|Observing the Jovian Decametric
The best conditions for detecting the emission occur during the early morning hours, between midnight and 7 AM, when the terrestrial ionosphere becomes more transparent thus reducing the amount of interference. During the summer months in the northern hemisphere (June-August), the amount of interference due to terrestrial lightning storms worsen the conditions for detecting the emission. Lightning discharges produce a lot of electrical noise which is picked up by low frequency antennas either from direct propagation or by reflection in the ionosphere . The best observing conditions for detecting the emission are from October to April, provided that the planet is visible during these months.
Antennas and receivers for detecting the emission.
The Jovian decametric emission can be detected with simple antennas such as a half-wavelength dipole or other low gain antennas such as the long-wire and full wavelength loop antennas. However, such low gain antennas may allow the detection of some of the strongest bursts only. An array of two half-wavelength dipoles, separated by about one half wavelength can provide enough gain to detect most of the storms and is relatively simple to build. Antennas with gains of 6-10 dB with respect to a half wavelength dipole are more suitable for the systematic detection and study of the emission. Five-element Yagi and dipole log periodic antennas usually have gains in this range and have been traditionally used for this purpose. These higher gain antennas connected to a short wave amateur radio receiver can easily detect most of the strong part of the Jovian decametric radio emission. All these antennas, including the simple half- wavelength dipole are directional. Jupiter must be within the beam of the antenna in order to receive the emission.
Most amateur short wave radio receivers have relatively narrow passbands and adequate noise figure and can be used for detecting the emission. The relative narrow band ( usually around 6 kHz) of these receivers will help in tuning away from radio stations. The receiver noise is usually just a fraction of the galactic background noise temperature. In order not to compress the output of the receiver which will limit the ability of detecting the emission, it is necessary to turn off or disable the AGC (automatic gain control) of the receiver.
An observing frequency between 18-24 MHz is recommended. At frequencies below 18 MHz strong interference from stations and lightning discharges is expected, which reduces the chances of detecting the emission. At frequencies higher than 24 MHz, the probabilities of detection drop sharply because of the drop in intensity of the emission (see histogram of occurrence probability).
A technical note.
The intensity of the emission can be more specifically expressed in terms of the parameter called flux density (power per unit area per unit bandwidth). As a reference, the minimum detectable flux density expected for an 8 dB gain linearly polarized antenna connected to a receiver having a 5 kHz bandwidth and a post detection time constant of 1 second is of the order of 5x10-22 wm-2 Hz-1 at a frequency of 18 MHz. Jupiter radio emissions are always received against the galactic background noise. The relatively high level of the galactic background noise is what limits the sensitivity of the system. Jovian decametric radio emission with peak flux densities in the range of 10-100x10-22 wm-2 Hz-1 are common. Expressing the flux density in Jansky (Jy), a unit more commonly used in radio astronomy, these peak flux densities are 100,000 to 1,000,000 Jy (1 Jy= 1x10-26 wm-2 Hz-1). In terms of power and voltage at the input of a receiver, 10x10-22 wm-2 Hz-1 is equivalent to a power of 1x10-9 microwatt or 0.23 microvolt over 50 Ohms.