Amateur radio astronomy, by Jim Sky, Radio-Sky Publishing
How does a radio telescope work ?
A radio telescope is like a radio receiver except that the signal is much weaker and must be recorded for processing.
Basically a radio telescope requires 8 stages as follows :
Radio Telescope Antennas
The antenna is the most obvious part of a radio telescope. It is analogous to the lens of an optical telescope. The antenna gathers the minute amount of radio frequency energy from the sky and transforms it to a tiny electrical current which, after much processing, we can then measure. Most radio telescope antennas are quite large due to the resolving power desired. Larger antennas may better focus the energy from a smaller region on the celestial sphere. This region of the sky to which the antenna is most sensitive may be thought of as the beam pattern of the antenna.
While the type of antenna most often thought of in relation to radio astronomy is the parabolic dish antenna, many other types of antennas are also used. Large arrays of dipole antennas have been used to discover pulsars and probe the noise storms of Jupiter. Long trough-like antennas, the cylindical parabolics, are still used in observatories around the world. Arrays of yagi antennas, horn antennas, Mills crosses, and many others have contributed to radio astronomy. Virtually any antenna which has a reasonably small beam pattern has been used.
Very often, amateur radio telescopes will keep the direction of the antenna fixed along the north-south line, or meridian. The antenna is adjusted in elevation to a given angle and the cosmic radio source allowed to pass through the antenna beam as the Earth rotates. This is called a meridian drift scan observation. As the radio source passes through the antenna pattern, an increase in energy is recorded as a rise and then a decline in the data recording device. Meridian drift scans offer the advantage that calculation of the source coordinates becomes a simple matter. The right ascension of the source is equal to local sidereal time at which the source passes.
Radio Telescope Pre-Amplifier Cosmic
radio signals are generally very weak. To measure them we have to amplify
them by factors of millions of times. The electronic components in our
radio telescope produce random electrical noise which also gets amplified
by this huge amount. It is easy to see that the noise from the early parts
of the chain of amplifiers gets multiplied by more than the later stages.
If we are not careful, this noise can totally hide the weak noise we are
trying to measure from the cosmic radio source. The role of the
preamplifier is to boost this incoming signal from the antenna many times
while adding as little noise as possible. The preamplifier is often called
an LNA or low noise amplifier. Special transistors are used in this stage
to accomplish this. Professional observatories also use cooling of the
amplifiers to very low temperatures, just a few degrees above absolute
zero to minimize the amount of noise contributed by the components. You
will sometimes hear amateur radio astronomers and other people interested
in receiving weak signals refer to the noise figure of an amplifier. We
won't get into the specifics of what this means but a noise figure of less
than .5 decibels (dB) is considered very good. The
Radio Telescope Mixer
Cosmic radio signals are generally very weak. To measure them we have to amplify them by factors of millions of times. The electronic components in our radio telescope produce random electrical noise which also gets amplified by this huge amount. It is easy to see that the noise from the early parts of the chain of amplifiers gets multiplied by more than the later stages. If we are not careful, this noise can totally hide the weak noise we are trying to measure from the cosmic radio source. The role of the preamplifier is to boost this incoming signal from the antenna many times while adding as little noise as possible. The preamplifier is often called an LNA or low noise amplifier. Special transistors are used in this stage to accomplish this. Professional observatories also use cooling of the amplifiers to very low temperatures, just a few degrees above absolute zero to minimize the amount of noise contributed by the components. You will sometimes hear amateur radio astronomers and other people interested in receiving weak signals refer to the noise figure of an amplifier. We won't get into the specifics of what this means but a noise figure of less than .5 decibels (dB) is considered very good.
The Radio Telescope Mixer
The job of the mixer is to lower the frequency of the signal from the preamplifier. We do this for a couple of reasons. First, it is hard to build good amplifiers, filters, and other other components for higher frequencies, (though this is getting easier with new technologies). Secondly, if we do all of our amplifying at the frequency which we are receiving, there is a good chance that some of the amplified signal will escape back into our antenna and produce feedback. Its the same problem you have when you get the microphone too near a public address speaker. The mixer combines the signal from the local oscillator with the signal from the preamplifier (input signal) and produces two additional outputs, one at the input signal frequency minus the local oscillator frequency, and one at the sum of these frequencies. We pick the lower of the two outputs by passing the mixer output through a filter in the IF amplifier.
The Radio Telescope Local Oscillator
The local oscillator produces a signal which is injected into the mixer along with the signal from the antenna in order to effectively change the antenna signal to a frequency which can be handled by the IF amplifier. Many radio telescopes use a quartz crystal derived local oscillator signal. Quartz crystal oscillators are quite stable and drift little in frequency. Because most radio telescopes are quite broadband in nature, a small amount of frequency drift in the local oscillator may be tolerable. One must be careful, however, that the drift is not so great that received signal frequency begins to infringe upon the bandpass of the antenna or any front end filters that precede the mixer. Also, it is possible to drift into an interfering signals frequency. When either of these things happen, it can result in a change in the output of the telescope which could be mis-interpreted as a real change in received cosmic noise.
Telescope IF Amplifiers
The intermediate frequency (IF) amplifier is a radio frequency amplifier which processes the output of the mixer. In addition to amplifying the signal, the IF amp usually has some form of bandpass filtering so that only a selected range of frequencies is allowed through. These filters are often of the SAW, crystal lattice, or ceramic varieties. Common, IF frequencies are 70, 45, 21.4, and 10.7 MHz, however there is no restriction to these frequencies. One difference between most radio telescope IF amps and those found in communications receivers is that communications receivers usually employ some form of automatic gain control, (AGC). AGC circuits need to be disabled in communications receivers modified for radio astronomy use as they tend to mask the subtle changes in signal strength we are trying to detect.
The amount of gain needed for the IF amplifier is determined by the signal level exiting from the mixer, the amount lost in the IF amp filter(s) and the appropriate level required for the square law detector which follows. It is sometimes difficult to determine all of these values ahead of time and so a variable attenuator is sometimes introduced at the IF amp output prior to the detector.
Telescope Square Law Detectors
If you become a dedicated amateur radio astronomer, you will no doubt hear much talk about "square law detectors". It is interesting that one of the simplest electronic configurations in the radio telescope show draw so much attention, but all of this attention is due to its' very important role. The radio frequency energy exiting from the earlier portions of the receiver alternates in polarity around some central voltage. If we could just hook up a DC (direct current) meter to this signal it would read zero volts because the positive and negative swings of the voltage would cancel each other out. In order to measure the intensity of the signal we thus must throw half of it away! We need a door which only allows passage of the signal in one direction and this is the semiconductor diode. Even the symbol we use for the diode suggests this quality, an arrowhead pointing towards a line.
There are several types of diodes. The types most commonly used by amateur radio telescope makers are the germanium diode and the schottky diode. If we pass just the right range of current through these diodes, the voltage we measure coming out of them will be the square of the input, and thus will be proportional to the power which is fed to them from the receiver. It is the power received by the radio telescope antenna that we want to measure and we owe to detector the ability to measure it. If this is a bit confusing, fear not. You will no doubt have time to sort it out in one of those discussions I refer to above.
Processors for Radio Telescopes
Once the radio frequency energy has been converted to a DC signal by the detector, we need to transform it in other ways that make it easier to record. Even though we have tried very hard to not introduce much additional noise from our receiver into the signal, there will usually be much more of this unwanted noise at this point than the actual noise we are trying to measure. In other words the output of the detector will consist of a lot of receiver noise added to a small amount of noise from the cosmic source. Lets say our recording system could measure from 0 to 5 volts. If we were to amplify our DC signal to fill most of this range, only a small change in the recorded output would be due to the source. What we need to do is remove most of the noise contributed by the receiver before we amplify the DC signal by a large amount. This function is provided by an offset circuit which simply subtracts a steady DC voltage from the signal voltage. This is easily accomplished with a operational amplifier connected as a voltage adder.
Even though we refer to the detected signal as a DC (direct current) signal, it still varies rapidly in intensity because it retains much of its noise character. The smoothing out of these rapid fluctuations is accomplished by an integrator. The integrator function is accomplished by using a capacitor as a holding tank for the incoming signal. Imagine that the signal is water coming through a hose and that the water pressure fluctuates in this hose. If we empty the hose in a large water tank and take the outflow of water from a small tap in the bottom of the tank, it is easy to imagine that the fluctuations in water pressure will be largely absent from our outflow tap. The integrator performs an additional service in that by averaging the signal over time it greatly increases the sensitivity of the measurement.
Lastly out DC processor amplifies the detected signal to a level where it matches the range of our recording device. The amplification function as well as the other functions of the DC processor is usually accomplished by use of integrated circuits called operational amplifiers. It is very important to use high quality "op amps" and other components in the DC processor.
Telescope Recording Devices
To be of any value, the output of a radio telescope must be recorded. For the simple radio telescope described here, what we want is a record of how strong the signal is over time. If we are using drift scan observations, we can relate the time a particular value of signal strength was recorded to where in the sky our antenna was pointed. The result is often called a strip chart. Below is a great example of a strip chart from the web pages of the radio observatory at the University of Indianapolis in Indiana. This is a chart of the radio source, Taurus A, taken as the rotation of the Earth moved the beam of their antenna across this region in the constellation Taurus.
In the old days, most strip charts were made on mechanical strip chart recorders. These devices had a pen which was deflected (in response to an applied voltage) across a moving continous sheet of paper. These devices are seldom used anymore and most data is collected and displayed on a computer. In order to make the transition from a continuously varying analog voltage (the output of the radio telescope) to digital information which the computer can process a special device is required called an analog to digital converter. These devices come in many forms. You can find out more about them here at websites listed hereafter. Special software graphs the data received from the analog to digital converter and it can be printed if the need arises.
Drift scans are nice, but what if you want to make a map of the skys radio emissions ? Well, all you have to do is plot drift scans of the sky at a series of elevations separated by somewhat less than the angular beamwidth of your antenna. If you live in the northern hemisphere, you could begin by pointing at an elevation close to the north celestial pole near Polaris, and running your telescope for 24 hours. If you had a beamwidth of say 10 degrees, you would then lower the elevation by about five to seven degrees and making a strip chart for that elevation. You would continue the process until the beam was point a bit above your horizon and then combine the data files to make a 2 dimensional map of the sky. In reality, there is quite a bit more to it than this. Calibrations half to be maintained and other factors like the interference and radio noise from the ground considered. Still, you get the idea.
If you intend to record sporadic events such as Jupiter's noise storms or meteor reflections with your radio telescope, you can use PC's sound card as a recording resource. Sound cards are great for recording sounds in these specialized cases, but remember, the output of the generalized radio telescope in our example is not sound, but a slowly varying voltage which corresponds to the amount of energy or antenna is picking up from the region of sky towards which it is pointed.
For more information
Publishing, Jim Sky's website
Weak signal communication
Primaluce Lab (radiotelescopes
SARA (Society of Amateur Radio
Homebrew Radio Telescope (PDF), WA8SME, QST, June 2009
Weak signal communication resource, Nitehawk
Primaluce Lab (radiotelescopes and accessories)
SARA (Society of Amateur Radio Astronomers)
Build a Homebrew Radio Telescope (PDF), WA8SME, QST, June 2009.