Separate Antennas Research Articles

Dynamic quenching of porphyrin triplet states by two-photon absorbing dyes: Towards two-photon-enhanced oxygen nanosensors

Two-photon-enhanced dendritic nanoprobes are being developed for two-photon (2P) laser scanning microscopy of oxygen [R.P. Briñas, T. Troxler, R.M. Hochstrasser, S.A. Vinogradov, J. Am. Chem. Soc. 127 (2005) 11851–11862]. In these molecular constructs, phosphorescence of metalloporphyrins is coupled to two-photon absorption (2PA) of electronically separate antenna dyes via intramolecular Förster-type resonance energy transfer (FRET). In the originally developed probes, competing electron transfer (ET) between the antennae and the long-lived triplet states of metalloporphyrins partially quenched the phosphorescence, reducing the probe's sensitivity and dynamic range. The rate of such ET can be reduced by tuning the redox potentials of the chromophores. In order to identify the optimal metalloporphyrin-2P antenna pairs, we performed screening of several phosphorescent Pt porphyrins (FRET acceptors) and 2P dyes (FRET donors) using dynamic quenching of phosphorescence. Phosphorescence lifetimes of Pt porphyrins were measured as a function of the dye concentration in organic solutions. The obtained Stern–Volmer quenching constants were correlated with the corresponding ET driving forces (Δ G ET), calculated using the Rehm–Weller equation. FRET pairs with minimal quenching rates were identified. The developed approach allows convenient screening of candidate-compounds for covalent assembly of 2P-enhanced triplet nanodevices. Systematic electrochemical measurements in a series of Pt porphyrins with varying peripheral substitution and conjugation pathways are presented.

Nested PIFAs for Dual Mode of Operation: GPS and Global Communications

This letter demonstrates how two planar inverted F antennas (PIFAs) can be combined in a nested configuration. A nested antenna results from integrating two antennas into one without modifying the dimensions of the original antennas. The nested antenna has two feed inputs for two different frequencies of operation and share a common ground plane. This letter will discuss the design of nested PIFAs with frequencies of operation at both global positioning system (GPS; 1.575 GHz) and global ( 2.45 GHz ) modes of communication. Two separate antennas were first individually designed for the respective modes of operation. The antennas were then integrated into a single nested antenna. The integration affects the radiation patterns and the frequencies of operation. This letter discusses the performance of the individual antennas and the antennas in the nested configuration.

On refining the name of one type of horn antennas

Advanced communications systems operating in the microwave band involve a wide use of horn antennas of different modifications. Horn antennas are applied as separate antennas, as feeds of reflectors, and elements of antenna arrays. However, the horn antennas most frequently used in practice and conventionally called pyramidal horns, in the view of the authors can be more accurately called wedge-shaped or wedge-like horn antennas: wedge is a more general variant of polyhedron, which is the basic component for designing the geometry of such horn antennas.

Link Envelope Correlation in the Backscatter Channel

High-frequency backscatter radio systems operate in the dyadic backscatter channel, a pinhole channel whose envelope probability density function and bit-error-rate performance are strongly affected by link envelope correlation - the envelope correlation between the forward and backscatter links of the dyadic backscatter channel. This paper shows that link envelope correlation is most detrimental for backscatter radio systems using co-located reader transmitter and receiver antennas and a single RF transponder antenna. It is shown that using separate reader antennas and multiple RF transponder antennas will decrease link envelope correlation effects and a near maximum bit-error-rate can be achieved with link envelope correlation less than 0.6.

Measuring the effect of helicopter rotors on GPS reception

AbstractThis paper describes an experiment which was performed using an offshore transport helicopter to investigate the impact of the rotor blades upon Global Positioning System (GPS) reception.The test aircraft was fitted with two separate GPS antennas which were positioned to isolate the effects caused by the main and tail rotors. Testing was undertaken with the aircraft on the ground and this allowed an assessment to be performed at different rotor speeds with accurate control over the relative geometry between the antenna, rotors and satellites.Recorded data from a measurement system incorporating three dissimilar GPS receivers (including a technical standard order (TSO)-C129() compliant aviation unit and a custom research receiver) was analysed to identify the effect of the rotors at the correlator level and to determine the impacts upon ranging accuracy, the availability of ranging measurements, and the receivers’ signal level estimates.Correlation data was used to demonstrate that the rotor blades were capable of generating both destructive and constructive interference effects, and the periodic nature of these oscillations was shown to correspond directly to the blade passing frequency. It was identified that signal degradation was not limited to satellite signal paths which intersected the rotor discs.No evidence was found for any increase in code measurement error due to the rotor interference, but it was demonstrated that there could be a significant impact upon a receiver’s ability to maintain continuous tracking of the satellite signals. The overall effect of this availability problem for a given installation and type of operation will be dependent upon satellite geometry and other factors which are beyond the scope of this study.The ability of a receiver to identify the presence of rotor interference was investigated by examining estimates of carrier-to-noise, and this revealed inconsistencies between the results from different receivers implying differences in the estimation algorithms employed. It was also identified that two alternative ‘textbook’ estimators do not give identical results in the presence of rotor interference and it is suggested that such data should therefore be interpreted with caution.

Stagger procedure to extend the frequency modulated interrupted continuous wave technique to high resolution radars

Frequency modulated continuous wave (FMCW) radars offer many advantages such as low peak power, low probability of interception, low interference to other systems and high range resolution. However, their major drawback is the isolation required between transmitter and receiver that typically leads to the use of two separate antennas for transmission and reception. Some schemes have been proposed to enable FMCW radars to operate with a single antenna. Frequency modulated interrupted continuous wave (FMICW) is a possible solution. However, some considerations must be taken into account before implementing this technique in high resolution radars because of the amount of bandwidth needed. The problems associated with the use of the FMICW technique in broadband radars are analysed and a staggering procedure is proposed to overcome them. This technique is tested using simulated data from a 1 GHz bandwidth millimetre wave sensor, currently under development. The results are compared against other single antenna configurations and the advantages of the staggering technique are shown.

The UWB indoor channel: large and small scale modeling

In this paper we examine large and small scale channel modeling for ultra-wideband signals. Specifically, we examine the impact of UWB signals on traditional channel modeling techniques. This examination is based on measurements taken in an indoor office environment as part of the DARPA NETEX Program. Our findings suggest that while traditional channel modeling approaches are appropriate, care must be taken in both applying the results and interpreting them. In particular, the wide bandwidths do not invalidate the traditional exponential decay versus distance approach to path loss, although it is important to separate antenna effects from path effects. Additionally, the traditional small scale approach which uses the time-invariant linear filter to represent the channel impulse response can still be applied to UWB signals. However, one must use the model with caution as "paths" from the model may no longer represent physical transmission paths but rather may simply represent frequency distortion of individual pulses. We also examine the path loss exponents obtained from measurements and compare them to previous measurement campaigns. Finally, small scale statistics such as delay spread and the energy per "path" are also examined and compared to previous results

Phylogenetic Analyses of the Core Antenna Domain: Investigatingthe Origin of Photosystem I

Phototrophy, the conversion of light to biochemical energy, occurs throughout the Bacteria and plants, however, debate continues over how different phototrophic mechanisms and the bacteria that contain them are related. There are two types of phototrophic mechanisms in the Bacteria: reaction center type 1 (RC1) has core and core antenna domains that are parts of a single polypeptide, whereas reaction center type 2 (RC2) is composed of short core proteins without antenna domains. In cyanobacteria, RC2 is associated with separate core antenna proteins that are homologous to the core antenna domains of RC1. We reconstructed evolutionary relationships among phototrophic mechanisms based on a phylogeny of core antenna domains/proteins. Core antenna domains of 46 polypeptides were aligned, including the RC1 core proteins of heliobacteria, green sulfur bacteria, and photosystem I (PSI) of cyanobacteria and plastids, plus core antenna proteins of photosystem II (PSII) from cyanobacteria and plastids. Maximum likelihood, parsimony, and neighbor joining methods all supported a single phylogeny in which PSII core antenna proteins (PsbC, PsbB) arose within the cyanobacteria from duplications of the RC1-associated core antenna domains and accessory antenna proteins (IsiA, PcbA, PcbC) arose from duplications of PsbB. The data indicate an evolutionary history of RC1 in which an initially homodimeric reaction center was vertically transmitted to green sulfur bacteria, heliobacteria, and an ancestor of cyanobacteria. A heterodimeric RC1 (=PSI) then arose within the cyanobacterial lineage. In this scenario, the current diversity of core antenna domains/proteins is explained without a need to invoke horizontal transfer.

A study on double resonance H‐type slot antenna

AbstractMobile communication terminals such as mobile computers will enhance their usefulness by using wireless LAN (2.4 and 5.2 GHz) and Bluetooth (2.4 GHz). However, if both bands are used, separate antennas are needed because the frequencies are different. It is not advantageous to install separate antennas. In this paper, an H‐type slot antenna is proposed as a dual‐frequency antenna that can be integrated with a mobile computer without any extrusion from the computer body. By changing the parameters of the H‐type slot antenna, each resonant frequency can be varied independently so that a design can be straightforward. In a model packaged with an A4 Notebook PC, it is found that the bandwidth for VSWR ≤ 2 is about 22% in the 2.4‐GHz band and about 16% in the 5.2‐GHz band. Regarding the radiation characteristics, it is demonstrated that the average gain is about −4 dBi for conceivable antenna use angle that is practically sufficient. © 2004 Wiley Periodicals, Inc. Electron Comm Jpn Pt 1, 88(2): 11–20, 2005; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ecja.20148

On Usefulness of Multipath Diversity for Multiple Antenna OFDM Systems

Orthogonal Frequency Division Multiplexing (OFDM) and Multiple Input Multiple Output (MIMO) technology hold much promise in realizing broadband wireless systems which are both power efficient and bandwidth efficient. This review article focuses on MIMO-OFDM wireless systems, and the usefulness of multipath diversity in such systems are addressed. With increasing data rate requirements for wireless access, the technology challenges of the physical layer designers are moving towards wideband wireless communications. One of the key issue to be confronted in such systems is the frequency selective fading of the wireless channel due to multipath propagation. Orthogonal frequency division multiplexing helps in transforming this frequency selective channel into multiple narrowband flat sub-channels which facilitate a computationally efficient equalization. Usually multipath delay spread is considered as an annoyance factor. However, such channels are rich in multipath diversity, which could potentially be exploited to yield performance benefits. Using simple frequency domain processing in OFDM systems with closely spaced antennas, this multipath diversity could be exploited to achieve performance close to that of space diversity systems using well separated antennas. With such closely spaced antennas, it appears as though there is sampling in the spatial domain. Linear dispersion codes are capacity achieving space-time codes designed by optimizing the information-theoretic capacity function. Multipath diversity available as frequency diversity could be exploited by linear dispersion codes when the dispersion is over OFDM sub-channels in frequency and space. In this article, we also review such space-frequency linear dispersion codes which are particularly useful at low rate and for low complexity systems.

Small printed integrated inverted‐F antenna for Bluetooth application

AbstractA printed integrated inverted‐F antenna is introduced which operates in the Bluetooth frequency band (2.4–2.485 GHz). The proposed antenna is a part of the radio frequency printed circuit board (PCB) of a cellular telephone or a personal digital assistant (PDA). The antenna being an integral part of the PCB reduces the cost and complexity of designing and manufacturing a separate antenna. The maximum dimension of the antenna on standard FR4 substrate is 25 × 5 × 1 mm3. The antenna operates within 2:1 voltage standing wave ratio (VSWR) and has a free‐space peak gain of 1.5 dBi. © 2002 Wiley Periodicals, Inc. Microwave Opt Technol Lett 33: 347–349, 2002; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.10317

Antenna coupled planar arrays of Josephson junctions

A novel approach to synchronization of arrays of Josephson junctions (JJA) is proposed. Each Josephson junction is placed in the center of a separate dipole antenna. Antennas at a distance of about a half wavelength from each other form a beam pattern with the main lobe perpendicular to the substrate plane. Two types of JJA were fabricated on YZrO 2 and sapphire bicrystal substrates. Self-resonant steps are clearly observed in I– V curves at temperatures 4–60 K. Besides the main resonance at 360 GHz, resonances at voltages corresponding to the second and third harmonics and to subharmonics at 1/2, 3/2, 5/2 of the main resonant frequency are observed. Radiation from such array was observed at bias voltages over 4 mV that corresponds to an oscillation frequency of up to 2 THz. The arrays demonstrated high sensitivity to submm-wavelength radiation in the frequency range 300–550 GHz.

Estimating stochastic gravitational wave backgrounds with the Sagnac calibration

Armstrong et al. have recently presented new ways of combining signals to precisely cancel laser frequency noise in spaceborne interferometric gravitational wave detectors such as LISA. One of these combinations, which we will call the ``symmetrized Sagnac observable,'' is much less sensitive to external signals at low frequencies than other combinations, and thus can be used to determine the instrumental noise level. We note here that this calibration of the instrumental noise permits smoothed versions of the power spectral density of stochastic gravitational wave backgrounds to be determined with considerably higher accuracy than earlier estimates, at frequencies where one type of noise strongly dominates and is not substantially correlated between the six main signals generated by the antenna. We illustrate this technique by analyzing simple estimators of gravitational wave background power, and show that the instrumental sensitivity to broad-band backgrounds at some frequencies can be improved by a significant factor of as much as $(f\ensuremath{\tau}{/2)}^{1/2}$ in spectral density ${h}_{\mathrm{rms}}^{2}$ over the standard method, where f denotes frequency and $\ensuremath{\tau}$ denotes integration time, comparable to that which would be achieved by cross-correlating two separate antennas. The applications of this approach to studies of astrophysical gravitational wave backgrounds generated after recombination and to searches for a possible primordial background are discussed. With appropriate mission design, this technique allows an estimate of the cosmological background from extragalactic white dwarf binaries and will enable LISA to reach the astrophysical confusion noise of compact binaries from about 0.1 mHz to about 20 mHz. In a smaller-baseline follow-on mission, the technique allows several orders of magnitude improvement in sensitivity to primordial backgrounds up to about 1 Hz.

An Assembled Digital Doppler Sodar and Its Application to the Tibetan Plateau Experiment (Tipex)

AbstractA new model for the Doppler sodar antenna was designed and implemented in order to make field experiments it move convenient. The newly assembled digital Doppler sodar is introduced in this paper. The results of comparison experiment illustrated that the wind speed and direction detected by the newly designed sodar are well consistent with those detected by tethersonde, and its functions are equivalent to those of Doppler sodar with a separated antenna previously developed in China and other countries. The newly designed Doppler sodar performed continuous detection for more than 40 days in Tibetan plateau experiment. The detecting results of the Doppler sodar showed that the upward plume motions exist in Dangxiong area, Tibet and that their vertical velocities can reach about 1m/s.

Multipath effects on the efficiency of interference compensation in VHF adaptive antenna systems

We consider the characteristics of signals in a multipath VHF communication channel which affect the efficiency of spatio-temporal processing of those signals. We present the results of experimental measurements of the efficiency of interference compensation with frequency-shift modulation under controlled conditions. Using the obtained data on various characteristics of the interference propagation channel, we analyze the factors decreasing the degree of compensation for such interference. We show that the multipath nature of the radio channel is the main factor leading to frequency and spatial selective fadings and uncorrelated distortions in the shape of the signal received by separated antennas.

A novel scheme for reception using an active microstrip antenna

An unique aspect of the injection-locked active microstrip antenna incorporating a Gunn diode, i.e., the variation of the device current within the lock band, is exploited for the demodulation of an FM microwave signal. A simple experimental arrangement has been presented to demonstrate the demodulation performance of the receiver even in a multichannel environment. Using two such separate antennas as transmitter and receiver, a two-way short-range communication system has been demonstrated that may be suited for certain commercial and military applications.

A New Approach to Interference Excision in Radio Astronomy: Real-Time Adaptive Cancellation

Every year, an increasing amount of radio-frequency (RF) spectrum in the VHF, UHF, and microwave bands is being utilized to support new commercial and military ventures, and all have the potential to interfere with radio astronomy observations. Such services already cause problems for radio astronomy even in very remote observing sites, and the potential for this form of light pollution to grow is alarming. Preventive measures to eliminate interference through FCC legislation and ITU agreements can be effective; however, many times this approach is inadequate and interference excision at the receiver is necessary. Conventional techniques such as RF filters, RF shielding, and postprocessing of data have been only somewhat successful, but none has been sufficient. Adaptive interference cancellation is a real-time approach to interference excision that has not been used before in radio astronomy. We describe here, for the first time, adaptive interference cancellation in the context of radio astronomy instrumentation, and we present initial results for our prototype receiver. In the 1960s, analog adaptive interference cancelers were developed that obtain a high degree of cancellation in problems of radio communications and radar. However, analog systems lack the dynamic range, noised performance, and versatility required by radio astronomy. The concept of digital adaptive interference cancellation was introduced in the mid-1960s as a way to reduce unwanted noise in low-frequency (audio) systems. Examples of such systems include the canceling of maternal ECG in fetal electrocardiography and the reduction of engine noise in the passenger compartments of automobiles. These audio-frequency applications require bandwidths of only a few tens of kilohertz. Only recently has high-speed digital filter technology made high dynamic range adaptive canceling possible in a bandwidth as large as a few megahertz, finally opening the door to application in radio astronomy. We have built a prototype adaptive canceler that consists of two receivers: the primary channel (input from the main beam of the telescope) and a separate reference channel. The primary channel receives the desired astronomical signal corrupted by RFI (radio-frequency interference) coming in the sidelobes of the main beam. A separate reference antenna is designed to receive only the RFI. The reference channel input is processed using a digital adaptive filter and then subtracted from the primary channel input, producing the system output. The weighting coefficients of the digital filter are adjusted by way of an algorithm that minimizes, in a least-squares sense, the power output of the system. Through an adaptive-iterative process, the canceler locks onto the RFI, and the filter adjusts itself to minimize the effect of the RFI at the system output. We have designed the adaptive canceler with an intermediate frequency (IF) of 40 MHz. This prototype system will ultimately be functional with a variety of radio astronomy receivers in the microwave band. We have also built a prototype receiver centered at 100 MHz (in the FM broadcast band) to test the adaptive canceler with actual interferers, which are well characterized. The initial laboratory tests of the adaptive canceler are encouraging, with attenuation of strong frequency-modulated (FM) interference to 72 dB (a factor of more than 10 million), which is at the performance limit of our measurements. We also consider requirements of the system and the RFI environment for effective adaptive canceling.

Application of ERS-1 wind scatterometer data to soil frost and soil moisture monitoring in boreal forest zone

The feasibility of the ERS-1 Wind Scatterometer (WS) for monitoring the boreal forest zone is investigated, concentrating on soil frost and soil moisture monitoring. The ERS-1 WS measures the target area with coarse spatial resolution (about 50 km) using three separate antenna beams and a wide angular range. The investigations are concerned with the boreal forest zone using data (1) from test areas located in Finland and (2) covering the whole northern European boreal forest zone. The seasonal behavior of WS data is studied and a semiempirical forest backscattering model-based inversion method for the retrieval of soil moisture and for soil frost monitoring from WS data is developed. The developed inversion method employs nearly simultaneous three-beam measurements and a varying incidence angle. Promising results were obtained in the monitoring of soil frost, and the retrieval of soil moisture also appears to be a feasible field of application. The applicability of the instrument for forest biomass retrieval using single images was found to be limited to long-term change detection.

Signal correlations and diversity gain of two-beam microcell antenna

The motivation of the present study is to investigate the possibility of using two independently directed antenna beams in a mobile base station to provide diversity gain and reduce fading problems in a microcellular environment. The signal correlation of the individual antenna beams is studied, and it is shown that correlation depends on beamwidth, separation angle of the antenna beams, and the location of the base-station antenna array. Signal correlations have been computed using narrow- and wide-beam antennas in two separate base-station antenna locations. Diversity gains of both antenna types have been optimized by selecting the beam orientations which minimize the signal correlation. The numerical simulations show that a considerable diversity gain can be obtained using this approach.

Global Positioning System with an Attitude: Method for Collecting Roadway Grade and Superelevation Data

The use of a specialized Global Positioning System (GPS) to conduct high-speed surveys of roadway alignment, grade, and cross-slope characteristics is discussed. The system uses a single GPS receiver that has 24 channels monitoring four separate antennas (six channels each). The collection of attitude (heading, pitch, and roll) is made possible through the relative orientation of the antennas. By mounting the system on a road surveillance vehicle, accurate grades, superelevation, and crown measurements can be made without differentially correcting the GPS data. However, to gather precise positional data that correspond to the roadway measurements, differential correction with a GPS base station at a fixed known point is required. The design and use of this attitude GPS unit are addressed. Accuracy specifications for static testing are provided along with techniques to maximize this accuracy. Kinematic data collection is depicted for a local road and a freeway off-ramp. The use of digital terrain modeling technology provides a promising graphic database representation of the roadway characteristics.