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How to begin in radio astronomy with lack of experience ,
not enough
knowledge in electronics and few money...
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This text is an attempt of answer to this question which a lots of people
ask us by e-mail, showing an interest for radio astronomy with sometimes a certain 'panic'...
NB: For the general questions of the style ' what is the radio astronomy? '
we send back towards sites dedicated to this subject. Your favourite browser will find
heaps of pages dedicated to the radio astronomy .
We are going to try to explain what can be done, and what one can expect from an amateur
point of view . We shall envisage things simply from our personal experiments.
1) Caution!!!:
It seems to us that it is necessary first of all to say :be realist :
the radio astronomy is a thankless science, do not think that you will
receive the beautiful pictures of galaxies which you see in books.
What you can observe is very interesting but gets closer to the kind of the results
obtained during the beginnings of the radio astronomy after WWII:on your graphic recorders
you will see a needle move and your delight will be total of what you think you are receiving...
(In our time as one often hears, the graphic recorder is replaced by a pc with a software
of acquisition, but for reasons which I shall explain I prefer the graphic recorders at least
at the beginning.) The amateur radio astronomy is surely more thankless than amateur's optical
astronomy.
One can think that if at the level amateur there is no discovery to be done, on the other
hand it seems to me that the amateurs could, by consulting to participate for example in
observations of daily surveillance in particular of the sun and do in a certain scientific work.
Having said that, heaps of things are possible, the proof we are doing that for more than
10 years, one is far from being tired, on the contrary. Courage!!!!
2) What can be received?
* which comes in the first one to the mind is probably the sun. Our star is a powerful
and close 'broadcasting station'. He can be received with modest means. On nearby frequencies
of 100 mhz you will receive in active period numerous bursts of a very varied aspects ,
in duration as well in power. This makes of the sun an object of choice, surely the easiest.
*the Milky Way, Cassiopea, and some most powerful radio-sources are much more monotonous.
They have a constant flux, and when you will have received them once it will be always similar.
*jupiter is interesting and easy to receive but we have no experience about it .
In fact, in a realistic way, what seems the most accessible and of nature to maintain
the observers curiosity is the solar radio astronomy. It is what we do.
3) Doe you need knowledge in electronics?
I am sorry to say that yes knowledge in electronics is needed. It is necessary to qualify
by saying that a very strong knowledge is not indispensable , but it is better to build
the material, know its defects, know how to repair it. In fact simple solutions exist,
who require only for handyman's level a little experienced, and not many sophistical equipment.
Please , know that there is an association: SARA Society of amateur Radio-Astronomers
with an european section, this international association publishes a brochure full of ideas
and the BAA also . Finally if you begin in electronics , please , do not do it
alone, go to a close radio-club from your home, and speak about your projects to the
radio hams that you will meet. Many radio astronomers amateurs are or were radio hams
and certain radio hams do not dare to try radio astronomy.
4) Doe you need other knowledge?
To install our devilish machines we have to learn and to make many things, it was
necessary to dig and to flow tons of concrete to build the pedestals of antennae, it is
necessary always to make a little of mechanics for these antennes: drilling, welding etc....
It is necessary to make computer programs...
For serious bibliography, please , go to the faculties of sciences and observatories of
your region, they are often subscribed to reviews as ' SKY AND TELESCOPE '
and often have books of radio astronomy. It was enough that a researcher is interested
in it one day so that he ordered the books.... We were surprised with all that we were able
to find in the faculty of sciences of Bordeaux in France , and at Floirac's Obervatory
that is close from our home.
We were also able to make several books come from foreign universities by the procedure
of loaning .
The radio astronomy is a complete sport....
5) What materiel is needed to begin?
It is probably the question asked us mostly. The answer is neither simple nor unique.
The material depends on what you will want to receive. We can give only general ideas, but
when you will have a more precise project, we could help you to refine it and to look for
the best technical approaches.
A) Choice of the frequency to be used: let us take the hypothese that you want to receive
the sun what is necessary to envisage?
First of all it is necessary to demystify the idea that radio - telescopes=antenneaes are
all parabolic, it is what one sees in professional obervatories, but it is about antennae
of very big dimensions (several dozens of metres of diametre, what makes that they have an
important gain even with rather low frequencies.). For us of the ' yagi 'antennas such as
tv antennas are enough with their gain which is often nearby of 10db.
Then also for the sun it is necessary to know that it is useless and even harmful in a sense
to use frequencies higher than 1000 mhz, because the bursts are most easily observable in vhf
near 100 mhz, it is necessary to use a frequency above fm . It is for it that reason we chose
a frequency of 169 mhz, is placed next 164 mhz used at Nançay, and which turned out exempt
from any broadcast tv or radio taxi: IT IS VERY IMPORTANT, the choice of the frequency depends
after all on local radio-electric conditions of pollution. The sun (and radio-sources) emits
a continuum, that is a brilliance with wide spectrum. Indeed on there are official
frequencies of radio astronomy, but the receivers on these frequencies are rare (for cause,
they are radio telescopes) ,very difficult to find, it is necessary to build them ,
what is not simple and is very costly. In fact one uses most of the time already made
receivers (tv or vhf receivers), modified or not.
B) Choice of the site: The quality of the reception depends also on simple things as the
presence of electric lines, or the presence of broadcasts, I think for example of the
radio hams or the cibists which send dozens watts. This is not a criticism but an observation,
if your neighbour is cibist or radio ham it is likely that you can never receive anything
from celestial bodies because the tiny power which they send you will not do be strong enought
in front of their watts!!! And your radio telescope with its preamplifiers with big gain
and your big antennae will be very perturbed even if you are not on the same frequency.
It will remain the solution of shared time. If unfortunately you live too near from a tv
transmitter or a relai of radio - telephones it will be very difficult .
An installation in town centre would also be perturbed by the motors and other parasites.
A good place is in the campaign, far from roads, lines of electricity, and in the middle
of the pine trees which establish an excellent absorbent, except upward what is exactly
what interests us (broad-leaved trees have not this effect there). The problem it is that
one does not generally choose these things.
C) Choice of the material:
To schematize, a radio telescope consists of following main elements:
*the antenna.
*the preamplifiers which must be the closest possible from the antenna
*le receiver strictly speaking.
*a conditioning system of the signal which makes the interface, adapts the signal received
beetween the receiver and the recording system.
* The recording system.
Let us detail a little:
*THE ANTENNA: indeed on it depends the frequency which you receive, if you want to receive
the sun in vhf on the tv band 160-220 mhz, an tv antenna is correct, in uhf on the band
460-860 mhz also (indicate that 604 mhz is an official frequence radio astronomy)
If you want to listen to Jupiter it is near 20mhz-25mhz, a dipole is often enough.
If you want to work on higher frequencies a parabola is indispensable. On the 11ghz band
a parabola of 1m diameter with a common lnb allows to receive the sun, but
as we said it, on these frequencies there is little or no bursts. You can often find
parabolae of big sizes sometimes a little dented, at scrap metal dealers. But it it
will be necessary to do some concrete for the pedestal and quite a lot of mechanics .
An antenna is directional in all the directions (azimuth / elevation), to begin, or
if your antenna is too important, you can try a meridian, directional frame only in
elevation, and let the sky scroll.
*THE PREAMPLIFIERS: they too depend on the frequency, one can use there vhf and uhf
tv or wide band preamplifiers, what is not advisable because their factor of noise is
high, and their total lack of selectivity risks to make that the receiver is saturated
by parasites in any kinds... It is better to envisage a preamplifier tuned on the
working frequency, the factor of noise will be better, and it will eliminate broadcasts
and parasites. On the other hand it will be necessary to find a radio ham or a club to
help you to build it. For Jupiter a tuned preamplifier is mandatory because the level
of Hertzian pollution with these frequencies. Higher in hyperfrequencies, it depends on
what you will find. Satellites front end exist on 4 ghz and 11 ghz .
*LE RECEIVER: it is evident that it determines frequency which you receive. In fact
it is not about an unique frequency, but more of a waveband, and it is necessary to
know that a receiver will be all the more sensitive as this band will be wide, but
it will be more vulnerable to interference. One always oscillates between these
problems: sensibility - protection with regard to the Hertzian pollution. And the optimal
choice depends on your local conditions. In vhf and uhf one can find aviation recievers
which are narrow band (100 khz), little sensitive (although!!!) But being protected
well against the disturbances, or to use receivers tv who are to wide band, more
sensitive, and more vulnerable. Let us indicate that in a tv the sound channel is
more narrow band (1mhz) than the video (8 mhz) .We use there vhf receivers aviation modified.
In decametric waves , a receiver of traffic should do the business. With satellites
front heads, you can either use the decoder and modify it, or build your own receiver
following the satellite rf head. In surpluses, one can find radio links between 1,6 and
2,2 ghz notably,or radars on 1.1ghz, 2.8ghz,or 9ghz .But here you will surely need help.
Here are master words for the construction of the receivers:
* total mechanics stability: metal system stand
* thermic stability: ventilation
* electric stability: stabilized power supply, if you use a tv set, it will probably
be necessary to rebuild the power supply.
* total screening, profesionnal connectors:type N or SMA, cables feeders of very good
quality , throw away without scruple the tv plugs and tv cables , N connectors and good
coax exist in 75 OHMS. Best is to build adapters 75 ohms / 50ohms and to use everything in
50 ohms, because it is on this impedance that are constructed the test equipment. Hunt the
bad contacts, for example if you use a tv set with an old rotactor , when you found the
good frequency: please , weld contacts, idem if you use an old ham receiver ...
*leave your receivers switched on 24 hours/24, they will be stable.
*LE SYSTEM OF CONDITIONING OF SIGNAL, adaptation between receiver's and recorder. It
is necessary to know that what goes out from the receiver , includes at the same moment
the noise of the receiver and that resulting from the observed celestial body. It is so
necessary to substract of this total noise, that of the receiver so as to keep only that
of the celestial body, to integrate the obtained difference, and to amplify it before
sending it to the recorder. This is made here by some good quality operational amplifiers
(op07 , op27). We can supply you with a schematic.
*THE RECORDER: two solutions to you:
* A paper graphic recorder, which one can find in surplus. With the speeds of paper used
(here 1mm / mn) the paper roll lasts 1 month .
* A pc with a software of acquisition: it is necessary to buy the interface and the software,
and it is as well expensive as the recorder. We do not like this solution because on one hand
a pc that stops (!!!!) well then say goodbye to the measures ..., on the other hand a pc
generates interference that perturb the receiver. It is better especially at the beginning
of your experiences not add interferences with the pc.
SOME IDEAS: with a vhf set, a systeme of conditioning of signal, and a recorder one can
envisage different things: with an antenna and a preamp you can receive the sun, the bursts,
Cassiopea and the Swan, you can make a rough map of the radio sky on your frequency by
letting the sky go through your system several lasting days and by varying the rise of your
antenna every day. With two antennae and two preamp you will do interferometry, possibly
on variable base if you move your antennae. With a tv set, a good antenna, a preamp some
people received the pulsar of the Crab, it demonstrates itself on the screen by an oblique
strip, result of the bips of the pulsar. A receiver tv in uhf, in the top of the uhf
band(860mhz) should be able to make a good receiver following a 11ghz front head .
Its output goes from 1ghz to 2 ghz, theorically , but surely much more. All this to
show to you what is accessible in a simple way...
6) How much that cost?
It depends.
If you want to use a receiver tv, it is in any well stocked waste reception center!!
An antenna tv are sold 30£, preamplifiers with weak noise costs the same price , you
should be able to build the electronics of the system of treatment of signal for 50£ by
using very good components (stability), and the simple graphic recorder should be for
50£-100£ from laboratories. Please , go to make a visit in the faculties of sciences,
heaps of things are there and ask only to get them free for you or only a bottle of
wine or champagne !!!!!. And so we are equipped, including for the parabola of
3.3 metres and it costed nothing or almost.
7) It is not a question, it is an advice: please , make use of help.
We hope that this small FAQ was able to help you .
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