Backfire Radiation from a Monofilar Helix with a Small Ground Plane

Numerical analysis of a helical antenna is made in the presence of a finite-grid ground plane. It is found that the transition from forward-fire radiation to backfire radiation occurs as the ground-plane diameter is reduced to the order of the helix diameter. The investigation of the backfire helix with a small ground plane shows that a circularly polarized wave is radiated over a frequency range of more than 1 to 1.3. An application of the backfire helix to primary feed for a paraboloidal reflector is also described, in which the far-field pattern is evaluated by the physical-optics approximatio

A Monofilar Spiral Antenna Excited Through a Helical Wire

The radiation characteristics of a monofilar spiral antenna excited through a helical wire are investigated using the method of moments. The spiral has a circumference of more than two wavelengths (2.3λ6, where λ6 is the wavelength at a test frequency of 6 GHz) to obtain a tilted beam. An input impedance of approximately 50 Ω is realized by appropriately selecting the diameter and pitch angle of the helical wire. The antenna shows a gain of approximately 8 dB relative to an isotropic source in the maximum beam direction (18° from the direction normal to the antenna plane) and a VSWR frequency bandwidth of approximately 12%

An Inverted FL Antenna for Dual-Frequency Operation

An inverted FL antenna (InvFLA) is analyzed to obtain dual-frequency operation at 2.45 and 5.2 GHz (wireless LAN system frequencies). The InvFLA is composed of inverted FL elements, a parasitic element, and a ground plate, where these lie in the same plane, i.e., the structure is a card-type structure having a co-planar ground plate. The antenna height above the ground plate is very small:$5.5~ mm=0.045$wavelength at 2.45 GHz. The analysis shows that the InvFLA has a 4.1% bandwidth around 2.45 GHz and a 31.8% bandwidth around 5.2 GHz, both for a$VSWR=2$criterion. The gain is calculated to be 0.9 dBi at 2.45 GHz and 1.7 dBi at 5.2 GHz, with a small gain variation in each of the VSWR bands

Circularly Polarized Conical Beam Formation by Backfire Helical Antennas

A backfire helical antenna (BHA) is numerically analyzed to form a circularly polarized conical beam. The conical beam whose radiation is null in the helical axis direction is obtained using the second mode operation of the BHA. The characteristics of monofilar, bifilar and quadrifilar BHA\u27s are compared and discussed. It is found that a second-mode bifilar BHA radiates a circularly polarized conical beam over a frequency range of 1 : 1.4. The use of the first mode of a bifilar BHA is also investigated to form a quasi-conical beam which has the weak radiation in the helical axis direction

A Spiral Antenna Backed by a Conducting Plane Reflector

An Archimedean planar spiral antenna is numerically analyzed in the presence of a conducting plane reflector. The analysis shows that the spiral antenna backed by the plane reflector has two distinct regions in the current distribution, which explain the radiation of a circularly polarized wave for the outer circumferenceCranging over about1.3 lambda 2.9 lambda, wherelambdais a free-space wavelength. Further consideration is given to a truncated spiral antenna whose outer circumference is on the order of1.4 lambda. The truncated spiral antenna maintains a decaying current distribution and radiates a circularly polarized wave over a 1:1.2 frequency bandwidth. It is also demonstrated that a power gain on the order of 8.5 dB is realized over the same frequency rang