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 24061 USA. E-mail: mharun@vt.edu

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Contents

1 Introduction 3

2 Prototype Antenna 3

3 s11 Measurement 5

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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 (138-174 MHz), 220 MHz (220-222 MHz), UHF (406-

512 MHz), and 800 MHz (764-862 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 1.165 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 s11 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 1.135 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

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Base length

Ext. length

Teflon

Relay

Connecting wire

Connecting wire

Power supply line

NMO mount

d

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 single-

coil 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 trunk-

mounted 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

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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 s11 Measurement

In this section we present the results of s11 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 s11 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 s11 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 s11 for the com-

plete 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 capaci-

tive 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

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Figure 2: Prototype reconfigurable antenna.

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Figure 3: Zoomed in to the middle section of the prototype reconfigurable antenna. (Theconnecting 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.

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100 200 300 400 500 600 700 800 900−20

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0

|s11

| (dB

)

Freq (MHz)

MeasurementNEC

Figure 4: s11 measurement for only the base antenna. Also shown for comparison is theresult 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. s11 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.

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100 200 300 400 500 600 700 800 900−20

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|s11

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Freq (MHz)

relay−openonly base

Figure 5: s11 measurement of the prototype reconfigurable antenna with the relay switchedoff. Also shown for comparison is the result obtained with only the base antenna.

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Freq (MHz)

d=0.5 cmd=3.5 cmd=5.5 cm

Figure 6: s11 measurement for varying separation between the base antenna and the exten-sion.

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relay−shortNEC

Figure 7: s11 measurement of the prototype reconfigurable antenna with the relay switchedon.

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|s11

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Aluminium−Panel GroundCar Mounted

Figure 8: s11 measurement of car mounted prototype reconfigurable antenna with the relayswitched off.

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100 200 300 400 500 600 700 800 900−20

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|s11

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Freq (MHz)

Aluminium−Panel GroundCar mounted

Figure 9: s11 measurement of car mounted prototype reconfigurable antenna with the relayswitched on.

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References

[1] S. Ellingson, “A Simple Reconfigurable Monopole for Multiband Public Safety Appli-

cations,” Project Report No. 17, Virginia Polytechnic Inst. & State U., July 28, 2011.

[online] http://www.ece.vt.edu/swe/asmr/.

[2] S. Ellingson, “Measurements of Elements of an LMR Multiband Antenna System De-

sign,” Project Report No. 5, Virginia Polytechnic Inst. & State U., Jun 30, 2010. [online]

http://www.ece.vt.edu/swe/asmr/.

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