Enhancement of Loop Antenna Reception Using Glow Discharge Plasma Core 1

Transcription

1 ISSN 6-78X, Plasma Physics Reports,, Vol. 8, No., pp Pleiades Publishing, Ltd.,. LOW-TEMPERATURE PLASMA Enhancement of Loop Antenna Reception Using Glow Discharge Plasma Core A. A. Azooz, Y. A. Al-Jawaady, and Z. T. Ali Department of Physics, College of Science, Mosul University, Mosul, Iraq aasimazooz@yahoo.com Received November 8, ; in final form, February, Abstract Experimental results on the behavior of air glow dc discharge column radio frequency antenna are presented. The effect of the discharge conditions on the loop antenna gain at five radio frequencies of 9.,.,.6, 6., and 9. MHz are studied. Increases in the loop antenna gain of up to db are observed. The increase in gain is observed to rise with increased plasma density at the onset of the discharge approaching a saturation limit as the plasma density is increased further. DOI:./S678X9. INTRODUCTION In spite of the fact that the concept of using the plasma glow discharge as a conductor antenna instead of metal components is not a new one [], literature works related to this problem started to appear only recently. Several works appeared in literature in recent years. Most these works are related to signal transmissions [ 9] rather than signals reception []. Plasma antennas enjoy several superior properties over conventional metal antennas. Easy configuration to produce the optimum matching by changing the plasma length, fast on/off setting, and good RF coupling, are few to name. Some of the disadvantages include higher noise, and reduced reproducibility []. Most of the experimental published work on the subject is concentrated on the microwave and high frequency radio waves [] bands. It can be said that the most suitable type of discharge for antenna purposes is the glow type. This is due to its relatively better stability and control properties. It can be also said that the positive column region in the glow discharge is the region which plays the more important role in this respect due to the higher charge density usually present in this region. It is our purpose here to present experimental data on the gain of a simple reception short radio waves antenna as related to the plasma discharge conditions.. EXPERIMENTAL SETUP The experimental setup is shown in Fig.. The transmission side consists of a. Watt variable frequency oscillator coupled to a one meter vertical wire antenna through a matching capacitor. The reception side involves the dc discharge tube surrounded by the The article is published in the original. small loop antenna. The discharge tube is a cm length,.6 cm diameter glass tube with the two ends flattened to the two metal electrodes via the two O- rings. The tube is connected to the vacuum system which consists of the vacuum pump, a pressure gauge and the release valve. The loop antenna consists of between three and five turns of insolated copper wire coil surrounding the positive column region of the discharge with radius equal to that of the discharge tube. The two terminals of the coil are connected to the measuring oscilloscope. The number of turns of the loop antenna is varied to produce the optimum signal reception at each of the frequencies measured when the plasma is off. The distance between the transmission antenna and the discharge tube is about. meters. This corresponds to ordinary simple near field transmission reception situation. The amplitude of the signal received under this condition is taken as the reference amplitude. The high tension voltage is then increased until the glow discharge plasma is produced. The breakdown voltage is recorded. The voltage is further gradually increased and the discharge voltage, current and the received signal amplitude are recorded maintaining the transmitter power and distance between transmitter and receiver fixed all the time. Given the fact that plasma discharge can produce some noise effect, the discharge conditions are selected such that signal to noise ratio (S/N) due to discharge is always grater than db.. RESULTS The results related to increase in signal amplitudes at five transmitted frequencies of 9.,.,.6, 6., and 9. MHz against increase of the dc discharge voltage are studied. The discharge pressures used are 8

2 86 AZOOZ et al. A Anode O ring Vacuum. m -m transmission antenna Shunt Loop antenna CRO VTV Coaxial cable MHz oscillator Matching capacitor Cathode Fig.. Experimental setup., 7, 8. and mtorr. These pressures correspond to pressure separation value (pd) in the range of slightly to the left, at, and slightly to the right of the minimum of the Paschen curve for air []. Typical results at 8. mtorr are presented in Fig.. Results at other pressures are much similar. The frequency values selected are those which produced the best transmission reception conditions when the plasma is switched off. The discharge voltages used at each pressure correspond to values between at the breakdown voltage and the limit when the discharge changes from the normal to the abnormal glow mode. The decibel received signal amplitudes are calculated using the signal amplitudes received when the discharge is off as reference signals. Received signal amplitude, db 6 P = 8. mtorr F = 9. MHz F =. MHz F =.6 MHz F = 6. MHz F = 9. MHz Discharge voltage, V Fig.. Increase in received signal amplitude against discharge voltage for all frequencies studied at pressure values of 8. mtorr. It is clear from Fig. that the received signal starts to increase as soon as the gas breakdown is initiated. Further increase in discharge voltages produces increase in the signal amplitudes at all pressures and frequency values. This increase becomes slower or may reach saturation values as the discharge conditions become close to the abnormal glow situation. The starting ignition voltage values at each pressure reflect the Paschen curve effect. Although all gain values show general increase behavior with discharge voltage at all pressures, the value of maximum gain increase achieved is both frequency and pressure dependent. To demonstrate this further, the maximum value of the gain at each frequency is plotted against the four pressure values in Fig.. It is clear here that the least affected signal is the 6. MHz. The maximum gain at this frequency shows a general increase with increasing pressure but never exceeds the value of about. db. At frequencies of 9. and MHz, the maximum gain is higher at lower pressure values with a general trend to decrease with increasing pressure values. The opposite effect takes place with the.6 and 9. MHz. It must be emphasized however that for the latter four frequency values there exist some pressure values when the gain is well in excess of db. In spite of the fact that plots of gain against discharge voltages do present experimental demonstration of the reception antenna gain increase, it may be convenient from physical point of view to relate the gain increase to plasma electron density. Detailed involved diagnostics measurements are needed to obtain the plasma electron density. However, and to a first approximation, systematic estimations concerning the plasma electron density can be obtained using discharge conductivity data. The plasma electron den- PLASMA PHYSICS REPORTS Vol. 8 No.

3 ENHANCEMENT OF LOOP ANTENNA RECEPTION 87 Maximum received signal amplitude, db Received signal amplitude, db F =. MHz P =. mtorr P = 7. mtorr P = 8. mtorr P =. mtorr F = 9. MHz F =. MHz F =.6 MHz F = 6. MHz F = 9. MHz Pressure, mtorr Fig.. Variation of the maximum increase in received signal amplitudes with pressure for all frequencies studied. Plasma density, m Fig.. Increase in the received signal amplitude against estimated plasma electron density at all pressure values at frequency of MHz. sity ne is related to the discharge conductivity σ through the relation [] is the electron col- Here, m is the electron mass and lision rate with neutrals, given by ν en σ= m ν m. () where δ is the collision cross section, v is the mean electron velocity, and N is the number of neutral molecules per unit volume. The approximate estimate of the plasma electron density in the positive column region is calculated using the plasma conductivity values obtained from the discharge I V data at each pressure value. It must be pointed out that the voltage values used in the conductivity calculations are those associated with the estimated voltage across the positive column. These values are assumed to be equal to the overall applied voltage minus 8% of the initial breakdown voltage. This ratio is considered to be a fair estimation of the voltage across the cathode fall region []. The electron density values calculated using this procedure are consistent with what one would expect in order of magnitude at least under the experimental conditions. Figure shows the increase in gain plotted against calculated plasma electron density for MHz frequency at different pressures. Results for other frequencies are much similar. Several features can be noted from Fig.. The first is that although and in all cases, the increase in the plasma density results in the increase of received signal amplitude, it seems that there is not much to be gained e m ν m = δ v N, () from increasing the density above ( ) m. The increase in gain tends to saturate above this limit for almost all pressures and frequency values. The noise picked up by the loop antenna tends to show large increase above this limit. The second feature that can be noticed from the data is the clear effect of the position on the Paschen curve. It is clear that for the two pressure values of 7 and 8. mtorr which are close to the Paschen minimum, the increase in gain starts to appear at much lower plasma density values, and at all frequencies. This is in contrast to the and mtorr which are slightly to the left, and to the right sides of the Paschen minimum. In these two cases one notice that the increase in gain starts to appear at significantly higher density values. Furthermore, the plasma density gain increase starting value at mtorr, which is to the left side of the Paschen curve, is always significantly higher than corresponding values at higher pressures. The change of pressure does not only affects the density starting value for the gain increase but it also affects the density values at which the increase in gain begins to slow down. The latter point seems to be being reached faster at pressure close to the Paschen minimum than those on the two sides of the Paschen curve.. DISCUSSION The above experimental plasma column effect on loop antenna gain looks slightly surprising at first. This is due to the fact that the cm plasma column can not be treated within the framework of wave approximation. However, it is well known fact that even short conductors ( L λ) can transmit and receive electromagnetic short waves. To check for the fact that the observed increase in amplitude of signal received is not PLASMA PHYSICS REPORTS Vol. 8 No.

4 88 AZOOZ et al. Signal amplitude, mv Frequency, MHz due to some change in the resonance position of the originally well tuned loop antenna, an additional experiment was carried out. This involved the study of the frequency response of the transmission reception process. The frequency response of the reception antenna is studied at the central frequency of about. MHz. The amplitude of the transmitted signal is kept constant by using another identical loop reception antenna which does not contain a plasma column. The signal received by the plasma cored antenna at plasma discharge currents of.,.,., and 6 ma are recorded in the frequency range of 6 MHz. Only results for.,, and 6 ma are plotted in Fig. for clarity reason. Results at other discharge currents are much similar. A simple series LRC equivalent circuit shown in Fig. 6 for the loop antenna is assumed. The data at each discharge current are ted to the relation V o = R V o ( Ra + Ro) + πfl πfc a No plasma I d = ma I d = ma I d = 6 ma Fig.. Frequency response of the loop antenna at plasma discharge currents of,,, and 6 ma for.-mhz transmitted signal., () L R a C V a Fig. 6. Equivalent circuit. R V where Va, Ra, and L are the source voltage, radiation resistance, and inductance of the loop antenna, respectively; C is the overall circuit capacitance; and R o is the kω effective oscilloscope input resistance. The parameters Va, Ra, L, and C are kept free during the ting process of all data. Lines in Fig. are ting results with 9% confidence level. The ted parameters in each case together with the resonance frequency are presented in the table. It is clear from the table that the only two parameters which are subjected to any significant changes due to the plasma formation are the antenna source voltage Va, and the antenna radiation resistance Ra. This may be explained by the fact that the loop antenna is affected only by the magnetic component of the transmitted electromagnetic field when there is no plasma. However, when the plasma core is initiated, electrons within the plasma will be forced to oscillate by the electric component of the field. This oscillation will induce additional induction in the loop antenna surrounding the plasma. The net result is the superposition of the two voltages resulting in enhanced reception signal. Results of s for the frequency response of the loop antenna I d, ma 6 mean std V a, mv f, MHz R a, MΩ L, μh C, nf PLASMA PHYSICS REPORTS Vol. 8 No.

5 ENHANCEMENT OF LOOP ANTENNA RECEPTION 89. CONCLUSIONS A glow discharge plasma core can produce considerable increases in the reception gain of a loop antenna in the short radio wave band. The value of the increase is pressure and plasma density dependent. Better gain improvement is obtained when the discharge gas pressure is close to the Paschen curve minimum. REFERENCES. J. Hettinger, US Patent No.,9, (July 8, 99).. W. M. Manheimer, IEEE Trans. Plasma Sci. 9, 8 (99).. R. A. Meger, J. Mathew, J. A. Gregor, et al., Phys. Plasmas, (99).. G. G. Borg, J. H. Harris, D. G. Miljak, and N. M. Martin, Appl. Phys. Lett. 7, 7 (999).. G. G. Borg, J. H. Harris, N. M. Martin, et al., Phys. Plasmas 7, 98 (). 6. J. P. Rayner, A. P. Whichello, and A. D. Cheetham, IEEE Trans. Plasma Sci., 69 (). 7. M. Chung, W. S. Chen, Y. H. Yu, and Z. Y. Liou, in Proceedings of the International Conference on Microwave and Millimeter Wave Technology, Nanjing, 8, p V. V. Kumar, M. Mishra, and N. K. Joshi, Prog. Electromagn. Res. Lett., 7 (). 9. L. X. Ma, Z. Li, H. Zhang, and C. X. Zhang, Prog. Electromagn. Res. Lett., 8 ().. I. M. Minaev, N. G. Gusein-zade, and K. Z. Rukhadze, Plasma Phys. Rep. 6, 9 ().. D. C. Jenn, Report No. NPS-CRC-- (US Naval Postgraduate School, Monterey, CA, ).. N. G. Gusein-zade, I. M. Minaev, A. A. Rukhadze, and K. Z. Rukhadze, J. Commun. Technol. Electron. 6, 7 ().. A. Lisovskiy and S. D. Yakovin, Plasma Phys. Rep. 6, 66 ().. Yu. P. Raizer, Gas Discharge Physics (Springer-Verlag, Berlin, 99), pp., 8. PLASMA PHYSICS REPORTS Vol. 8 No.