THE SIMPLEST RADIO TELESCOPE...
NOTE:the next pages are very detailed from the technical point
of view,there are lots of photographs and schematics. Depending on the speed of your internet connection
it may take a rather long time to download. We are sorry but close ,hight resolution pictures are
the best way th show how things are done !!!
We receive a certain number of telephonic calls and e-mail asking how to start simply
and without knowledge in electronics. To practically illustrate
the FAQ we developed what follows based on a satellite system of reception.
Let us say that such descriptions already exist on the Internet and that our assembly does
not have any originality.This assembly can interest a beginner by his simplicity or a
club of astronomy or a professor within the framework of a manipulation in electronics.
It is very evolutive , starting with a system that can be installed in a few minutes
but with lack of stability and sensitivity ,up to a very stable and sensitive system
with a computer controlled data acquisition system .
The basic system consists in using a system of satellite reception without any modification.
Le lnb receives between 11 and 12 ghz and transforms this frequency band into a
frequency band covering 1 to 2 GHz. In the case of the satellite reception,
the precise frequency of reception is fixed by the demodulator.A' Satfinder' is
intercalled in the coaxial cable.
The Satfinder is a small tool used in the beginning to locate the satellites.
It is a wide band amplifier that receives between 1 and 2GHz and contains a detection
with a diode.
The maximum of the signal detected by the diode is indicated by the meter
that it contains and by a buzzer. There are various models, buy the least expensive...
and especially not the digital models.
The connections are immediate: assemble the lnb on the parabolic reflector , an end the
coaxial cable with "F" plug is connected with the lnb, the other end this coax is connected
with the 'F' plug of Satfinder labeled ' to lnb', to the other "F" plug of Satfinder
connect another coax whose other end will go to the demodulator. This being installed,
power on the demodulator, regulate the small button of Satfinder until the needle moves a bit
when the antenna is directed to vertical far from any obstacle.
And by maintaining the shade of the sun in the vertical axis of the parabola, seek the
sun while sweeping upwards or downwards. Attention the precision of pointing is 1 degree
with a parabola of 90 cm. You found the sun? Very well. Note the position of the needle
of the modulation meter of Satfinder.
Now direct your antenna to the bottom, towards the ground and look at:le modulation meter
indicates a very strong signal??? What did you receive? Quite simply the radiation of our
planet, or more precisely the radio radiation of thermal origin. Without knowing it
or almost you come to observe two objects : our star and our planet. With your radio telescope
look around you, the trees, people, various places of the sky .According its temperature
any physical body emits something. At this frequency (11ghz) you receive waves of approximately
3 cm, but that occurs on all the frequencies. The light radiation is the same thing,
think of the color that takes a piece of metal when you heat it: the brown one... with the red...
with the yellow... with white is similar. It is the thermal radiation of the universe.
A receiver measures the thermal radiation of all that surrounds us, it measures the remote
temperature of it. It is a thermometer.
By measuring the temperature of the ground , you carried out
a calibration of your radio telescope. Not too bad for a beginner!!!
Here are you have just entered the field of radio astronomy.
Thanks to the rather extraordinary electronic components that there is in a satellite
receiver which makes it possible to receive the emissions TV of a satellite
at a distance 36000Km, the system which you have is much more sensitive than much
radio telescopes used at the beginning of the evolution of this science 60 years ago.
It suffers however from many defects, we will try to improve it.
First of all you have noticed that in this application the demodulator is not used
for nothing that power the Satfinder, in fact it is used to feed the unit: on the
' F' plug of the demodulator there is a continuous voltage of 13-18 V which feeds the lnb
and Satfinder by the coaxial cable. The first modification will consist in opening Satfinder
and to weld two wires a black (ground) and a red (+) onto the ' F' plug marked ' to receiver',
the red is welded onto the center pin and the black on the adjacent ground . The wire is
outputed by a hole (cf images). Here are you can leave away the demodulator and replace it
by a small 15V 1A dc power suply well regulated . You can also use batteries to do
mobile radio astronomy...
The second modification will consist in reading measurement it on a measuring
apparatus more precise than the meter of Satfinder or to send it towards a system
of recording. For that locate the buzzer, there are two wires a red and a black.
The black it is the ground and the red conveys a tension proportional to the received signal.
Cut two wires by leaving one or two centimetres . Remove the buzzer, you need
some more plug BNC or Cinch with the to output the signal. Between the two
wire red and black weld two resistances of 1.2k in series (= with the continuation one
of the other). The point medium of resistances is welded with the center pin of the BNC.
The Ground of the plug is welded with the black wire. Look at the photographs...
the tension of the buzzer proportionnal to the received signal is on the BNC. With
two resistances in series we divided it by two as it is too much for many systems of
recording.
On the bnc you can use now a digital voltmeter to have a precise measurement.
The complete system now includes a parabolic reflector, a lnb,a modified satfinder ,
a voltmeter and a power supply .
The cost of this assembly is very weak. One should not hesitate to benefit from the '
promotions. Reflectors parabolic with lnb is for 50 euros, a ' satfinder' for 10 euros,
a small universal controller is for 5E. For the 15v power supply kits exist.
An evolution possible and not expensive is the digitalization of the signal: one replaces
the voltmeter by a system of acquisition which uses an old PC. This system of acquisition can
use the same power supply than the system above.
DATA ACQUISITION
This assembly presents limitations due to its simplicity. It presents primarily a lack of
stability related to several factors: lnb and satfinder not thus thermostated there is a
slow thermal drift, in the detecting satfinder with simple diode and not thermically
compensated, use of "F" plugs whose quality is not what is necessary to make radio
astronomy and general instability of Satfinder whose adjustment of gainis too precise .
Also as it is a broad band receiver it is sensitive to jammings of the satellites is
necessary that learn how to know where they are in the sky.
Here how to replace the unstable satfinders.
IMMEDIATE RESULTS: Monday 3/10/2005 we used this receiver with others (1440 MHz and 151Mhz)
to study the eclipse. It functioned well and thanks to him we could see a reduction of 50% of
flow on this frequency at the precise time of maximum eclipse. This shows what one knows :
on this frequency band what one observes in radio corresponds to the visible solar
disc in optics. The passage of the moon straightforwardly cut the beam of solar flow.
The phenomenon was very brute and fast.