A SIMPLE DECAMETRIC RADIO TELESCOPE FOR JUPITER
by Bob Greef (all pictures courtesy of Bob except where noted otherwise)
As a keen amateur astronomer I was used to observing visually with my local astronomical society, the Breckland Astronomical Society, with the club’s 19 inch reflector or my own 70 mm refractor and 8 inch reflector. This often ended in frustration due to cloudy skies.
On joining a second local society, the Norwich Astronomical Society as well as Breckland, I became intrigued by the pair of radio dishes they had erected at the observatory. Questions revealed that a former member had obtained these dishes had erected them then moved away from the area.
I thought to myself “this is the answer to cloudy skies” and proceeded to gather information about observing at wavelengths other than visual. One of my early discoveries on Internet searches was the NASA Radio Jove Project that encourages high schools to observe the decametric emissions from Jupiter using relatively simple and reasonably cheap equipment.
I downloaded all the plans
for the antenna and receiver then sought out some assistance before
committing to the purchase of the necessary components. One of the
members of both societies is a professional radio and TV engineer and
keen amateur radio enthusiast. I had spent some time doing visual
observation and CCD imaging with him and enlisted his help.
Diagram of antenna.
Among his archives was a photocopy of an article from the December 1989 edition of Sky and Telescope entitled ‘Build a 21 MHz Jupiter Antenna’ by David Rosenthal that described the construction of a low cost antenna designed to be used with a short wave receiver to listen to Jupiter’s decametric emissions.
This looked very
attractive as a first step on the road to radio astronomy so I built
my own modified version using the Sky and Telescope plans. The
antenna was linked to a cheap multiband radio receiver via some
ordinary television co-axial cable. This was achieved by removing
most of the supplied telescopic antenna and fitting a co-axial
connector in its place.
Picture showing TV coaxial connector replacing radio's antenna.
Connection of coaxial cable to antenna.
The only things I had to
buy were the copper wire, the co-axial cable and the little multiband
radio, everything else was to hand as left overs from domestic and
garden projects. The total cost of the materials I have estimated at
well under £50 including the radio.
A radio telescope for under 50 pounds!
My first target was the
Sun and as we had just passed solar maximum there were still periodic
outbursts of sunspots, which generate massive radio emissions that
could be clearly heard via the radio loudspeaker. Swinging the
antenna across the Sun revealed the wide angle that the radio Sun
occupies in the sky.
Trace of the sun with this telescope.
This was last September
and as Jupiter became an evening object in the late autumn skies I
began to listen to the sounds of Radio Jove via my simple set up.
Trace of Jupiter with this telescope.
This is not without problems, the main one being that the 18 to 25 MHz band is crowded with terrestrial transmissions. This can get to be so bad that no observations are possible and at other times fortunately the ionosphere becomes less reflective and Radio Jove can be easily heard.
The other big problem is that Jupiter does not transmit constantly at decametric wavelengths. The orbit of the satellite Io through the magnetically created torus of matter that surrounds Jupiter creates spectacular but only sporadic bursts of emissions that are still very much the subject of study and quantification. Tables for predicting the likelihood of Jovian radio emissions may be downloaded from the University of Florida’s website.
To complete the apparatus I have recently begun to feed the sounds via the earpiece jack to a simple audiocassette recorder then play them back through the sound card of my PC. This allows me to run the emissions through Radio Sky’s Radio-Sky Pipe software to create a visual and measurable record.
This software in its basic
form is available as a free download from Radio Sky’s website.
I have included some of my better Radio-Sky Pipe traces in these
pages and they may be compared with verified traces from the Radio