UWB Syste for Tie-Doain Near-Field Antenna Measureent B. Levitas #, M. Drozdov #, I. Naidionova #, S. Jefreov #, S. Malyshev *2, A. Chizh *3 www.geozondas.co # Geozondas Ltd., 6, Shevchenko Str., LT-3, Vilnius, Lithuania levitas@geozondas.co * Lab. of Seiconductor Optoelectronics, Stepanov Institute of Physics 22, Logoiski trakt, 229, Minsk, Belarus 2 alyshev@ieee.org, 2 chizh@ieee.org Abstract An UWB syste for tie-doain near-field antenna easureents with new tie-doain data pre-processing algoriths has been proposed. UWB syste developed is suitable for planar, cylindrical, and spherical near-field antenna easureents. It is shown that near-field tie-doain easureents using developed UWB syste with optically-fed antenna probe are in a good agreeent with conventional farfield antenna easureents. I. INTRODUCTION The ain advantage of tie-doain antenna easureents is the possibility to characterize antenna in the ordinary laboratory roo without usage of anechoic chaber []. To avoid data corruption during tie-doain antenna easureents it is enough to ensure, that reflections fro surrounding objects are outside of easureent tie window. However, outdoor test ranges are usually used for large antennas, because their far field zone starts far fro aperture. Other possibility to easure such antennas is to use near-field easureent setups. Near-field easureents can be carried out not only in frequency doain, but in tie doain also [2],[3]. In this paper UWB syste for tie-doain near-field antenna easureents with new tie-doain data preprocessing algoriths and optically-fed antenna probe has been proposed. II. UWB SYSTEM In near-field tie-doain easureents a short UWB pulse is used as a test signal and digital UWB sapling oscilloscope is used as receiver (Fig. ). The signal received by sapling receiver is pre-processed to correct hardware tie drift and change of delay of tie window. After Fourier transfor of signal all well-known frequency doain algoriths can be used for further data processing [4]-[6]. There are three ain near-field easureent ethods: planar, cylindrical and spherical. The equipent for all three setups is alost the sae, naturally there is only difference in positioning devices used. For planar near-field easureents syste XY scanner is used (Fig. 2), for cylindrical easureents vertical scanner and aziuth positioner are used (Fig. 3), and for spherical easureents aziuth and elevation positioner is used (Fig. 4). The difference is that in cylindrical easureents antenna under test (AUT) is positioned on foa holder and rotated, and antenna probe is oved only along vertical axis. For spherical near-field easureents a 2D positioner with foa AUT holder and coinciding aziuth and elevation centrus of rotation was developed. Sapling oscilloscope with sapling head in tiedoain easureents is counterpart of network analyzer in frequency-doain easureents. To speed up easureents pulse repetition rate of pulse generator and sapling converter was increased up to 7 MHz. Antenna can be easured in frequency band 2 GHz. Fig.. Planar tie-doain near-field antenna easureent schee The ain advantage of the tie-doain near-field antenna easureents is the possibility to easure antennas without anechoic chaber. Moreover, the hardware is cheaper and the easureent linearity is better in coparison to the frequency-doain ethod. On the other hand, the tiedoain near-field antenna easureents have saller dynaic range and higher frequency base error in coparison to the frequency-doain ethod. One of the probles in realization of tie-doain nearfield antenna easureent schee is a hardware tie drift, which is equivalent to positioning error in near-field easureents. There are several sources of tie drift in tiedoain easureents and different ways to decrease or
Fig. 2. Planar tie-doain near-field antenna easureent setup Fig. 3. Cylindrical tie-doain near-field Fig. 4. Schee (left) and photo (rigth) of aziuth and elevation antenna easureent setup positioner used for spherical near-field antenna easureents reove it entirely. First source of tie drift is bending of cables. At least one cable (RF cable in Fig.) is not stationary during near-field scan, so easureent errors arise because of cable oveents. To eliinate such tie drift it is necessary to use phase stable RF cables or optically-fed probe antennas [7]. It is worth noting that for spherical easureents it is possible to realize easureent setup without RF cables oveents using positioner with rotary joints. To correct tie drift of pulse generator and sapling receiver an RF cable (RF cable 3 in Fig.) is added into the easureent setup. That kind of tie drift is very slow but is accuulated during long easureents. To eliinate its ipact on easureent results it is necessary to perfor preprocessing drift correction. Two pulses are generated by pulse generator without tie drift between the because of the reference pulse is a branching of ain. At the receiver there is zero drift between channels because of coon sapling pulse. Using those properties tie drift in the ain easureent channel can be corrected by control of signal position shift in reference channel. Signal phase correction proceeds through processing of the reference signal, easureent of the tie drift, and phase correction. Reference signal (V).2 -.2 -.4 -.6 -.8 -. 2 4 6 8 Fig. 5. Signals in reference channel Processing includes low pass filtering and pulse cutting. After cutting a convenient for of pulse is obtained. To have clear correlation function of two reference signals in further steps of algorith, threshold is used to obtain only part of Reference signal cut (V). -. -.2 -.3 -.4 -.5 2 4 6 8 signal as shown in Fig. 5. During near-field scan the shape of reference signal cut doesn t change. After each acquisition correlation function R of first signal cut s with current signal cut s is calculated: R t IFFT FFT s FFT s, () where FFT and IFFT denote Fast Fourier Transfor and Inverse Fast Fourier Transfor accordingly. Signal tie drift is equal to shift of correlation function R axiu. To receive resolution better than signal sapling step a parabolic approxiation of correlation function top is carried out. Phase of pulse obtained in easureent channel is corrected using calculated drift value. The result resolution is about. of sapling step. To shift the signal in easureent channel by tie lesser than sapling step its spectru in frequency doain is calculated using the following forula: i 2 n s n s n e N, n N, (2) where N is the total nuber of saple points, is tie drift calculated by eans of () for each point. Sliding windowing function is used as one of preprocessing steps. During planar and cylindrical near field scan the delay of received pulse is changing. Delay of sliding windowing function ust be changed accordingly, as shown in Fig. 6. Miniu tie window duration depends on shape of spectru of received signal. It can be shown, that when frequency response has a step, after tie-doain easureent slope duration f > / T. As result, iniu duration of windowing function: Tin f f2, (3) where f and f 2 are slope durations of the beginning and the end of the receiving signal band pass accordingly. It is known [4], that iniu distance between antenna probe and AUT ust be larger than 3λ ax, where λ ax is the axiu wavelength of easureent bandwidth. There will be no reflections in tie window if T < 6 ax /c. Taking in account forula (3), the following inequality for tie window can be obtained:
f f2 T 6ax c, (4) It is worth noting that for narrow band signal the requireents (4) are unrealizable. Resulting signal (V) Windowing function Received signal (V) Fig. 6. Usage of windowing function in tie-doain near-field easureents For easureent error estiation the coparison with far field easureent results is used. Fig. 7 shows coparison of double ridged TEM horn antenna GZ26DRH radiation pattern obtained fro far-field and cylindrical near-field easureents. The difference in vicinity of 8 is caused by influence of vertical feed cable in far field. In near field this influence is out of tie window. 5-2 3 4.5 Center of the near-field scan 2 3 4-2 - 2 3 4 2 3 4 - -2-25 -3-35 -4-45 5 2 3 4 Fig. 7. Coparison of double ridged TEM horn antenna radiation patterns (H-plane) obtained by eans of far-field easureents and cylindrical near-field easureents at the frequency 6 GHz 2-2.5 2 Edge of the near-field scan tie delay 5 ps 2 3 4 cylindrical near-field easureent using bow-tie probe far-field easureent - 5 5 Many factors which have a great influence on the accuracy of the results of the near-field antenna easureents, are connected with the easuring probe itself [8]. Aong the, the ost iportant are the errors of probe positioning, probe relative radiation pattern and polarization properties. The usage of fiber-optic cable with optically-fed probe instead of coaxial cable with conventional antenna probe allows to eliinate the dependence of radiation pattern on fed cables position and utual coupling of the probe with its feeder [9]. The block diagra for optoelectronic generation of UWB pulse is shown in Fig. 8. Pulse generator GZ7 (Geozondas Ltd.) eits negative Gaussian-like pulse with aplitude of 4 V and full width at half axiu of ps. Electrical pulse switches the pigtailed laser diode fro a state below threshold into inversion, and the laser generally eits several relaxation oscillations. Under proper width and aplitude of the input electrical pulse, it is possible to extract a first optical spike in the relaxation oscillations, so that the wavefor of the output optical pulse is deterined by the photon lifetie in the laser diode cavity. By adjusting the aplitude of the input electrical Gaussian pulse to 9 V the laser diode odule eits single optical pulse with aplitude of 25 W and full width at half axiu of 27 ps. For adjusting the aplitude of initial electrical pulse the variable attenuator has been used. The pigtailed laser used is uncooled InGaAsP/InP ulti quantu-well distributed-feedback laser with coaxial TOpackage under zero bias condition. This laser has eission wavelength of 3 n, resonance frequency higher than GHz, and side-ode suppression ratio higher than 5 db. The single ode fiber-optic cable is used to transit short optical pulse to the UWB probe, which eits the UWB pulse. Pulse generator GZ7 Variable attenuator DFB laser diode single ode optical fiber optically-fed UWB probe Fig. 8. Optoelectronic UWB pulse generation schee UWB pulse Fig. 9 shows prototype of the optically-fed UWB probe for tie-doain near-field antenna easureents. Optically-fed UWB probe developed is based on pigtailed photodiode integrated with bow-tie radiator. The diensions of the bowtie radiator are 33. The photodiode is soldered onto the backside of the antenna. The bias voltage is supplied to the photodiode through two resistors, soldered at the both edges of bow-tie radiator. Since reverse-biased photodiode consues a very low electrical power the two.5 V batteries have been used in series as power supply. III. OPTICALLY-FED UWB PROBE
single ode optical fiber optical connector Fig. 9. Optically-fed UWB probe prototype bow-tie radiator battery pigtailed photodiode The pigtailed photodiode used in the UWB probe is InGaAs/InP p-i-n photodiode with coaxial TO-package. The photodiode has a sensitive area diaeter of 4 µ, spectral sensitivity range of 85 65 n, and 3dB-bandwidth of 6 GHz. For coupling of the photodiode with single ode optical fiber the tapered fiber lens has been used. To achieve high coupling efficiency the active fiber alignent and index atching resin between the diode and the fiber lens has been used. This results in high responsivity of the pigtailed photodiode ( A/W @ 3 n), which ensure low optoelectronic conversion loss in the UWB probe. The axiu aplitude of the generated UWB pulse is liited by axial photocurrent generated by the photodiode, which is usually not ore than tens of illiaperes. Thus using this technique it is possible to generated UWB pulse with aplitude not ore than several volts, which is sufficient for ost UWB applications. Fig. shows antenna pattern at the frequency 4 GHz of the optically-fed UWB probe and teporal wavefor of the UWB pulse generated. One can see that optically-fed UWB probe has unifor radiation pattern within angle range of 4º, while conventional bow-tie antenna fed by coaxial cable through SMA connector has unifor radiation pattern within only 5º. - -2-3 -4-6 Aplitude (arb. un.) 4 2-2 -4 3 4 5 6-7 bow-tie antenna probe optically-fed UWB probe -8-4 -3-2 - 2 3 4 Fig.. Radiation patterns (H-plane) of the optically-fed UWB probe and bow-tie antenna probe for near-field antenna easureents at the frequency 4 GHz. The inset shows easured teporal wavefor of the UWB pulse generated by the optically-fed UWB probe Fig. shows radiation patterns of the ridged horn antenna P6-23 easured in far field and in near filed using opticallyfed UWB probe and conventional bow-tie antenna fed by 7 coaxial cable. The planar near-field scanning square was, and distance between scanning plane and antenna under test was 6 c. The figure shows that differences between easureents in far field and in near field is uch less in the case of usage of the optically-fed UWB probe. One can see that errors in the near-filed easureents using conventional bow-tie antenna fed by coaxial cable significantly increase for the angles ore than 5º. Thus, optically-fed UWB probe developed allows to carry out tiedoain near-field antenna easureents with high accuracy. It should be noted that ripples for easureents with optically-fed UWB probe are caused by Gibbs phenoenon due to scan plane liitation ( x ). - -2-25 planar near-field easureent using bow-tie antenna probe planar near-field easureent using optically-fed UWB probe far-field easureent -3-4 -3-2 - 2 3 4 Fig.. Ridged horn antenna P6-23 radiation pattern (H-plane) obtained by eans of far-field easureents and planar near-field easureents at the frequency 4 GHz using two different probes IV. CONCLUSIONS Tie-doain near-field antenna easureent syste operating in the frequency range of 2 GHz has been developed. UWB syste for tie-doain near-field antenna easureents includes novel pulse generator and sapling receiver with 7 MHz pulse repetition rate, positioner with coinciding aziuth and elevation centrus of rotation, and optically-fed UWB probe. Antenna easureents can be carried out in planar, cylindrical and spherical surface without usage of anechoic chaber. Hardware tie drift correction algoriths have been presented. Requireents for tie window duration are calculated based on AUT frequency response slope. It is shown that tie-doain near-field antenna easureents are in a good agreeent with far-field antenna easureents. REFERENCES [] B. Levitas, D. Ponoarev, Antenna easureents in tie doain, Proc. of IEEE Antenna and Propagation Society International Syposiu, pp.573 576, vol., Jul. 996. [2] T. Hansen, A. Yaghjian, Planar near-field scanning in the tie doain, IEEE Transactions on Antennas and Propagation, vol.42, n.9, pp.28 3, Sep. 994 [3] T. Hansen, Forulation of spherical near-field scanning for electroagnetic fields in the tie doain, IEEE Transactions on Antennas and Propagation, vol.45, n.4, pp.62 63, Apr. 997 [4] C. Balanis, Antenna theory. Analysis and design, 2nd ed., 997
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