A New Method of VHF Antenna Gain Measurement Based on the Two-ray Interference Loss

Transcription

1 7 nd Intenational Confeence on Atificial Intelligence and Engineeing Applications (AIEA 7) ISBN: A New Method of VH Antenna Gain Measuement Based on the Two-ay Intefeence Loss ZHIEI YANG, JI SUN, YONGXIN ZHANG and PENGCHENG GUO ABSTRACT Antenna gain is an impotant index of wieless communication system, how to accuate measuement of antenna gain is a difficult poblem. Thee ae many factos affecting the VH antenna gain test accuacy in the outdoo test site. Gound eflected wave is the most impotant souce of eo. A new method is poposed in this pape to measue the antenna gain by using a two-ay intefeence loss to modify, which impoved the antenna gain accuacy. KEYWORDS Gain measuement, Gound eflected wave, Two-Ray intefeence loss. INTRODUCTION At pesent, antenna gain test is based on the fee space sight popagation model in the outdoo test field. But due to the existence of gound wave, the signal test values tend to be lowe o highe than the nomal values, which is the main eason causing the diffeence of test esults. Gound eflection model has a matue theoy in adio wave popagation, but the eo of two-ay intefeence loss caused by gound eflection has not seen in the public liteatue. When the gound eflection model is applied to the measuement of two-ay intefeence loss in antenna measuement, two coections ae needed: one is the calculation of the eflected phase, and the othe is the calculation of the field intensity of the eflected wave. TWO-RAY INTERERENCE LOSS MODEL Two-ay Intefeence Theoy Thee ae mainly two signals to each the eceiving antenna: one is diect wave which popagate in line of sight [], and the othe is gound eflected wave, as shown in figue. Zhifei Yang, zhifeiyang@63.com, Ji Sun, Yongxin Zhang and Pengcheng Guo, Luoyang Electonic Equipment Test Cente of China, Luoyang 473, China 83

2 igue. Signal popagation between antennas. The total signals is a composition of all incoming signals, it may be inceased o cancelled. The phenomenon of contay to the elation between the tansmission powe and the distance of the electomagnetic wave due to the gound wave intefeence, which is called the electomagnetic two-ay intefeence. Phase Diffeence Calculation The two-ay intefeence is mainly caused by the phase diffeence between the eflected wave and the diect wave. The phase diffeence is composed of two pats: the diect wave and eflected wave path length caused phase diffeence and suface eflection caused phase diffeence. It can be poved that the diffeence of the wave path between the eflected wave and the diect wave is equal to the diffeence in the geometic length between them []. Because the test distance is elatively shot, the influence of the eath cuvatue is not consideed. As shown in igue, the popagation path of the diect wave is: d ( h h ) () t The length of popagation path of eflected wave is: s d ( ht h) () The diffeence between the eflected wave and the diect wave is: (3) The phase diffeence caused by the wave path diffeence is: (4) The eflected phase diffeence devoted to the popagation of adio waves is usually given vale of π in the monogaph [, 3], which is no longe applicable to the antenna test. 84

3 反射相位 ( ) 垂直极化 3MHz 6MHz 水平极化 3MHz 6MHz MHz MHz 3MHz 6 4 MHz MHz 3MHz 擦地角 ( ) igue. Reflection phase of ultashot wave in medium dy gound. As shown in igue, we simulated the eflected phase of ultashot wave in accodance with the enel eflection phase [].The electical paametes ae selected by medium dy gound: the dielectic constant value is 5, and the conductivity value is 3-3. It can be seen fom the figue of the eflection phase size and polaization of electomagnetic wave, the fequency of the incident wave, the gazing angle and the electical chaacteistics of the gound. To sum up, the phase diffeence between the eflected wave and the diect wave is: (5) Reflectivity Calculation The gound eflectivity is defined as the atio of the field intensity to the eflected wave at the eflection point. As shown in igue 3, we simulated the eflectivity of ultashot wave in accodance with enel [] eflectivity. The selected electical paametes ae as follows: the dielectic constant value is 5, and the conductivity value is 3-3. It can be seen fom the figue on the gazing angle eflectivity and polaization of an electomagnetic wave, the incident wave and the electical chaacteistics of the gound is almost independent of the fequency 水平极化.6 反射率.5.4 垂直极化.3.. 3MHz 6MHz MHz MHz 3MHz 擦地角 ( ) igue 3. Reflectivity of ultashot wave in medium dy gound. 85

4 Calculation of Two-ay Intefeence Loss At the eceiving point of the antenna, the equations fo calculating the field intensity of the diect wave and the eflected wave ae espectively expessed as: e( R) Eexp{ j( t )} (6) e( R) E exp{ j( t s )} (7) whee ω is angula fequency, is diect wave distance, s is eflection wave distance, is eflection phase, E and ae the amplitude of diect wave s and E eflected wave s field intensity at the eceiving point espectively. In the monogaph [4], the equation is given: E R E (8) Thee is debate about whethe the equation is coect. The eflectivity is the atio of the amplitude of the eflected field to the incident electic field intensity at the eflection point, but not at the eceiving point of eflection wave and diect wave field intensity. It is a micoscopic and patial quantity and can not be genealized to the whole wold. The above analysis shows that the equation (8) is not coect, and the amplitude of the eflected wave field at the eceiving point needs to be ecalculated. The amplitude of field stength calculated by powe and enegy density is: E 3PG t t (9) The amplitude of incident wave field at gound eflection point is: 3PG t t E () s The amplitude of eflected wave field at gound eflection point is: E 3PG t t E R s () R As shown in igue 4, the eflected wave is equivalent to an image adiation souce at the distance fom the gound eflection point so that the amplitude of the eflected wave field at the eceiving point can be calculated: 86

5 E 3PG t t R s s () whee s is the gound incident wave distance, s is the eflected wave distance. om equation (), () can be deduced as follows: R ' E E RE s sr (3) With the elationship between E and E, the synthetic wave field stength can be calculated. The field stength of the composite wave is expessed as the vecto sum of the diect wave field intensity and the field intensity of the eflected wave. er ( ) e( R) e( R) E j t R j ' exp{ ( )} [ exp( )] (4) Accoding to the Eule equation, the two-ay intefeence loss is as follows: L er ( ) lg e ( R) R R ' ' lg[ cos( )] (5) R s igue 4. Equivalent adiation souce of eflected wave. 87

6 ANTENNA GAIN MEASUREMENTS The Taditional Method Taditionally, in addition to the antenna unde test, one exta antenna is needed. Then, one is used as the tansmitting antenna, the othe is used as eceiving antenna. This is called the same antenna method. The test distance is d, the opeating fequency is f and the input powe is P t. All of the paametes above ae known, and the gain of the antenna unde test can be calculated by equation (6) as long as the eceived powe P of the eceiving antenna is measued [5]. G ( P P L L L )/ (6) t f Whee, G is the gain of the antenna unde test (dbi), P is the powe eceived by antenna Rx(dBm), P t is the powe tansmit by antenna Tx(dBm), L f is the popagation loss of signals in fee space(db), L is the feed cable loss at tansmitte(db), L is the feed cable loss at eceive (db). The Poposed Method The undestanding of L is the main eason fo the diffeence in test esults. In the elevant technical data of antenna measuement, L f is calculated in tems of fee space popagation loss. L L 3.45 lg f lg d (7) f bf Obviously, this undestanding does not take into account the influence of gound eflected waves, and the pesence of eflected waves will cause geat measuement eos. o example, thee is a eflected wave of db smalle than the diect wave signal at the eceiving antenna stand. Though, the eflected wave is elatively small, but afte adding to the diect wave, then thee may be a change in the 8%~%. Theefoe, the signal space popagation loss should be undestood as the sum of fee space popagation loss and two-ay intefeence loss, and the coected gain equation is expessed as follows: G ( P P L L L L )/ (8) t bf Whee, L bf is popagation loss in fee space, L is two-ay intefeence loss. An Example of Measuement We choose a flat open gound as the testing gound. Antenna unde test is a 3m length whip antenna. Its woking fequency is 3MHz~9MHz. Antenna gain measuements ae accomplished using the two antenna method. The tansmitting and 88

7 eceiving antennas ae both mounted m above the table top on a fixed wooden stand to minimize petubation of the electomagnetic fields. The test distance wee m and m espectively. We select two-ay intefeence loss model fo vetical polaized wave in medium dy gound. The electical paametes ae as follows: the dielectic constant value is 5, and the conductivity value is 3-3. The two-ay intefeence loss at the m and m test distances is calculated by equation (5), and the esults ae shown in igue 5. Based on these values, the measued data at the test distance of m and m is calculated by the taditional method and the poposed method, and the diffeence between two esults ae shown in igue 6. It can be seen fom figues 6, the esults modified by two-ay intefeence loss model has a bette ageement than the taditional method. The pefomance of the poposed method is times highe than that of the taditional method. The 9% gain diffeences ae within db,which ae vey close to the theoetical value 二径干涉损耗 (db) 二径干涉损耗 (db) 频率 (MHz) 频率 (MHz) igue 5. Two-ay intefeence loss a)at a distance of m b)at a distance of m 测试增益差值 (db) - 测试增益差值 (db) 频率 频率 (MHz) (MHz) igue 6. Compaison of gain diffeence a) by the taditional method b) by the poposed method. 89

8 CONCLUSION The outdoo test field of antenna is difficult to each the condition of fee space test field, so it is not suitable to test the antenna gain accoding to the fee space tansmission model. This pape modifies the gound eflection popagation model in eflection phase and eflection wave field calculation, the intoduction of the two-ay intefeence loss to eliminate the influence of gound wave eflection on the antenna gain test, impove the accuacy of the test. REERENCES. Yixi Xie, et al. Pinciples and applications of adio wave popagation. Beijing: People's Posts and Telecommunications Pess, 8, p Qing Dai, Jijun Huang, Jinjun Mo, et al. Moden micowave and antenna measuement techniques. Beijing: Electonics Industy Pess, 8, p Yuankun Wang, et al. Intoduction to adio popagation. Beijing: National Defense Industy Pess, 4, p Li Li, et al. Antenna and Popagation. Beijing: Science Pess, 9, p L. Huitema, T. Reveyand, et al. Definition of effective antenna gain and how to measue it. IEEE Tans. Antennas Popagat, Vol. 6 (3) No. 9, p