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Technical Information

The ultimate swl dx....listening for the Jovian Decametric Emissions.
By Felix Scerri VK4FUQ

Broadband radio noise in the HF spectrum coming from the giant planet Jupiter was first detected by earthbound radio astronomers in 1955.
I remember reading about these radio emissions when I was a boy, but it was only in the last year or so that I've finally actually heard these eerie sounds from that giant planet, propagating across millions of miles of empty space coming out of my HF radio's loudspeaker .
A simple half wave dipole and a good receiver is all that is basically required to receive the radio noise. More about the specifics of reception shortly, but first, some background into what causes this radio noise.

Jupiter is an amazing celestial body. It is one of the gas giant planets (along with the planet Saturn) and orbits the sun at a mean distance of about 500,000,000 miles. It is an extremely large planet, having a diameter of about 88,800 miles. Sorry about the imperial units, they make more sense to me! It is surrounded by a large number of moons, one of which, named Io, is thought to be responsible for the broadband decametric (meaning tens of metres of radio wavelength) radio noise from Jupiter. Io is in a very close and tight orbit around Jupiter and is subject to enormous gravitational stresses as a result. The exact mechanism that produces the radio noise is still not completely understood, but it is known that interactions between the planets powerful magnetic and radiation belts and Io, results in a very large ion current flow between the moon and the planet. The active volcanoes, so beautifully photographed by the Voyager and Gallileo space probes are a very visible testament to the enormous stresses experienced by Io.
Jupiter is a very active planet in the radio and radiation sense generally, it radiates considerably more energy in total than it receives from the sun. Astrophysicists still guess at the exact mechanism driving Jupiters powerhouse. As well as the HF frequencies, the planet also emits radio noise on much higher frequencies as well. These decimetric noise emissions have been extensively studied by radio astronomers. As far as the HF decametric noise is concerned, it is worth considering that these noise bursts are easily detectable here on earth with simple antennas and receivers, it gives one an idea of the amazing power levels involved in their generation, to be audible at such a great distance. It is truly awe inspiring.
These decametric noise bursts exist between the frequencies of around 5MHz and an upper frequency of about 40MHz. The peak emission frequency range is around 8MHz.
Unfortunately, 8MHz is not a good frequency to listen to, as ionospheric attenuation is generally severe at these frequencies. The frequency range around 20MHz is considered the best for listening, due to reduced ionospheric attenuation and lack of man made interference.
Now the specifics of listening requirements. Many different antennas can be used, but as an indication of a bare bones setup that performs well, I'll mention what I use. My antenna is a simple half wave folded dipole cut for a frequency of 20MHz.
The folded dipole is the ideal antenna to use in this application because they are a little "quieter" than a standard dipole and have a wider operational bandwith, which is useful, although the noise emissions from Jupiter are very broadband. I have mine fed with a short length of 300ohm ribbon, then into a 4:1 balun, then into RG58 coax straight into my receiver. Any reasonable receiver will do, if tunable to around 20MHz. The AM mode is preferred.
With a simple dipole, don't expect S9 signals. In my experience, the signal levels are quite audible but wouldn't register on an "S" meter even if amplified many times. For this reason a quiet receiving location is a very desirable asset. I have heard of yagi's, phased arrays, and other "gain" antennas being used, but they are not strictly necessary with a quiet location.
The position of the antenna poses an interesting question, remember we are trying to receive signals from outer space, therefore we need to look "up". With a simple dipole antenna this requirement is satisfied by placing the antenna at a little lower than a half wavelength above the ground in order to produce an "upward" looking lobe. My folded dipole runs in a roughly nne-ssw line (not too critical) and is in the form of a very wide inverted V.
Now, when to listen and what does one hear(hopefully). Firstly,the planet needs to be in the sky. Night time is probably preferable, but I have successfully copied the noise bursts mid morning on several occasions. The Io events (as they are termed) occur quite frequently but not continuously. For those with access to the internet, typing the words "jovian decametric noise" on your favorite search engine, will reveal many useful sites on this subject and some provide prediction tables (just like tides!) and even prediction software. If one is feeling lucky one could just set up a receiver and listen. You could be lucky and hear the noise bursts! I did once. There is also an Astronomy net on 3.543MHz at 1200hrs UTC, Friday evenings. The subject of radio noise from Jupiter is frequently discussed there, and the members are only too willing to assist. As the frequency of 20MHz is used by the time signal station WWV, I listen just off to one side. Actually, when terrestrial propagation is poor, as evidenced by a weaker than usual WWV, this is a good time to listen for the Jovian decametric noise, as ionospheric attenuation will be lower than usual.
What do they sound like? The noise bursts most often heard are termed "S" for short bursts and" L" for long bursts. The "S" bursts have a "ssssssh" type of sound with a rapid onset and a very quick decay. They are often heard in bursts of a few at a time in quick succession. The "L" bursts are often classically described as sounding like ocean waves breaking on the shore.
In my experience, that is exactly what they sound like, a similar "ssssssh" sound, but with a slow rising/falling profile. Many of the internet sites provide sound samples for easier identification. My experience is that, once you've heard them, you'll instantly know it. They are quite distinctive. You won't hear any voice identification, CW, or digital transmission, just the audible results of the interaction between the moon Io and the giant planet Jupiter.
Sometimes during an Io event either "S" or "L" bursts are dominant, sometimes both occur during the one event. I've heard both. A truly remarkable listening experience.

73's Felix Scerri VK4FUQ.
31st March 2002


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