Overview
This is a work-in-progress. Despite the title and my intentions, it's probably going to be too complex to include in the Simple Electronics Projects section of this website.
I have tried - and failed - to detect meteors by tuning a radio to a transmitter over the horizon. Perhaps I really needed to have been using an external Yagi aerial as described here. That must count as one of the simplest RTs imaginable, albeit very limited in scope and more RADAR receiver than RT.
Simpler still, an AM radio can be used to infer the presence of the ionosphere through its ability to receive distant signals at night. It can also be used to detect thunderstorms and extreme solar weather.
Of course, by radio telescope, we usually mean one intended for astronomical use.
So, perhaps an old CRT TV that showed static 'snow' originating from the Big Bang when nothing was being transmitted could count as one? Except, that's true of only a small fraction of that snow, so unless there's some way of removing the rest of it (the noise), you can't point at the screen and say 'That's the Big Bang!' after all.
What I'm looking for is the most basic Radio Telescope that can be used to clearly detect naturally-occurring extra-terrestrial radio waves.
First though, a (rather lengthy) diversion, to look at an amateur project from 50+ years ago.
A Radio Telescope at School in the early 1970s
I remember an amateur radio telescope at Wellsway School, possibly built more as a challenge than as an instrument for serious use, as I recall it was relatively short-lived. I had no involvement with it, sadly. Perhaps I could rectify that and make and document an updated version, hopefully simplified and cheaper? Let's see...
Recollections
date ~1972/3
built by Head of Physics Mr. Lane & senior pupils
aerials: one, possibly two, on the flat roof of a single storey building containing the Physics lab; of wooden-framed construction; right-angled isosceles triangle cross-section; parallel wires running the length of the frame along the two sides adjacent to the right angle to form a reflector; element running the length of the frame inside the reflector.
equipment included a plotter or pen recorder (HP?) in the Physics lab
according to comments that were written on paper from the recorder, showing the signal and time of day, it picked up the school's bells that sounded throughout the day (!), the Sun certainly and possibly the Milky Way and Jupiter*.
The inspiration
It occurred to me that the inspiration likely came from an electronics hobbyist magazine of the time and after some research, I was able to find a series of 10 articles in Practical Electronics magazine (archive here) from 1971/2, called Radio Astronomy Techniques by pioneering amateur radio astronomer F.W. Hyde. The aerial is of the same type as the one I remember and is known as a corner reflector antenna (not to be confused with a corner reflector!). Given the content and timing of these articles, I'm in no doubt they were the inspiration for the School's RT.
The basic RT in these articles is intended for solar observation at 137 MHz (i.e. in the VHF band), although it seems it is capable of detecting deep space radio sources and artificial satellites as well. It comprises:
aerial (x2 for interferometer): 90 degree corner reflector antenna 11 feet in length with two 6-foot sides, with wire mesh or single wire for reflecting 'surfaces' and two folded dipoles. Gain > 12dB (i.e. ratio of power signal out to in of >10**(12/10) ~16x).
pre-amp: gain > 16dB (10**(16/10) ~40x);5 to 10 MHz bandwidth. Commercial one for use with 144 MHz amateur band can be suitably modified. Total gain of reflector and pre-amp therefore > 28dB (10**(28/10) ~630x).
converter 'for the operating frequency' to change the VHF signal to a lower frequency that the receiver can detect. 'Commercial television converters can be modified for the purpose'. Example circuit diagrams provided.
'standard' communications receiver. Typically, it seems such receivers can receive signals from 0.5 to 30 MHz, hence the need for the converter.
recorder - pen or tape
DC amplifier for recorder if not provided. Example circuit diagrams provided.
stabilized power supply
In addition, a signal or noise generator is recommended for testing each stage. Example circuit diagrams provided.
Here are links to the articles - they should take you to the right page in the document (possibly after a short delay). If not, the page numbers are provided - first the printed page number and in brackets the actual page number in the pdf document.
1/10: Intro + aerials
p. 462 (p. 24 of 92 in the pdf document)
2/10: Various radio telescope systems described and design suitable for amateurs described
p. 586 (60/84)
3/10: Increasing resolving power and some special methods of observation (interferometers...)
p. 658 (52/84)
p. 730 (44/92)
5/10: Solar radio observatory: pre-amplifier, converter, receiver and recording system
p. 830 (56/92)
6/10: Setting up, testing, starting observations
p. 930 (68/100)
7/10: Forming an interferometer/interferometers and their construction
p. 1009 (59/108)
8/10: The phase-switched interferometer
p. 63 (65/92)
9/10: More sophisticated units for phase-switching; improving the single aerial system
p. 134 (48/92)
10/10: Jupiter, other observations, producing radio map of the sky
p. 221 (55/100)
Comments on the project
This is a complex project, even with off-the-shelf kit and minimal soldering. It seems best suited to the HAM radio enthusiast who would have had experience of aerial-construction and of integrating the stages in a radio communication system and who would most likely already have some of these stages to hand. As such, the project would perhaps have been more at home in the pages of Practical Wireless magazine.
It's not quite clear to me why the frequency of 137 MHz was chosen, especially as in the outline design in the second article, the author identifies another 3 that use standard communications equipment, as being suitable: 184 MHz, 144 MHz and 30 MHz.
The footprint of each aerial was large - 11 feet long by 6 feet wide, by 6 feet high if inclined to the horizon - and a challenge for the amateur to build and accommodate, I would have thought. Why not choose a higher frequency and make the size of the aerial correspondingly smaller?
A second aerial could be added to form an interferometer. There would need to be wide separation between the aerials (36 feet IIRC), making this RT configuration even less suitable for the average garden. Not so much of a problem perhaps for some schools and colleges...
The author of the articles refers to the focal line of the aerial containing the dipole element. As the aerial isn't parabolic in cross-section, perhaps it should be described more accurately as a fuzzy focal tube.
According to the second article, the aerials must be aligned East-West. Clearly resolution in this direction is not the same (less than?) as North -South.
The author says that with a single aerial, it should be possible to detect stronger radio sources from our galaxy, such as those in Andromeda, Cygnus and Taurus.
*The aerial wasn't capable of picking up the lower frequency radio emissions from Jupiter. Perhaps I'm mistaken, or there was some wishful thinking.
The School must have adapted the design, as my recollection of the dimensions of the aerial differ from those in the articles. First, I'm pretty sure the sides were no more than 3 feet wide. If that was indeed the case, then the frequency being looked at was at least double, say ~275 MHz, still just in the VHF band. A higher frequency and hence shorter wavelength than specified would mean smaller aerials, making the project easier to construct (albeit with reduced margins for error) and to accommodate and to carry up to the roof. Second, the ratio of sides to length was IIRC more like 1:3 or 1:4 rather than the ~1:2 in the article. My understanding from the first article is that the ~1:2 ratio is a minimum, so perhaps that ratio of the aerials at school was indeed higher, possibly for stability?
One final thought: I do wonder if the reason that the School's project was abandoned after little use was if there were problems with observing at a higher frequency.
Other aerials
As for other types of aerial suitable for the amateur, the PE articles also mention:
a Yagi aerial, dimensions provided (1971, p. 466). There is a picture, but no further details, of an array ('compact and mechanically simple') of 4 such aerials for observing the Sun at 200 MHz (1972, p. 68).
' a pair of full wave dipoles with reflector', dimensions provided (1971, p. 466). To my mind, this is a curious design, looking like two folded (half wavelength) dipoles with reflectors that have been re-arranged somewhat. The distinction between full and half wavelength in this context is lost on me, but perhaps a pair of folded (half wavelength) dipole and reflector (think Yagi, but without the director elements) may be worth looking at?
A 'modified Kooman array', consisting of 'a group of dipoles with a reflecting sheet of netting or parallel wires' (1971, p. 467) .
a near-closed loop aerial, 4 foot 6 inches in diameter, with reflector, for observing Jupiter at ~20 MHz (1972, p. 221). Full instructions provided, although results seem vague. Unfortunately, this type of aerial seems to have been discredited, for this purpose, as per this analysis (you will need to scroll down) of the performance of an aerial of similar, albeit smaller, design.
Conspicuous by its absence is the half-wavelength dipole (see below). While it would have been much longer than the corner reflector aerial of the PE RT, for the fortunate amateur with enough space, if the aerial were sited high up in a suitable location, it could well have intruded less, occupying little volume. Also, it would have been easier to construct and potentially of greater use to the novice radio astronomer.
An updated PE RT
Now back to the present day, when converter and receiver can be replaced with a Software-defined Radio (SDR), running on a computer. Similarly, the need for a recorder and DC amp can be met with software.
That would simplify things an awful lot, but the aerials would still need a lot of space. The author says he chose the design for its superior performance to the ordinary dipole and reflector and also because it can be adapted readily for higher frequencies.
So, perhaps a higher frequency and smaller footprint version could be worth considering? There's no guidance in the articles about doing this, so really it still looks like a project for the more experienced amateur radio enthusiast.
However, I do have some doubts about such a project, since no-one seems to be using aerials like this these days*.
That really leaves the Yagi aerials and array - see below.
Popular RT projects for the beginner today
i. Satellite dish (SHF band)
The satellite dish RT seems to be limited, being capable of picking up the Sun and satellites only, but does have the advantage of having a simple off-the-shelf aerial and being of comparatively easy build.
ii. Horn antenna (UHF band)
The horn antenna RT comprises a DIY horn aerial of metal, and (optional?) pre-amp and SDR. It detects emissions from neutral hydrogen from sources in our galaxy. What's really cool is that it can distinguish between the spiral arms of the Milky Way! The horn takes up relatively little space as it receives radio waves of a short wavelength (21 cm) and can therefore be brought indoors when not in use. Because of the short wavelength, margins for error will be tighter than at longer wavelengths.
iii. Half-wavelength dipole (HF band)
Specifically NASA's Radio Jove Project, which is aimed at school and college students. It is intended mainly for observing the Sun and Jupiter, but is also capable of picking up galactic background radio waves. It looks well-documented and kits are available. The aerial is a simple half wavelength dipole and is documented here. For observing Jupiter, a pair of aerials is required, forming an interferometer. There is no pre-amp, no balun and the aerials are connected directly to a SDR.
It receives radio waves at 20.1 MHz and needs an aerial around 7m long. Finding an appropriate site, especially for the two aerials needed for the interferometer will likely limit its appeal to most amateurs today working in isolation.
The frequency of the NASA RT looks likely to have been chosen as it is in the middle of the range of low frequency radio emissions from Jupiter. If we're willing to forgo receiving those, could we detect emissions from the Sun and our galaxy with a much shorter half wavelength dipole? Unfortunately not, as the intensity of the galactic background falls off substantially with frequency - see here and here, which is going to make isolating this signal from other background noise increasingly challenging as we shorten our dipole.
While it seems we won't be able to pick up the galactic background with a shorter dipole, could we pick up a strong galactic radio source instead? If so, I can't find evidence of anyone else doing this.
A Yagi array?
Other than the mention in the PE articles of the Yagi aerial and array, so far I've only come across this (scroll down to near the end, where it says 'Addendum...') very brief outline design from some years' ago, produced by someone with reservations about using a dipole aerial, such as that used in NASA's Radio Jove. It looks worth a go, especially as the author refers to an array of 1 to 4 aerials, so we could just start with one and work our way up.
However, given the dearth* of such projects online now, it would seem unlikely this would qualify as a basic RT.
Proceed with caution then, perhaps gaining useful experience first like this:
start with an easy project, perhaps meteor detection as above
then try to receive signals in an amateur band (perhaps 137 MHz, say, in tribute to the PE RT) from ground-based stations;
next try to receive from amateur high altitude balloons
next try receiving from satellites (also this); even here aerials can be very simple, absurdly so even, see e.g. here.
Once some experience has been gained of constructing aerials, putting together a receiving station and then using it to obtain positive results, we should be in a much stronger position to attempt a RT.
To test it, perhaps identify a suitable object of interest, point the aerial at the appropriate location on the meridian and then wait for it to pass. If no signal is received, this being a directional aerial, there may be some tweaking with the aim. If still no signal is received, then consider upgrading the aerial (by adding more elements if appropriate) or adding another to (form) an array. If that doesn't help, then perhaps look at electronics to remove noise or amplify the signal. Once a radio source is tentatively detected, see if it is still there one sidereal day later. If it is, that will be our positive result. Depending on the complexity of the build, it could be considered a basic RT or not. If no positive result is obtained, then its unsuitability as a RT will have been demonstrated within the constraints of the project, but some fun will have been had in the meantime.
Stay tuned!
*See document A BASIC PRIMER ON SETTING UP AN AMATEUR RADIO TELESCOPE in Links below and the author's comments about favouring UHF vs VHF, which would also account for the absence of an equivalent to the PE RT today.
Links
British Astronomical Association, Radio Astronomy Section: https://britastro.org/sections/radio-astronomy
Society of Amateur Radio Astronomers: https://www.radio-astronomy.org/
(inc. A BASIC PRIMER... : https://www.radio-astronomy.org/node/118)
UK Radio Astronomy Association: https://www.ukraa.com/