Antenna elements

Fig. 9 Schematic representation of the scintillation proximity assay of (S)-propanolol using imprinted microspheres. The light green area represents the aromatic antenna element, (a) The bound, tritium-labeled (S)-propranolol triggers the scintillator to generate a fluorescent light, (b) When the tritium-labeled (S)-propranolol is displaced by the unlabeled (S)-propranolol, it is too far away from the scintillator antenna to transfer efficiently the radiation energy therefore, no fluorescence can be generated (as described in [62])...

A cylindrical prototype was developed at PNNL that operates from 27-33 GHz using a vertically oriented linear array, as shown in Fig. 9. This system uses switching and transceiver technology similar to the prototype shown in Fig. 4. However, this system is composed of 384 antenna elements versus 128 for the previous system. This system collects a 360° scan in 6-10s. Image reconstruction is then performed using a conventional PC coupled to an array of high-speed co-processors. Example cylindrical images collected with a laboratory transceiver and scanner are shown in the lower half... 

Radio antenna element Ceramic seal and indifferent electrode plate... 

Radio antenna element Feed-through Electronics... 

One of the attractive features of the Opto-VLSI-based tunable time delay architecture is its ability to generate multiple RF delays without the need for RF splitters. Furthermore, the amplitude weight of each generated RF delay sample can simultaneously be controlled. This architecture offers excellent flexibility in applications such as phased-array null steering because multiple true-time RF delays for each antenna element can simultaneously be synthesized using computer generated holograms. 

Note that the number of multiple time delays that can be generated simultaneously depend on (i) the spectral width of the ASE source, (ii) the maximum delay time (i.e., the maximum wavebands separation), and (iii) the size of the active window of the Opto-VLSI processor. The larger the size of the active window of the Opto-VLSI processor, the larger the number of time delays that can be generated. On the other hand, the more wavebands are required (more nulls), the smaller the maximum waveband separation that could be achieved, thus limiting the null angle which can be synthesized. Note, however, that for a certain number of nulls, the required number of wavebands for each antenna element is fixed. In this case the ASE source should have sufficient spectral width to ensure the synthesis of arbitrary null angles. 

Photonics-based broadband phased-array antenna beamformers have been extensively investigated over the last decade for applications ranging from modern microwave radar to wireless communication systems. A broadband phased-array antenna requires the generation of variable true-time delays at each antenna element to realize beam or null steering. Several approaches have been adopted to realise tunable true-time delay units, including the use of in-... 

Broadband null-steering beamformers are much more difficult to realise than beam-steering beamformers. Theoretical analysis of broadband null steering of phased-array antennas shows multiple variable true-time delays are needed for each antenna element, while only one variable true-time delay for an antenna element is required for broadband beam steering. An N-element smart antenna can synthesise (N-1) nulls only, and this requires the beamformer to simultaneously generate (2n-1-1) delayed versions of the RF signal received by the antenna. 

Generally, for an N-element broadband phased array, the synthesis of (N-1) broadband nulls can be achieved if the beamformer of the antenna can adaptively generate and combine (2 -1 -1) delayed versions of the RF signals received by the antenna elements, as illustrated in Fig. 10(b). 

A variety of phased array systems have been fabricated based on polsrmeric electrooptic materials (141,142). One configuration is based on the photonic phase shifter shown in Figure 10. This provides a very linear phase shift as a function of control d-c voltage. Optical signals of controlled phase are thus sent to various radiating antenna elements. The optical signals are converted to radiofrequency signals by diode detectors. 

The efficiency of an antenna element depends on the radiation resistance Rmd 3nd the ohmic resistance R. If a dipole, for instance, has peak current Jin at the feed point, then the radiated power is I /lR i, whereas the ohmic power loss is dependent on the current values at the various positions along the antenna wire. The ohmic power loss per unit length of the wire is the given by... 

Isotropic source A fictitious antenna element that radiates equal intensity in all directions. This source is usually the reference antenna element to which gain or directivity of an actual antenna is compared to. 

Generation and Detection of the Terahertz Radiation Using a Photoconductive Antenna Element... 

Several different types of wide-band terahertz-radiation generation element are available these include a photoconductive antenna element, a nonlinear optical effective element (NOE), and a surface outgoing radiational-type semiconductor device. In this subsection, brief descriptions of the most commonly used PCAs are given. 

Figure 19.3 Schematic illustration of the photoconductive antenna element (PCA). For details, see text.

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