What frequencies can I use to hear Jupiter?

Jupiter emits radio signals from just below 40 MHz down to a few kilohertz.  Actually, the planet may be detected at higher frequencies with very large radiotelescopes, but those emissions are not the ones we are interested in here. The radio noise storms of interest can be heard from about 15 MHz up to a practical limit of about 38 MHz.  Below 15 MHz the signals are severely attenuated or refracted away by the Earth's ionosphere. At the upper limit the strength of the signals tapers off rapidly.  The emissions we can hear are often referred to as decametric noise storms, because the waves are tens of meters long. Okay, it is possible to hear Jupiter from 15 to 38 MHz, but what are the optimal frequencies?  The consensus seems to be that 18 MHz up to about 28 MHz is a good place to listen.  A good rule would be to pick the lowest frequency in this range which was not being hindered by ionospheric refraction.  Unfortunately, during times of high sunspot activity, (expected to peak in the year 2000), the ionosphere can remain excited all night long.  One way to judge if the ionosphere will get in your way is to check a ham band just below where you intend to listen.  If you hear amateur radio signals coming in from distant places on the ham band, then you know that the ionosphere is reflective enough that it is bouncing those signals. If the ionosphere can keep the amateur radio signal under its blanket in this way, then it may be bouncing away the signals from Jupiter, and you may not hear them. (Though this is not a hard and fast rule!)  The amount of bending that the ionosphere imparts to a radio wave is inversely proportional to the frequency of the signal. Thus, you can often escape the ionospheric effect by going higher in frequency. Understanding the behavior of the ionosphere is then crucial to your success. You should understand that the ionosphere changes in behavior throughout the day, and is usually much more active while the Sun is above the horizon.

As a practical matter, you will have to pick only as many frequencies to monitor as your equipment will allow. The real limitation is the range of frequencies over which most antennas will operate effectively. A good directive antenna is usually only good for a range of about 500 kHz or so. There are some exceptions, log periodic yagi arrays, and arrays of conical helices are quite broad banded but are not commonly owned by amateurs because of their size. You will probably settle on one or two frequencies and build the best antenna you can for those. 18 MHz and 24 MHz might be good choices.