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Introduction Ultra-wideband (UWB) communication systemshave the promise of very high bandwidth reduced fading from multipath low power requirements. The main concept behind UWB radio systems isthat they transmit pulses of very short duration, asopposed to traditional communication schemes,which send sinusoidal waves The role that UWB antennas play in all of this isthat they have to be able to transmit these pulsesas accurately and efficiently as possible.
UWB Frequency-Domain SignalLink Characterization The transmit antenna is excited with a continuous wave signalwith the frequency f . The relevant parameters for the frequency-domain linkdescription are: AMPLITUDE OF TRANSMITTED SIGNALUTX(F) IN [V]; amplitude of receive signal Urx(f) in [V]; radiated field strength at a position r ETX. transfer function of the transmit antenna. transfer function of the received antenna. characteristic transmit antenna impedance. characteristic receive antenna impedance. Antenna gain. Distance between Tx-Rx antenna.
the transmit antenna is excited with a impulse. The elements of the UWB time domain linkcharacterization are: AMPLITUDE OF TRANSMIT SIGNAL UTX(F) IN [V]; amplitude of receive signal Urx(f) in [V]; radiated field strength at a position r ETX. transfer function of the transmit antenna. transfer function of the received antenna. characteristic transmit antenna impedance. Distance between Tx-Rx antenna.UWB Time-Domain Signal LinkCharacterization
Antenna CharacterizationParameter1) Peak Value of theEnvelope: The peak valueof the analytic envelopeh+(t) is a measure for themaximal value of thestrongest peak of theantenna’s time-domaintransient response envelope.2) Gain in FrequencyDomain: The gain infrequency domain is definedlike in narrow-band systems.
Antenna CharacterizationParameter (Contd.)3)Envelope Width: The envelope width describes thebroadening of the radiated impulse and is defined as the widthof the magnitude of the analytic envelope at half maximum(FWHM).4) Ringing: The ringing Tt of a UWB antenna is undesired andusually caused by resonances due to energy storage or multiplereflections in the antenna. It results in oscillations of theradiated pulse after the main peak.5) Transient Gain: The transient gain gT is an integral qualitymeasure that characterizes the ability of an antenna to radiatethe power of a given waveform uTx6) Group Delay: The group delay of an antenna characterizesthe frequency dependence of the time delay.
UWB ANTENNA PRINCIPLESThe ultra-wide bandwidth radiation is basedon a few principles:1. traveling-wave structures2. frequency-independent antennas3. self-complementary antennas4. multiple resonance antenna5. electrically small antennas.
Traveling-Wave Antennas Traveling-wave antennasoffer for the guided wavea smooth, almost notrecognizable transitionwith the fieldsaccelerated to free-spacepropagation speed co Typical antennas aretapered wave guideantenna e.g horn antenna,Vivaldi antennas Aperture coupled Vivaldiantenna. (Left) Top view;(right)bottom view with feedline.
Frequency-Independent Antennas If wavelength and size ofantenna is scaled by samefactor than radiation patternwill remain same this typeof antenna are calledfrequency independent.
Self-Complementary Antennas It is an arbitrary shapedantenna Antenna constitute half ofan infinitely extendedplanar-sheet conductor suchthat the shape of itscomplementary structure isexactly identical with thatof the originalstructure with two terminalsfor the simplest case. The self-complementaryantenna has constant inputimpedance independent ofthe source frequency andthe shape of the structure.Truncated fractal antenna toshow the principle of self-complementary antennas.
Self-Complementary Antennas(Contd.) Type of self complementary antenna depends on numbers ofterminal, number of reference plane and others They also have constant-impedance property independent ofthe source frequency and the shape of the structure forrespective classes of structures with various grades ofcomplexity. This general principle is called “Principle of Self-Complementarity”. (This is also called “Mushiake Principle”on the Internet.) This principle is applicable also to thestructures other than antennas
Multiple Resonance antenna Multiple resonance antennasare combinations ofmultiple, narrow-band,radiating elements. Each element for example, adipole covers a limitedbandwidth, e.g., 20% of thetotal UWB bandwidth.Typical candidates are theLog-Per and fractalantennas.Log-Per antenna with a coaxialconnector feeding the inner triplateline.
Electrically Small Antennas
Conclusion Ultra-wide-band as an emerging technology requires for theantenna characterization a thorough knowledge of thebehaviour in time domain, in frequency domain, and, incertain cases, in the spatial domain. It has been shown that forultra-wide-band, certain antenna classes can be definedaccording to their radiating characteristics.