A basic radio telescope has the following attributes:

• A sensitive radio which can receive one of the frequency identified for radio astronomy
• The radio has "No" AGC (automatic gain control) or has the ability to turn it off.
• It uses a high gain low noise pre-amp and a directional antenna which can be pointed at the sky.
• A Data Logger is used to record total signal strength of the radio over a period of time

The signal strength of the radio is recorded over a period of time, signal levels will increase and decrease as a radio emitting object passes across the sky. Over number of days it is possible isolate radio astronomical observations from terrestrial and satellite interference.

Of cause everything is always a little more complicated. But it is possible and there are many examples of success to be found on the internet. The most famous of these is the NASA RadioJove Projects.

Radio interferometry is a powerful tool that can be used for a number of diverse applications. A radio interferometer consists of more than one antenna tuned to receive radio emissions from the desired frequency.

The antenna must be spaced more than 10 lambdas (baseline) apart East to West and following the natural rotation of the earth. The signals from the antenna is then cross-correlated in a Summing Amplifier at the input of the main radio receiver. As a radio emitting object passes above it produces a "fringe pattern" in the receivers measured signal strength, as the wave front of the radio emitting object passes in and out phase between each antenna.

From the Hans Michlmayr - Amateur Radio Astronomy Website

This fringe pattern can then in theory be analysed to produce a result ranging from an image of a distant astronomical object to the location of a nearby terrestrial or extra-terrestrial radio emitter.

The main limitation to the Amateur Radio Astronomer is not really the electronics but more the available space on your property. This is because in order to obtain a suitable fringe pattern the two antenna must be spaced at least 10 wave lengths (or 10 lambdas) apart. For example at 74Mhz VHF the distance between the to receiving antenna is more then 40 metres.

Radio Interferometry
Phase Interferometry
Interferometry

Phase Switching is a system used to increase the discrimination and sensitivity of an interferometer; where an extra half-wave path difference is switched in, at well defined frequency, between the two interfering signal sources. In-phase signals then become out of phase and vice versa, so that the signal output becomes modulated by the switching frequency, and can be more easily filtered from the internally-generated noise. (The discrimination is highest for sources which are small compared with the interferometer-fringe spacing)

In the block diagram a phase switch is used to introduces either 180° or 360° (equivalent to 0° zero degrees) of phase shift into the right-hand transmission line, at a specific frequency easily demodulated by the receiver detector.

This switching signal generator is a square wave which changes periodically from one state to the other, many times per second, so that at one instant the first interference pattern is obtained, and at the next a second pattern. The same switching signal is applied to a phase-sensitive detector, which acts in synchronism with the phase switch and so subtracts the second pattern from the first. The resulting pattern is shown in the third part of Figure 1.; below, each maximum of the first pattern appears as a positive peak, but each maximum of the second pattern appears as a negative peak.

The instrument known as a phase-switching interferometerwas invented by Sir Martin Ryle in 1951, and is one of the major innovations in radio astronomy for which he was awarded the Nobel prize in 1974. This principle and later versions are now very widely used in radio astronomy.

You might ask, why go to so much trouble? Why not just use the simple adding interferometer and eliminate the phase switch and phase-sensitive detector? Although it is true the same information about a cosmic source is available in either type, the phase-switching instrument can be made much less sensitive to variations inherent in the receivers' electronics. Also, the phase switch causes the cosmic signal to be modulated at the switching frequency before it is amplified in the receiver. The noise generated by the receiver has the same general character as the cosmic noise, and this switch modulation acts as an identification tag helping to distinguish it from the receiver noise.

Also, the phase-switching interferometer responds less to extended sources, such as the general background radiation of the Milky Way, which might otherwise obscure the fainter radiation from weaker and small-diameter sources. In summary phase switching interferometers or more current versions, the correlator interferometer, are vastly superior to a simple adding interferometer and now universally used in preference to it.

"Phase switching is a clever trick how to make a basically additive interferometer behave like a multiplicative one. A plain additive interferometer has some undesired traits, like it outputs the fringes on a big DC pedestal, and needs the channels to be reasonably amplitude balanced to give good results. The phase switching is a way to avoid these." - Marko Cebokli

Information based on articles from Sky & Telescope by G.W. Swenson & W. Swenson, Jr.

Natural radio emissions from space cover the total range of the electromagnetic spectrum. However, on the earth's surface the majority of this spectrum is blocked by the earth's magnetic field and atmosphere only allowing few regions to pass. In the radio spectrum the earth’s atmosphere becomes increasingly transparent above 18Mhz and then increasingly opaque at around 40Ghz.

Atmospheric Transparency to different wavelengths of the Electromagnetic Spectrum

Any frequency above 18Mhz free from terrestrial and satellite interference can be used for radio astronomy. The lower segments of the spectra are used for solar and Jupiter observations; the 73, 150 and 406 MHz segments are quite popular for pulsar, and the 1.4 Ghz band and above is used for spectral line or energy measurements.

The following frequencies below are generally accepted spectral regions for radio astronomical observations and so have just chosen list the official regions as a reference, most accessible for an amateur radio astronomer.

• 25.550 – 25.670 Mhz
• 37.5 – 38.25 Mhz
• 73 – 74.6 Mhz
• 150.05 – 153 Mhz
• 322 – 328.6 Mhz
• 406.1 – 410 Mhz
• 608 – 614 Mhz
• 1.4 – 1.427 Ghz
• 1.6106 – 1.6138 Ghz
• 1.66 – 1.67 Ghz
• 2.655 – 2.700 Ghz
• 4.8 – 5 Ghz
• 10.6 – 10.7 Ghz
• 18.28 - 18.36 GHz

Amateur Radio Astronomy Frequency Choices

Most natural cosmic sources have spectra that fall off with frequency, so even if you keep the same antenna aperture (effective area) the signals will decrease with frequency.

Consequently the lower the frequency that is still transparent to the ionosphere (e.g. above 18Mhz) the greater the energy (signal strength) that can be collected by a specific gain of antenna. Said another way the better chance you have in detecting it.

What you gain by going up in frequency is:
- a narrower antenna beam (if you keep the same antenna area),
- less man made interference,
- more transparency to the ionosphere,
- a bigger possible bandwidth (if your hardware can eat it)

The relation between gain and effective area is

G = 4 * PI * A / L2 or A = G * L2 / 4 / PI

Where G is gain (linear, not dB), A is the effective area, PI is 3.14... and L2 is wavelength squared. Units for A and L2 are not important, but both must be given in the same units. The same area means more gain at a higher frequency, and the same gain means less area at a higher frequency.

Consequently from this reasoning the best choice of frequency would then be the lowest frequency that is free of interference that can be installed on the land area available to the Amateur Radio Astronomer. Land area becomes even a greater concern with interferometry as the antenna must be space apart East to West by 15 or more wavelengths to achieve a suitable fringe pattern.

In selecting an antenna for Radio Astronomy it is important to achieve a high degree of gain, low noise and have a a reasonable bandwidth for the frequency chosen.

After a review of many antenna designs, I have settled on a 16 Element Collinear Broadside Array as the most cost effective and high gain system to deploy on the 74Mhz band.

Element dimensions for 74Mhz
Each 1/2 wave feeder element = 1.948M
Each 1/2 wave reflector element = 2.064M
Spacing between feeder and Reflector = 0.771M
Overall dimensions = 6M X 4.2M X .771M

This is the basic electrical layout of the antenna it has balanced feed-line with an output impedance of between 300 to 400 ohms and an expected gain of 16dbi and a beam width of about 30 degrease.

Each element is made of aluminium tubing and can be mounted directly using a simple U-clamp to welded steel frame without insulators. Affixed in the centre of the 1/2 wave element the impedance is at its lowest, so an insulator is not required.

Collinear Broadside Arrays can also be connected in pairs simply, without the need of complex impedance matching networks.

A receiver antenna aperture or effective area is measured as the area of a circle to incoming signal as the power density (watts per square metre) x aperture (square metres) = available power from antenna (watts).

Antenna gain is directly proportional to aperture and generally antenna gain is increased by focusing radiation in a single direction, while reducing all other directions. Since power cannot be created by the antenna the larger the aperture, the higher gain and narrower the beam-width.

The relation between gain and effective area is

G = 4 * PI * A / L2 or A = G * L2 / 4 / PI

Where G is gain (linear, not dB), A is the effective area, PI is 3.14... and L2 is wavelength squared. Units for A and L2 are not important, but both must be given in the same units. The same area means more gain at a higher frequency, and the same gain means less area at a higher frequency.

Simply increasing the size of antenna does not guarantee an increase in effective area; however, other factors being equal, antennas with higher maximum effective area are generally larger.

It seems obvious to optical astronomers that a parabolic dish antenna that is many wavelengths across, will have an aperture nearly equal to their physical area. However other antenna such as a Yagi and Collinear arrays my not look to be the same at first glance but they do achieve the same result using other means at radio frequencies.

A few years back I bought some large VHF Broadside Antennas at a stock-take sale. So during one weekend, I thought I'd just weld up a stand and see if I could use them as a simple radio telescope.

Using a WinRadio WR-3700e Wide-band receiver and small preamp, I scanned the VHF spectrum within the antenna's designed frequency range for a radio quiet area where I could conduct a meridian drift scan of the sky.

After a week of scanning and logging, the quietest frequency identified was 231.58Mhz which was reasonably clear either side by about 1Mhz. The scans where conducted at a receive sensitivity less than 1uV, the bandwidth was 17Khz and was logged at 1khz steps, AGC (Automatic Gain Control) was disabled.

The purple line indicates the maximum signal strength detected over the 24 hours and similar the green the average and orange minimum. Note that over the 24 hour period, some RFI interference occurred at different times of the day. Nevertheless this could be reduced using a band-pass filter at a latter stage.

The audio of the receiver was connected to a computer using the sound card and was logged over 24 hours from 7pm to 7pm using the the famous amateur radio astronomer software Radio-SkyPipe and the results so far are not to bad for a first attempt.

The antenna was aligned approximately by eye to line up with the sun as it passed over, if there are any extraterrestrial objects it is difficult to verify, as no planning at all was attempted.

The next trace was recorded from 9pm to 9pm where the antenna was tilted directly vertical to zenith.

The last is an example of the noise generated by heat in the equipment without an antenna over a similar 24 hour period. Note that the noise floor has been amplified significantly.

The two antenna where stacked to offer a 32 element collinear phased array and should in theory offers a gain of around 20dbi and a beam width of about 18 degrees. Although the gain may seem low compared with microwave radio telescopes the all important aperture size is over 4 metres square. Also note that galactic noise is very bright compared to microwave in the VHF spectrum.

Most interferometers for radio astronomy are built on an east-west baseline and although this is not strictly necessary the operation of an east-west interferometer is the easiest to visualise. Also it is the easiest to interpret as the fringes recur more rapidly than those of any other baseline.

So how long should the baseline be?
In the vicinity of the meridian a baseline n wavelengths long would produce n/4 fringes per hour when a source on the celestial equator is observed. For example if the wavelength was 4 Meters (e.g. 74Mhz) then a baseline of 80 metres or 20 wavelengths would have a fringe rate approximately 5 fringes per hour or a fringe every 12 minutes.

Using a method called “Meridian Drift Scan Observation” it is possible to build up an image of the sky at radio frequency not visible using optical telescopes. As seen below.

The Radio Sky: Tuned to 408MHz Credit: C. Haslam et al., MPIfR, SkyView

Drift scans plot the sky line by line using the earths rotation from East to West then adjusting the antenna every 24 hours over a series of elevations separated by somewhat less than the angular beamwidth of your antenna. 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 to make a 2 dimensional map of the sky. In reality, there is quite a bit more to do this, but this is the basic Idea.

Here are some more examples:

The Infrared Sky (and more)

Most natural signals (i.e cosmic sources) are almost always non-polarised (which is the same as "random polarised"), so the use of any single polarisation method either linear or circular will achieve the same result. The slight polarisation present in such signals do not bring any significant "power advantage" so in practice linear polarised antennas are preferred more in Radio Astronomy as they are more practical to construct for a specific gain over a circular polarised antenna.

Polarisation can however carry interesting information about the source, so radio astronomers sometimes want to measure this. However it is quite difficult to do, because the signal characteristics are so weak, and below a few 100 MHz, the polarisation information is usually too mixed up by the ionosphere to be of any practical use.

Information about Antenna Polarisation.