# Microstrip heatsink antenna cooled with sapphire layer fo integrated RF transmitters

Atef Al Nukari, Yann Mahe, Serge Toutain, Yves Scudeller

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Atef Al Nukari, Yann Mahe, Serge Toutain, Yves Scudeller. Microstrip heatsink antenna cooled with sapphire layer fo integrated RF transmitters. 6th Europen Conference on Antennas and Propagation, Mar 2012, Prague, Czech Republic. pp.EuCAP 2012, A01.3. [hal-00663519](https://hal.science/hal-00663519v1)

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https://hal.science/hal-00663519v1

Submitted on 19 Jan 2026

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# Microstrip Heatsink Antenna Cooled with Sapphire Layer for Integrated RF Transmitters

A. Alnukari, Y. Mahe, S. Toutain

Lunam Université - Université de Nantes, UMR CNRS 6164
IETR, Ecole polytechnique, La Chantrerie, Rue Christian
Pauc, BP 50609, 44306 Nantes Cedex 3, France

yann.mahe@univ nantes.fr

Y. Scudeller

Lunam Université - Université de Nantes
Institut des Matériaux Jean Rouxel-CNRS, Ecole
Polytechnique, La Chantrerie, Rue Christian Pauc, BP

50609, 44306 Nantes Cedex 3, France

***Abstract*** This paper reports the design and performance of Sapphire-based active heatsink antenna for Integrated RF transmitter. The antenna consists of a high-thermal-conductivity Sapphire layer serving as electromagnetic emitter and heatsink. We present the topology of such antenna as its electromagnetic and thermal performance. The antenna has been investigated by simulations and measurements. It is evidenced that the Sapphire layer has no substantial influences on the gain, radiation pattern and return loss of the antenna. Heat removed by the Sapphire layer was found as high as 1 Watt per centimeter square while the thermal resistance of the transmitter was of 5.5 °C/W. The transmitter could operate over a wide temperature range.

***Keywords*** RF device, RF transmitters, heatsink antenna, thermal analysis

## I. INTRODUCTION

RF front ends must be miniaturized to create high data bit rate terminals and provide numerous accesses to multimedia services. In recent years, new integration concepts using Low Temperature Co-fired Components (LTCC) [1] and System on Package [2] (SoP) have been proposed. Such concepts are becoming more promising solutions to develop the future miniaturized communication modules. But more compactness introduces not only electromagnetic but also thermal problems which have to be taken into account. So, generally, thermal management plays an essential role in the design of Microwave and RF devices.

This paper reports the design and performance of Sapphire-based microstrip heatsink antennas for RF power transmitter. The different options for dissipating heat in a device can be classified into two main categories: active and passive solutions. We have already proposed a solution of heatsink antenna that can be classified into the first category since it is necessary to force the circulation of a coolant fluid [3]. This solution, although with promising performance, is not usable in all configurations such as in the case of the transmitter module closest to the antenna of a base station. It is also interesting to propose alternative passive solutions. These solutions are generally based on the use of large surfaces to maximize heat exchange with the environment. This exchange surface in the case of integrated transmitter modules is limited to the antenna surface in direct contact with the surrounding

air. Less the antenna surface, less natural convection with air. This is the main limited factor of this solution. Such a concept has been previously validated with a patch antenna built "on air" [4]. Thickness and diameter of the heat spreader were 10 mm. Dimensions of the antenna were 67 x 54 mm² in length and width for a 2GHz working frequency. The wire-fed patch heatsink antenna can carry away 0.22 Watt per centimeter square of antenna as thermal power with power transistor maximum temperature of 80°C.

In multilayer technology, it is possible to consider a high thermal conductivity dielectric substrate as a radome for the antenna. In this case, this layer is designed to carry away heat to be dissipated by the device package. This concept is particularly interesting in embedded transmission module to increase the heat exchange surface. The use of substrates such like aluminum nitride (AlN), Sapphire or even diamond, given their excellent thermal conductivity properties may be considered. Also, heatsink antenna prototype using Sapphire layer has been developed and its performance discussed.

## II. TOPOLOGY OF MICROSTRIP HEATSINK ANTENNA COOLED WITH SAPPHIRE LAYER

A description of the transmitter module is shown in Figure 1. The structure incorporates a solid conducting cylinder acting as a heat sink that connects the patch antenna to the "hot spot" constituted by the power transistor which is used as the power amplifier. This heat sink is placed in the symmetry plane of the patch antenna, in order to minimize the effect of this metallic post on the electromagnetic fields distribution.

<figure><img src="image_2.png" /><figcaption>Figure 1. Topology of the heatsink antenna cooled with Sapphire layer</figcaption></figure>
Thus the antenna acts as a cooler attached to the power amplifier by carrying away the dissipated power and improving heat exchange with the surrounding medium using antenna surface. If the size of the heat spreader become quite large compared to the wavelength then electromagnetic performance of the antenna (gain, radiation pattern) could be affected. Otherwise the size of the heat spreader defines the operating frequency. The Sapphire layer is placed on the top side of the antenna. Such a layer can create an efficient thermal path towards the package where an added cooling system can dissipate the heat. Dimensions of the 2GHz patch antenna realized on a FR4 substrate are 33.9×27.12mm² in length and width with a 500 µm layer thickness substrate. This layer acts as a “radome” which can be optimized in thickness in order not to disturb the electromagnetic performances of the antenna. Investigated simulations show that there is no significant influence of layer thickness (between 40 and 1000 µm) on the gain of such an antenna. The relative permittivity of Sapphire is taken as 8.9.

### III. PERFORMANCE ANALYSIS

#### A. Simulation

Figure 2 shows the variation of S11 versus frequency and the gain of such an antenna at its working frequency (here 2GHz), as simulated on HFSS.

<figure><img src="image_3.png" /><figcaption>Figure 2. (a) Reflection coefficient S11 as a function of the frequency, (b) Gain at 2GHz operating frequency, for microstrip heatsink antenna cooled by a layer of Sapphire, as simulated</figcaption></figure>

Here also we can observe that the presence of the Sapphire don't strongly modify the performance of the antenna. Indeed, the relative permittivity of Sapphire ($ε_r$ = 10) different will modify resonance frequencies, but this influence can be easily taken into account by adjusting the antenna length to match the

operating frequency. A second resonant frequency was additionally observed at 1.36 GHz corresponding to wire-patch configuration [5], and it is possible to maintain the fundamental resonance at 2GHz. The spatial gain is very few disturbed by the presence of the Sapphire and the maximum gain is in the same order of magnitude of 4 dBi.

Figure 3 shows the temperature distribution on the structure presented in Figure 1, as simulated on COMSOL Multiphysics. The temperature of the package case is taken as 20°C. The thermal power removed microstrip heatsink antenna by Sapphire layer must be less than 9 Watt to ensure a maximum temperature of 80°C into the power component. The thermal efficiency of the proposed heatsink antenna is about 0.98 Watt per centimeter square which is quite close from the efficiency of the active heatsink antenna [3].

<figure><img src="image_4.png" /><figcaption>Figure 3. Temperature distribution of Microstrip Heatsink Antenna cooled with Sapphire layer, as simulated</figcaption></figure>

#### B. Measurements

A prototype of the heatsink antenna cooled with a layer of high thermal conductivity (Sapphire) was fabricated as the structure presented in Figure 4. The Sapphire layer is placed on the top side of the antenna using silver lacquer to ensure high efficiency thermal contact with the top plate of the patch antenna. The antenna is associated with the heat spreader (metal cylinder) by tin-plumb solder. In addition, a thin wire with a resistance of 18 Ω is wound around the cylinder (thermal resistance) to simulate the heat source. To obtain a thermal power of 9W, the wire is fed by a voltage $V = \sqrt{P} \times R = \sqrt{9 \times 18} = 12.73$ V.

<figure><img src="image_5.png" /><figcaption>Figure 4. Microstrip heatsink antenna cooled with Sapphire layer, (a) top view, (b) bottom view, as fabricated</figcaption></figure>
Figure 5 shows the reflection coefficients $S_{11}$ as a function of the frequency, measured for the microstrip heatsink antenna with and without Sapphire layer. The resonant frequencies are slightly shifted. This can be explained by the fact that in simulation, the relative permittivity of Sapphire is considered as $ε_r = 10$, while in reality, the Sapphire is an anisotropic material with different permittivities in two perpendicular directions. It is of the order of 9.4 in perpendicular and 11.6 in parallel [6]. However, these resonance frequencies can be easily repositioned by adjusting the length of the upper plate of the antenna. The measurement results (Figure 5) are consistent with the simulation (Figure 2.a), there are however, additional losses on the frequency response, associated with the presence of the Sapphire layer. This can be caused by the imperfect contact between the patch antenna and the Sapphire plate in the practical realization.

<figure><img src="image_6.png" /><figcaption>Figure 5. Reflection coefficient $S_{11}$ of the antenna as a function of the frequency, (O) with Sapphire, (V) without Sapphire, as measured</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>Figure 6. (a) Patch antenna radome by Sapphire layer under test, (b) Measurement Setup</figcaption></figure>

A metal stand was especially fabricated and connected to the edges of the antenna board using silver lacquer. This support is then placed on a metal plate cooled by water at fixed temperature (20° C), circulating in metal plate. This is done using a temperature controlled water-pump, as shown Figure 6. The metal plate and the water-pump constitute the cooling system. The measurements are then realized using infrared imaging system and then are compared with simulations. The measurement setup used to evaluate thermal performance of the microstrip heatsink antenna cooled with Sapphire layer and to confirm simulations is shown in Figure 6. It consists of power supply to feed thermal resistance (heat source) and the infrared imaging system.

Figure 7 shows the temperature distribution on the patch antenna for a thermal dissipation power of 9W, first without the layer of Sapphire (Figure 7.a) and then with the layer of Sapphire (Figure 7.b). The temperature of the used water thus that of the metal stand is 20° C while the ambient temperature is about 26° C.

<figure><img src="image_8.png" /><figcaption>Figure 7. Temperature distribution on patch heatsink antenna connected to metal stand has temperature of 20°C for dissipated thermal power of 9 W. (a) antenna without Sapphire layer, (b) antenna with Sapphire layer, as measured</figcaption></figure>

Figure 7 shows that the maximum temperature on the antenna without a layer of Sapphire is near to 140° C, while the maximum temperature on the antenna with Sapphire layer is near to 80° C. This means that an improvement of about 60° C is achieved using the layer of Sapphire. The total thermal resistance of the transmitter is done as in
$$R_{total} = \frac{\Delta T}{Q} = \frac{T_{max} - T_{substrate}}{Q}. \qquad (1)$$
$$\alpha + \beta = \chi. \qquad (1)$$

Where Q is the thermal power, $T_{max}$ $T_{substrate}$ are the maximum temperature on the antenna and the temperature of the substrate measured for a given thermal power Q.

The thermal power removed by the thermally active patch antenna cooled with a layer of Sapphire is 9 W for a maximum temperature of 80° C at the antenna and therefore into the power component which is connected to the antenna. This result is consistent with that obtained by simulation. Measurements show that the antenna is capable of removing a thermal power as high as 1 Watt per centimeter square. The total thermal resistance of the transmitter without Sapphire layer is near to 11.2 ° C/W, while the total thermal resistance of the transmitter with Sapphire layer is near to 5.5 ° C/W. The transmitter can thus operate over a wide temperature range without any additional cooling device.

## IV. CONCLUSION

In this paper the topology of an active microstrip heatsink antenna cooled with Sapphire layer is presented. No major influences on the electromagnetic performances of the antenna (gain, radiation pattern and return loss) are consequently observed by the presence of different heatsinks, while a higher thermal efficiency is observed to remove thermal dissipated power by promoting heat exchange with the surrounding medium. This is directly linked to the choice we've made placing the heat spreader, combining the power component with the antenna, in a zone where the amplitude of electric field

is weak. The thermal power removed through the microstrip heatsink cooled with Sapphire layer is as high as 1 Watt per centimeter square.

This new topology of thermally active microstrip heatsink antenna cooled with dielectric layer has high thermal conductivity, presents very good heat dissipation capacity associated with a greater compactness, similar to the topology of thermally active microstrip heatsink antenna cooled with fluid. However, this technique is a passive one because it does not require the use of forced circulation of a heat transfer fluid in the module. Otherwise, an external cooling system connected to the package case of the module is required.

For instance, it will be possible to integrate 10 W power amplifier with PAE of 50% using only such antennas, which is the double power and compact factor compared to [4].

## REFERENCES

[1] S. Pinel and al, "3D integrated LTCC module using µBGA technology for compact C Band Front end module" MTT S Symposium 2002, Vol. 3, pp. 1553 1556.

[2] Rao. R. Tummala and al, "Gigabit wireless: system on package technology", Proc. IEEE, vol.92, N°2, Feb.2004, pp.376 384.

[3] A.ALNUKARI, S.TOUTAIN, Y.SCUDELLER, Ph.GUILLEMET, "Microstrip heatsink antenna for integrated RF transmitters", European Conference on Antennas & Propagation, EuCAP'2010, Barcelone, 12 16 Avril 2010.

[4] A. Alnukari, P. Guillemet, Y. Scudeller and S. Toutain, "Active heatsink antenna for radiofrequency transmitter", IEEE Transactions on Advanced Packaging, Vol.33, Issue.1, pp.139 146, February 2010.

[5] Ch. Delayeaud, B. Jecko, "Antenne fil plaque monopolaire," European Patent N° 94 926 276.0, September 1994.

[6] Jerzy Krupka, Krzysztof Derzakowski, Michael Tobar, John Hartnett and Richard G Geyer, "Complex permittivity of some ultralow loss dielectric crystals at cryogenic temperatures", Meas. Sci. Technol., Vol.10, No.8, pp.387 392, February 1999.