EMBEDDED ANTENNAS FOR UAV COMMUNICATIONS Selvanayaki K1, Rahul2, Samudra Das Gupta1, Dilip Y1

1 Scientist, Aeronautical Development Establishment (ADE), DRDO, Bangalore-75 2 Junior Research Fellow, Aeronautical Development Establishment (ADE), DRDO, Bangalore-75

E-mail: Selvanayaki@ade.drdo.in; Phone: 080-25057890 Abstract

The concept of multiple antennas embedded as

part of UAV structure for meeting Omni

directional azimuth coverage of UAV

communication system is studied bringing out the

advantages of using such a concept. This study

brings out number of antennae elements required

for a typical Long-Ez UAV structure. A suitable

antenna element to meet the required coverage is

designed, simulated , fabricated & tested and the

results are presented. A typical wing leading edge

was fabricated and used to test the perturbation

caused by embedding the antenna as part of the

fabricated structure. The results are promising

and implementing such a concept is essential for

the current demanding needs of high data rate

communication systems and enhanced stealth

capability.

Index terms

Unmanned Air Vehicle (UAV), UAV structure,

wing leading edge

I Introduction

Conventionally, UAV antennae are externally

mounted in order to have better Line Of Sight

(LOS) with the ground systems and also to reduce

radiation pattern perturbation due to its proximity

with the UAV structure with different material

characteristics. Main requirement is achieving the

Omni directional coverage. The externally

mounted UAV antennae need to be

aerodynamically shaped in order not to disturb

the airflow and provide extra drag to the aircraft.

Such externally mounted UAV antennae design

need special care as they come under the category

of environmentally exposed and such care comes

with additional weight penalty. Present work

brings out the major advantages of going for

UAV structurally embedded antennae. It studies

the locations of antennae on a UAV structure,

brings out the number of antenna elements and

their required radiation characteristics to meet

Omni directional coverage as embedded inside

for a Long-Ez aircraft. A suitable antenna

element to meet the radiation requirement and the

bandwidth requirement is designed and radiation

characteristics are shown by simulation and by

experiment. The embedding of the antenna is

studied by positioning the antennae as per the

requirement in different sections of the UAV and,

by using the computational electromagnetic tool

the radiation pattern perturbation analysis is

carried out showing that it is possible to achieve

required antenna radiation performance. As the

implementation requires the concept to be taken

up during the aircraft fabrication itself, a sample

case of embedding an antenna in a typical wing

and the measurement results are shown. Proving

this paves the way for drastic reduction in Size,

Weight and Power (SWaP) requirement of UAV

communication system.

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II Multiple Embedded Antennae for UAV

Communication System and its advantages

RF link analysis of UAV communication system

should consider realistic onboard antenna gain as

mounted on to the UAV structure. Conventional,

single Omni antenna on elevated structure

mounted on a Long-Ez aircraft typically gives a

gain of -6dBi. Accordingly, for the given range,

for the given receiver sensitivity, and for the

given ground antenna gain the required

transmitting power is worked out to have enough

fade margin. Recent communication systems

demand for high data rate which in turn demands

for more transmitting power if conventional

single Omni antenna is used. Hence, if multiple

high gain antennas can be used, then the

requirement of increasing the transmitting power

can be eliminated. The number of antennas can be

anything depending on the gain requirement. In

this case, a long-Ez aircraft is considered and

depending on the masking effect, the available

transmitting power and the required fade margin

the number of antennae is decided. The following

gives the formula for finding the required

transmitting power

(1)

where Pt = Transmitting Power

Pr = Received Power

Gt = Airborne antenna gain

Gr = Ground antenna gain

FSL= Free Space Loss

FM=Fade Margin

20 10 20 10 92.5

(2)

where d is in Km, f is in GHz

For a typical data link for 200Km range at 5GHz,

the given receiver sensitivity is -85dBm @

16Mbps data rate, the given ground antenna gain

is 32dBi. To achieve minimum fade margin of

7dB with the conventional Omni antenna gain of

-6dBi in presence of the UAV, the required

transmitting power is 70W. This is a huge power

requirement and hence mechanically steerable

directional antenna system is used in many

unmanned aircrafts. But this comes with

additional weight penalty due to the mechanical

steering arrangement, radome to accommodate

the antenna etc., In addition to the weight penalty,

it gives additional drag to the aircraft. However,

if multiple directional antennae could be used

with switching capability meeting a gain

requirement of 2.5dBi in 360�, the required

transmitting power is only 10W. Hence, the

concept of having multiple antennae embedded as

part of the UAV structure is ideal for any UAV in

this era of high data rate communication systems

where new thinking is required at every design

input. In addition to Size Weight, and

Power(SWaP) saving it will also help in radiating

only in a narrow angle and hence will enhance the

stealth capability. Micro strip patch antenna is

chosen as the candidate antenna to be embedded

as it can meet the minimum gain required over

±45� angle. Intuitively, it might appear that 4

patch antennae mounted at 0 �, 90 � , 180 � , 270 �

will be enough to cater for Omni coverage

requirement as shown in figure 1. But the issues

are analysed in part IV.

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Figure 1: 4 patch antennae to give 360� coverage

III Antenna element for embedding inside the aircraft and the performance

The requirement of the antenna element for embedding as part of the UAV structure is that the gain shall be

greater than 2.5dBi after embedding inside the UAV structure over the beam width of ±45�. The most suitable

candidate antenna is patch antenna whose peak gain is approximately 7dBi with 3dB beam width of ±45�. A

wing leading edge is considered and the details are as shown in figure 2 where a=20cm, b=20cm,c=9cm with

the length of 1m.

Figure 2 Figure 3 Figure 4

Wing leading edge is modelled in FEKO as 2mm thick GFRP material with loss tangent of 0.019 and dielectric constant of 4.4. Figure 3 shows radiation pattern of wide band patch antenna & figure 5 shows the perturbed radiation pattern of wide band patch antenna at 5GHz when embedded inside the wing. It is evident that the shape of the wing leading edge perturbs the radiation pattern and the details are seen in the elevation and azimuth pattern as embedded in figure 5 & 6 and the minimum gain seen is 4dBi for ±45� coverage.

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Figure 5: Azimuth pattern perturbation Figure 6: Elevation pattern perturbation

Figure 7 Figure 8

Figure 7 shows the measurement set up for testing the fabricated antenna as embedded in the wing leading edge and figure 8 shows the measured normalized azimuth radiation pattern showing the perturbation due to the presence of the wing leading edge. In figure 8, Red plot shows the radiation pattern in the presence of the wing and blue plot shows antenna alone without wing. Similar to the simulation results, the measured results show negligible perturbation due to the wing embedding in the azimuth.

IV Perturbation Analysis results of multiple embedded antennae

Upon identifying suitable element for embedding, the radiation perturbation analysis of multiple antenna

elements is carried out for the Long-Ez aircraft as shown in figure 9. After iterations, it was found that aircraft

masking calls for 2 more additional antennae to meet the requirement. For the Long-Ez aircraft shown in

figure 9, to have 360� coverage in the azimuth there are 6 elements considered to be embedded as part of the

structure radiating as shown in red.

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Figure 9: Long-Ez aircraft with the radiating elements to meet the Omni coverage in azimuth

For the analysis, PEC model of the aircraft is considered except at the locations of the antenna elements radio transparent GFRP material is considered. Figure 10 shows the antenna radiation pattern as mounted on to the aircraft for single antenna element.

Figure 10

Figure 11

Figure 11 shows perturbed radiation pattern when multiple antennas are embedded and one can be switched on based on antenna selection logic. The lowest gain is greater than 2.5dBi which is the minimum required gain to meet the given fade margin of 7dB. It is also to be noted that the wing leading edge mounted (front

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looking)antennae suffered (green and blue) suffered aircraft masking and so the idea of positioning two antennae are mandatory. However tail looking two antennae didn't suffer masking effect and so one is sufficient. In effect 5 antennae is good enough to have 360� coverage.

V Conclusion

The concept of having multiple antennas embedded as part of UAV structure for UAV communication system is studied and the advantages are brought out. A sample directional antenna performance as embedded inside the UAV wing leading edge is simulated and experimentally verified to be a promising candidate for structurally integrated antenna. The simulation results of antenna radiation perturbation for multiple embedded antenna for a PEC long-Ez aircraft are presented and minimum gain is shown to meet the requirements. This study also brought out the advantages of going for multiple high gain embedded antennae than single Omni directional antenna.

VI Reference

1. "Light weight agile beam antennas for UAVs", Wyman Williams, Chris Burton, EMS Technologies Inc.

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