A Prototype Simple Reconfigurable Antenna for the. Multiband LMR Antenna System

Transcription

1 A Prototype Simple Reconfigurable Antenna for the Multiband LMR Antenna System Mahmud Harun and S.W. Ellingson April 4, 2012 Bradley Dept. of Electrical & Computer Engineering, 444 Durham Hall, Virginia Polytechnic Institute & State University, Blacksburg, VA USA. 1

2 Contents 1 Introduction 3 2 Prototype Antenna 3 3 s 11 Measurement 5 2

3 1 Introduction In this report we document some initial measurements of a simple reconfigurable monopole antenna designed for use in a multiband land mobile radio (LMR) antenna system. The concept of the antenna is described in [1]. The idea is to use the monopole with an antenna tuner to achieve a useful level of performance in each of the bands of interest; namely, VHF-Low (25-50 MHz), VHF-High ( MHz), 220 MHz ( MHz), UHF ( MHz), and 800 MHz ( MHz). It is shown in [2] that a single 23.5 cm long monopole is suitable for the UHF and 800 MHz bands, but unacceptable for the VHF Low through 220 MHz bands. In order to achieve a usable level of performance in the VHF Low through 220 MHz bands, a switchable extension to this 23.5 cm long monopole is proposed in [1]. The concept is shown in Figure 1. The extension is m in length, supported by a section of Teflon tube. When switched off, the extension is disconnected, and the monopole is essentially identical to the original monopole. When switched on, the extension is connected, yielding a longer monopole. Section 2 describes the prototype reconfigurable monopole antenna, and Section 3 shows results of s 11 measurements at the antenna terminals when connected to a 50 Ω load. 2 Prototype Antenna Figure 2 shows the prototype reconfigurable antenna. The upper 20.5 cm of the base antenna is hollow brass rod of 13/32 in diameter. The lower 3 cm is a NMO-to-3/8 in thread adapter. One end of a 3/8 in bolt screws on to the adapter and the rest of the bolt is inserted into the hollow brass rod. The extension is m long solid aluminum rod with a diameter of 1/8 in The connection between the base and the extension is 30 mm in length. The middle section of the antenna is shown in detail in Figure 3. A hollow teflon tube of 3/8 in diameter is inserted inside the brass rod and it extends out of the top of the brass rod as shown in Figure 3. At the other end of this teflon tube the extension is attached. The 3

4 Ext. length Connecting wire Relay Teflon Connecting wire d Power supply line NMO mount Base length Figure 1: Design of the reconfigurable antenna. base and the extension are then connected to the relay using single conductor wires. The total length of the antenna is 1.4 m. The switching between the two sections is done using a NAiS Model AGN2104H singlecoil latching relay. Since the relay is of the latching variety, no power is required except to change state. A voltage of about 2.5 V across the coil is sufficient to activate the latch; applying a signal of the opposite polarity changes the state. The power cable for the relay is drawn through the inside of the brass rod. The ground plane is 1.79 m 1.19 m, constructed from 3 aluminum panels which are bolted together. The ground plane is located approximately 1 m above an asphalt surface. For the purposes of this study, this should be a reasonable surrogate for a vehicular trunkmounted installation. A hole is drilled through the ground plane to hold an NMO mount, and the adapter at the end of the antenna is then connected to this mount. All measurements are made using a Rhode Schwartz FSH3 spectrum analyzer with 4

5 tracking generator option, fitted with an FSH-Z2 VSWR bridge. The test setup is calibrated to the end of coaxial cable with a NMO mount connector; thus the measurements account for the monopole as well as the NMO-to-3/8 in. thread adapter. 3 s 11 Measurement In this section we present the results of s 11 measurements at the reconfigurable antenna terminal when it is connected to a 50 Ω load. In order to check the validity of the experimental setup, the performance of the base antenna alone is first tested. (A similar test and the results have already been reported in [2].) The results of s 11 measurement are shown in Figure 4. Also shown is the result obtained using a simple NEC-based method of moments computer simulation, in which the monopole is divided into 13 segments. NEC results assume an infinite ground plane. It is observed that the measurements show reasonable agreement with the NEC results, considering the limitations of the NEC model. The antenna exhibits resonance at around 300 MHz which, is expected since the antenna is about λ/4 long at this frequency. However, there is some oscillation observed in the s 11 measurement; especially at the higher frequencies. This behavior can be attributed to the reflection of the waves between the antenna and garage present in the proximity. Now that we are confident about our experimental setup, we measure the s 11 for the complete reconfigurable antenna with the relay switched off. The results are shown in Figure 5. Also shown is the result for the base antenna alone. It is observed that the performance of the reconfigurable antenna in the relay switched off mode is different from that observed for the base antenna alone case. Resonance at multiple frequencies other than around 300 MHz is produced for the reconfigurable antenna. The behavior can be attributed to the capacitive reactance introduced by the dielectric teflon present between the antenna segments. In order to verify this theory a parametric study is performed where the distance d between the base and the extension (as shown in Figure 1; but with the relay, connecting wires and 5

6 Figure 2: Prototype reconfigurable antenna. 6

7 Figure 3: Zoomed in to the middle section of the prototype reconfigurable antenna. (The connecting wires were made much shorter for the original measurement ) power supply lines removed ) is varied and the s11 measurements are recorded. The results are shown in Figure 6. It is observed that the distance between the base and the extension affects the oscillation; as the distance increases resonance at frequencies other than 300 MHz vanishes. Increasing the distance between the antenna segments decreases the capacitance and therefore the behavior starts to follow the base only case. For the next set of measurements the complete reconfigurable antenna is tested with the relay switched on. The results are shown in Figure 7. Also shown is the result generated from a NEC simulation (segment length of 1.8 cm used irrespective of frequency). The measurement agrees with the NEC predictions of resonant frequencies; however, s11 is higher for the measurement than that observed in NEC results; especially in the UHF and 800 MHz band. This behavior can again again be attributed to the close proximity of the garage. 7

8 s 11 (db) Measurement NEC Freq (MHz) Figure 4: s 11 measurement for only the base antenna. Also shown for comparison is the result obtained with NEC. In the last set of measurements the reconfigurable antenna is mounted on the trunk of a four door sedan car using a trunk-lip NMO mount. s 11 is again measured both with the relay switched off and on. The results are compared with the aluminium panel ground case. The results are shown in Figures 8 and 9. It can be observed that the results are comparable; i.e., the aluminium panel ground is indeed a good surrogate for the car-trunk. 8

9 s 11 (db) relay open only base Freq (MHz) Figure 5: s 11 measurement of the prototype reconfigurable antenna with the relay switched off. Also shown for comparison is the result obtained with only the base antenna. 9

10 s 11 (db) d=0.5 cm d=3.5 cm d=5.5 cm Freq (MHz) Figure 6: s 11 measurement for varying separation between the base antenna and the extension. 10

11 s 11 (db) relay short NEC Freq (MHz) Figure 7: s 11 measurement of the prototype reconfigurable antenna with the relay switched on. 11

12 s 11 (db) Aluminium Panel Ground Car Mounted Freq (MHz) Figure 8: s 11 measurement of car mounted prototype reconfigurable antenna with the relay switched off. 12

13 s 11 (db) Aluminium Panel Ground Car mounted Freq (MHz) Figure 9: s 11 measurement of car mounted prototype reconfigurable antenna with the relay switched on. 13

14 References [1] S. Ellingson, A Simple Reconfigurable Monopole for Multiband Public Safety Applications, Project Report No. 17, Virginia Polytechnic Inst. & State U., July 28, [online] [2] S. Ellingson, Measurements of Elements of an LMR Multiband Antenna System Design, Project Report No. 5, Virginia Polytechnic Inst. & State U., Jun 30, [online] 14