Determination of the Noise Floor of the WHISPER instrument on Cluster

<p>The WHISPER (Waves of HIgh frequency and Sounder for Probing of Electron density by Relaxation) instrument is part of the WEC (Wave Experiment Consortium) of the ongoing Cluster mission, launched almost 20 years ago in 2000. It uses the long double sphere electric dipole antennas of the EFW (Electric Field and Wave) instrument to measure the electric field spectra in the frequency range 2-80 kHz.<span> </span></p><p>The characteristic signatures of natural or actively triggered waves indicate the nature of the ambient plasma regime and, combined with the spacecraft position, reveals the position of key magnetospheric boundaries encountered during a specific time interval.</p><p>Most of the time, WHISPER observes natural waves in the 2-80 kHz band, using one of the two EFW antennae pairs: Ey or Ez, which can have different characteristics. After successive technical problems with the EFW sensors, the receiving antenna had to be changed on 3 of the 4 spacecraft over the years, and Cluster 1 (C1) WHISPER measurements are currently being made with only one probe (the spacecraft acts as the second probe).</p><p>In order to understand the signals observed by the WHISPER experiment, one needs to know the instrumental noise floor, around which the measurements are meaningless. It is a frequent feature of wave detectors that this noise floor is temporally static - it does not degrade with time. However, what this noise floor actually is, and how its behaviour changes around the failure of probes and switching to other probes is the purpose of this study.<span> </span></p><p>The NATURAL spectra were processed orbit by orbit, in order to avoid strong emissions from any one region, then the minimum value for the low-energy spectra was picked out for each frequency. Individually, these show a large amount of variation, but the median of 20 orbits gives a very reproducible curve showing how the noise floor varies with frequency. The periods before, during and after probe failures and configuration changes were investigated and show the limited changes to the curves and strength of interference lines.</p>

Reconfigurable Magneto-Electric Dipole Antennas for Base Stations in Modern Wireless Communication Systems

Magneto-electric (ME) dipole antennas, with the function of changing the antenna characteristics, such as frequency, polarization, or radiation patterns, are reviewed in this paper. The reconfigurability is achieved by electrically altering the states of diodes or varactors to change the surface currents distributions or reflector size of the antenna. The purpose of the designs is to obtain agile antenna characteristics together with good directive radiation performances, such as low cross-polarization level, high front-to-back ratio, and stable gain. By reconfiguring the antenna capability to support more than one wireless frequency standard, switchable polarizations, or cover tunable areas, the reconfigurable ME dipole antennas are able to switch functionality as the mission changes. Therefore, it can help increase the communication efficiency and reduce the construction cost. This shows very attractive features in base station antennas of modern wireless communication applications.