научная статья по теме ENHANCEMENT OF LOOP ANTENNA RECEPTION USING GLOW DISCHARGE PLASMA CORE Физика

Текст научной статьи на тему «ENHANCEMENT OF LOOP ANTENNA RECEPTION USING GLOW DISCHARGE PLASMA CORE»

ФИЗИКА ПЛАЗМЫ, 2012, том 38, № 10, с. 916-920

НИЗКОТЕМПЕРАТУРНАЯ ПЛАЗМА

ENHANCEMENT OF LOOP ANTENNA RECEPTION USING GLOW DISCHARGE PLASMA CORE © 2012 г. A. A. Azooz, Y. A. Al-Jawaady, Z. T. Ali

Department of Physics, College of Science, Mosul University, Mosul, Iraq e-mail: aasimazooz1@yahoo.com Поступила в редакцию 08.11.2011 г. Окончательный вариант получен 20.02.2012 г.

In this work, 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 9.3, 10.0, 13.6, 16.5, and 19.1 MHz are studied. Increases in the loop antenna gain of up to 5 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.

1. 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 [1], 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 [2—9] rather than signals reception [10].

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 [11]. Most of the experimental published work on the subject is concentrated on the microwave and high fre-

quency radio waves [12] 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.

2. EXPERIMENTAL SETUP

The experimental setup is shown in Fig. 1. The transmission side consists of a 0.1 Watt variable fre-

1 meter

transmission

antenna

# matching capacitor

MHz

oscillator

0

Fig. 1. Experimental Setup.

recieved signal amplitude, dB 6

P = 28.5 mTorr -O-F = 9.3 MHz ■ F = 10.0 MHz ■■•■■F = 13.6 MHz -O-F = 16.5 MHz -O- F = 19.1 MHz

ENHANCEMENT OF LOOP ANTENNA RECEPTION

maximum recieved signal amplitude, dB

5

4 -

917

1200 1400 1600

plasma density (/m3)

Fig. 2. Increase in received signal amplitude against discharge voltage for all frequencies studied at pressure values of 28.5 mTorr.

25

26

27

28

-O- F = 9.3 MHz • F = 10.0 MHz F = 13.6 MHz -O-F = 16.5 MHz -O- F = 19.1 MHz

J_I_I

29 30 pressure, mTorr

Fig. 3. Variation of maximum increase in received signal amplitudes with pressure for all frequencies studied.

3

2

quency oscillator coupled to a one meter vertical wire antenna through a matching capacitor. The reception side involves the dc discharge tube surrounded by the small loop antenna. The discharge tube is a 15 cm length, 3.6 cm diameter glass tube with the two ends flattened to fit 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 1.2 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 30 db.

3. RESULTS

The results related to increase in signal amplitudes at five transmitted frequencies of 9.3, 10.0, 13.6, 16.5, and 19.1 MHz against increase of the dc discharge voltage are studied. The discharge pressures used are 25, 27, 28.5 and 30 mTorr. These pressures correspond to pressure x 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 [13]. Typical results at 28.5 mTorr are presented in Fig. 2. 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.

It is clear from Fig. 2 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 again the four pressure values in Fig. 3. It is clear here that the least affected signal is the

Fig. 4. Increase in received signal amplitude against estimated plasma electron density at all pressure values at frequency of 10 MHz.

16.5 MHz. The maximum gain at this frequency shows a general increase with increasing pressure but never exceeds the value of about 2.4 db. At frequencies of 9.3 and 10 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 13.6 and 19.1 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 4 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 density ne is related to the discharge conductivity a through the relation [13]:

u —-, w

mv

m is the electron mass and vm is the electron collision rate with neutrals and is given by

V m =S( v) N, (2)

5 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 conduc-

signal amplitude, mV

frequency, MHz

Fig. 5. Frequency response of the loop antenna at plasma

discharge currents of 0,1, 4,6 mA for 14.5 MHz transmitted signal.

tivity 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 85% of the initial breakdown voltage. This ratio is considered to be a fair estimation of the voltage across the cathode fall region [14]. 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 4 shows the increase in gain plotted against calculated plasma electron density for 10 MHz frequency at different pressures. Results for other frequencies are much similar.

Several features can be noted from Fig. 4. 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 from increasing the density above (2—3) x 1010/m3. 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 27 and 28.5 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 25 and 30 mTorr which are slig

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