? - Which Observing program ? We are going to answer to these questions in the next chapter. Next chapter
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