Understanding Link Dynamics in Wireless Sensor Networks with Dynamically Steerable Directional Antennas

Abstract. By radiating the power in the direction of choice, electronicallyswitched directional (ESD) antennas can reduce network contention and avoid packet loss. There exists some ESD antennas for wireless sensor networks, but so far researchers have mainly evaluated their directionality. There are no studies regarding the link dynamics of ESD antennas, in particular not for indoor deployments and other scenarios where nodes are not necessarily in line of sight. Our long-term experiments confirm that previous findings that have demonstrated the dependence of angleof-arrival on channel frequency also hold for directional transmissions with ESD antennas. This is important for the design of protocols for wireless sensor networks with ESD antennas: the best antenna direction, i.e., the direction that leads to the highest packet reception rate and signal strength at the receiver, is not stable but varies over time and with the selected IEEE 802.15.4 channel. As this requires protocols to incorporate some form of adaptation, we present an intentionally simple and yet efficient mechanism for selecting the best antenna direction at run-time with an energy overhead below 2 % compared to standard omni-directional transmissions.

Fundamental limitations for antenna radiation efficiency

Small volume, finite conductivity, and high frequencies are major imperatives in the design of communications infrastructure. The radiation efficiency η r impacts on the optimal gain, quality factor, and bandwidth. The current efficiency limit applies to structures confined to a radian sphere ka (k is the wavenumber and a is the radius). Here, we present new fundamental limits to η r for arbitrary antenna shapes based on k 2 S , where S is the conductor surface area. For a dipole with an electrical length of 10 -5 our result is two orders of magnitude closer to the analytical solution when compared with previous bounds on the efficiency. The improved bound on η r is more accurate, more general, and easier to calculate than other limits. The efficiency of an antenna cannot be larger than the case where the surface of the antenna is peeled off and assembled into a planar sheet with area S, and a uniform current is excited along the surface of this sheet.Morteza Shahpari, David V. Thie

Measurement of surface conductivity of graphene at W-band

Graphene has excellent mechanical and conducting properties. Scattering parameter measurements were made in W‐band (75‐110 GHz) on 25 mm × 25 mm monolayer graphene film printed on polyethylene terephthalate (PET) substrate. From near field measurements using WR10 horn antennas the surface conductivity was de‐embedded after calibrating the s₁₁ data against the reflection of a copper plate of same cross‐sectional dimensions as the DUT and calibrating the s₂₁ data against horn‐to‐horn measurements. A monolayer graphene strip (20 mm × 2 mm) printed on PET substrate (25 mm × 25 mm) showed higher reflection when it was aligned with the E field of the incident radiation (as compared to H field alignment). The surface conductivity of graphene was de‐embedded from s₁₁ measurements using standard transmission line theory and from s₂₁ measurements using cascade matrix theory. The measured real part of the surface conductivity of graphene ranges between 0.7 and 1.8 mS/sq.Farhat Majeed, Thomas Fickenscher, Morteza Shahpari, David V. Thie