Antenna impedance measurements in a magnetized plasma. II. Dipole antenna

Abstract

This paper presents experimental impedance measurements of a dipole antenna immersed in a magnetized plasma. The impedance was derived from the magnitude and phase of the reflected power using a network analyzer over a frequency range of 1MHz–1GHz. The plasma density was varied between 107and1010cm−3 in weakly (ωce<ωpe) and strongly (ωce>ωpe) magnetized plasmas in the Space Physics Simulation Chamber at the Naval Research Laboratory. Over this range of plasma conditions the wavelength in the plasma varies from the short dipole limit (λ≫L) to the long dipole limit (λ∼L). As with previous impedance measurements, there are two resonant frequencies observed as frequencies where the impedance of the antenna is real. Measurements have indicated that in the short dipole limit the majority of the power deposition takes place at the lower resonance frequency which lies between the cyclotron frequency and the upper hybrid frequency. These measured curves agree very well with the analytic theory for a short dipole in a magnetoplasma. In the long dipole regime, in addition to the short dipole effects still being present, there is resonant energy deposition which peaks at much higher frequencies and correlates to 1∕2 and 3∕2 wavelength dipole resonances. The wavelengths in the plasma predicted by these resonances are consistent with the antenna radiating R and L-waves.