Aperture coupled

Wide-Band Micro Strip Antenna Design

- Srivatsa Bhargava J (4610-510-081-05891) MTech , CEDT, IISc Bangalore.

Aim: Parametric Study, design and implementation of single patch, wide band Aperture coupled microstrip patch antenna for 2GHz frequency range.

Abstract: Microstrip patch antennas are well suited for integration in to many applications owing to their conformal nature. There are many wide banding techniques used for the MSAs. But many wide banding techniques such as using slots in the patch require an inductive coupled feed (probe feed). But complete planar (2D) processes for manufacturability needs capacitive coupling. Capacitive coupling with coplanar feed network has the drawback that the feed network interferes with the radiation properties of the antenna. So the coupling techniques with the feed lines in the plane other then the antenna are more suitable. Aperture coupled feed and proximity feed are to such feed techniques. Of these, aperture coupled feed which makes use of thick antenna substrates is the most convenient as it has only single ground plane. Apart from this aperture coupling provides a greater radiation pattern symmetry and greater ease of design for higher impedance band width owing to a large number of design parameters. In this type of feed by using multiple patches bandwidths up to 70% are reported(how ever only single patch design is attempted in this project).

So the parametric study, design, fabrication and testing of a with wide band aperture coupled antenna has been attempted in this project. And an aperture coupled antenna with 500 MHz band width was designed at 2GHz range thus realizing 25% band width.

Band width considerations:

In this project, 10 dB return loss band width is considered for design. The 10dB return loss corresponds to a VSWR of 2. Therefore the impedance circle for the antenna has to stay with in the VSWR =2 circle on the smith chart for the desired range of frequencies. So, the entire design problem thus boils down to identify the parameters of the ACMA geometry which control the impedance circle and tune the design parameters accordingly.

Design methodology:

The design procedure evolves from the analysis of the geometry. The ACMSA has been analyzed with different models such as transmission line model, modal expansion model, intergral equation model etc.., Of these transmission line model provides us with better intuition as to which dimension affects which parameter, but is inaccurate and the other two are more accurate and mathematically rigorous but provide us with very little design intuition. So the transmission line model of the ACMSAs was studied and is briefly outlined below:

An exploded view of a simple aperture coupled microstrip patch antenna is shown below:

The patch on the topmost substrate is the radiating element and the slot in the ground plane couples the energy from the microstrip feed line (beneath the bottom most substrate) to the patch. This transmission line model is applicable only when, dominant mode contribution of the patch is sufficient which is true for thin substrates.

The equivalent circuit model of the ACMSA is shown below:

The patch is modelled as to consisting of two finite radiating slots. The impedance provided by these slots is deduced from the infinite radiating slot model as:

Where G and B are for unit length.

So, the reflected impedance at the centre of the slot ie., the feed point can be calculated by transforming these impedances to the centre of the slot (since we generally couple to the patch through the aperture at the center) and adding the corresponding admittances.

The slot impedance describes the energy stored near the slot. It is inductive in nature since the slot is electically small(in case of non-resonant apertures. But if we use resonant aperture the slot length will be equal to nearly half of the guided wavelength. So Yap can be modelled as:

Where Yos is the characteristic impedance of the slot.

A transformer describes the coupling from the slot to patch. The turns ratio of the transformer model is as shown below:

Jo bessel’s function of 0th order

The final simplified input impedance can be deduced as given below:

From this the condition for resonance will be :

That is increasing length of the aperture requires a decrease in the patch suceptance and this decreases the resonant frequency. This has more effect on the function of the antenna if we are using a resonant aperture.

From this analysis the following qualitative conclusions can be drawn:

For non resonant slots if the slot length is small the patch is under coupled. And resonant resistance of the antenna is less that characteristic impedance of the feed line thus , as slot length increases the coupling to the line increases thus increasing resonant resistance and the size of the loop. The resonant resistance and coupling can be increased by decreasing the substrate thickness. Thus a wide range of resistance and reactance are possible by proper design of length and width of the aperture. Any additional antenna reactance can be cancelled out by properly choosing the length of the feed line stub extended from the centre feed point. So instead of directly using analysis results qualitative parametric study of the ACMSA is done and the results are used in the design.

Parametric study of Aperture coupled microstrip patch antenna:

There are two types of slots which can be used in the ground plane. They are resonant slots and non- resonant slots. There are nearly a dozen parameters in this aperture coupled antenna structure which decide the performance of the antenna. Thus these parameters provide us enough flexibility to get a wide

bandwidth. These parameters and their affects on the antenna performance are discussed below:

1) Antenna substrate dielectric constant:- This parameter affects the bandwidth of the antenna directly. The substrate’s loss tangent factor also has an impact on the radiation efficiency of the antenna. Lower the permittivity of the antenna substrate wider the impedance bandwidth and lesser the surface wave excitation.

2) Antenna substrate thickness:- This parameter directly affects the bandwidth and coupling level of the antenna. Thicker the substrate better the impedance bandwidth but lesser the coupling level. So this parameter design involves the trade off between the bandwidth and coupling between the feed line and the radiating patch for a given aperture size.

3) Microstrip patch length:- This decides the resonant frequency of the microstrip patch antenna.

4) Microstrip patch width :- The width of the antenna affects the resonant resistance of the antenna. Wider the patch lower the resistance. Square patches generate higher cross polarization levels and hence must be avoided unless circular/dual polarization is required.

5) Feed substrate dielectric constant:- This has to be selected to get good microstrip circuit qualities. Higher permittivity of the substrate leads to less leakage of the power out of the microstrip line due to resistance. So high permittivity of substrate is used for feed network and lower permittivity is used for antenna substrate. This feature( different permittivities for feed network and antenna) is possible only in structures using multiple planes such as aperture coupled feed, proximity feed but is not possible in coplanar feeds.

6) Feed substrate thickness:- Thinner micro strip substrates result in lesser spurious radiation from the feed lines.

7) Slot width:- This parameter affects the coupling level from the feed lines to the patch. Generally the ratio of the slot length to the slot width is kept typically as 10:1.

8) Slot length:- Coupling level is primarily decided by the slot length. Primarily two types of slots are used in ACMSA design they ate resonant and non resonant type based on the length of the slot. If the slot length

is comparable to the half of the wavelength of the antenna it si called as the resonant slot. If smaller length slots are used it is non resonant slot. If non-resonant slots are used the edges of the slot are elongated to get more uniform field distribution to get more coupling. For this slots of H, dog bone and hourglass shapes are used. For non resonant slots.

9) Feed line width:- this parameter decides the characteristic impedance of the feed line. So chosen to get the required Zo.

10) Feed line position with respect to slot:- For maximum coupling the feed line must be placed perpendicular to the centre of the slot. Skewing the feed from the slot will reduce the coupling.

11) Position of patch w.r.t slot :- For maximum coupling the patch should be centred over the slot. Moving the slot in H-field direction has little effect on the antenna performance but if it is moved in E-field it leads to reduction in coupling.

12) Tuning stub length :- Used to tune the excess reactance of slot. Typically is of length 0.5 times the guided wavelength. Shorter subs move the impedance circle downwards towards capacitive part of in smith chart.

Design:

Since the aim of the project is to use a single patch design. We have a resonant cavity due to patch and if we decide to use a non-resonant aperture to feed the power to it, we will be left with only one resonant cavity in the structure. And to get a wide bandwidth with this, we have to make its Q-factor very low ie., we have to introduce heavy losses in the cavity this introduces the trade off between coupling and bandwidth of the antenna.

So, to ease this trade off a resonant aperture shall be used in the design since it introduces another additional cavity. Thus, making it a two cavity structure. Now, the parameters of the cavities have to be designed and adjusted to stagger tune these two resonant cavities to get higher band width.

In the designed antenna, the patch resonance decides the higher frequency response in the band (ie., patch is the higher frequency resonant cavity )

and the aperture resonance decides the lower frequency band(ie., aperture is the lower frequency resonant cavity ). The thickness and permittivity of the antenna substrate decide the losses caused in the aperture cavity and thus decide its Q-factor. Based on the above discussed theory two designs one having slightly different ground structure are attempted both were simulated and the first one of them was fabricated. The design details and simulation results and measured results are presented below.

In both the designs air/foam(permittivity=1/1.07) has been used as antenna substrate with a thickness of 17mm. FR4 material(permittivity=4.4 and thickness=1.58mm loss tangent=0.0023) is used for feed line substrate and material with permittivity=2.5, thickness=1.58mm and loss tangent=0.0023 is used for the radome substrate of the antenna.

Design-I

The 3D view of the design is shown below followed by the layout of the design:

Layout:

Parameters:

Patch: Lp=48mm Wp=66mm patch substrate(radome): er=2.5 thickness=1.58mm loss tangent=0.0023

Aperture: La=40 Wa=7mm antenna substrate : er=1.07 thickness=17mm loss tangent=0.0009

Feed line: width=3mm stub length=5.5mm feed line substrate: er=4.4 thickness=1.58mm loss tangent=0.0009

Simulation results:

S11 vs frequncy plot

In the S11 versus frequency plot we can clerly see that there are two resonances happening. The 10dB impedance band width is seen to be from 1.751GHz to 2.24GHz thus yeilding 489MHz bandwidth amounting to 24.69% bandwidth.

This antenna structure was fabricated and tested as shown below:

The measured S11 versus frequency graph is as shown below:

It can be seen that here there is a shift in the 10dB impedance band and the bandwidth is from 1.842GHz to 2.299GHz thus yeilding 457MHz bandwidth amounting to 23% bandwidth. The minor deviation of the measured reults from the simulation results are due to fabrication tolerances.

The simulated input impedance circles of the antenna in the smith chart are shown. It can be observed that for the frequencies with in 10 dB impedance bandwidth the ipedance pints lie within the VSWR=2 circle. To increase the bandwidth one can introduce another cavity tuning which properly results in another loop within the VSWR=2 circle thus increasing the band width.

The simulation results of the radiation pattern at 2.1GHz are shown below:

It can be observed from the simulation plots that the gain of the antenna is 8.185dB and the front to back ratio is 8.185-(-5.251)=13.436dB

The measured radiation pattern at 2.1GHz is shown below:

The red colored trace is for co polarization radiation pattern and the blue one us for cross polarization radiation pattern. It can be seen that the designed antenna is a linearly polarized antenna.

The 3D view of the radiation patter is shown below:

The simulated gain of the antenna in the impedance bandwidth range is shown below:

The gain is seen to vary by nearly 1.3dB across the impedance bandwidth.

Some other important simulation results are presented below:

Design-II:

This design was simulated an it was observed that introducing additional slots in the ground plane has the affect of slightly shifting the band downwards with same patch dimensions thus increase the percentage bandwidth. The design details and simulation results are presented below:

Layout:

Patch: Lp=48mm Wp=66mm patch substrate(radome): er=2.5 thickness=1.58mm loss tangent=0.0023

Aperture: La=40 Wa=7mm antenna substrate : er=1.07 thickness=17mm loss tangent=0.0009

Feed line: width=3mm stub length=6mm feed line substrate: er=4.4 thickness=1.58mm loss tangent=0.0023

Additional slot dimensions: 15mmx15mm each

It can be observed that all the dimensions are same except the difference of 0.5mm in the stub length.

From the above figure it can be seen that the impedance band width is from 1.69GHz to 2.189GHz ie., 499MHz yielding 25.94% bandwidth with similar patch dimensions as above design.

A few other relavent observations are presented below:

Conclusions:

Aperture coupled microstrip patch antenna was designed for 25% bandwidth and was fabricated and was measured to have 23% band width. The design used foam substrate with 17mm thickness. The structure designed was only a two cavity structure but to increase the bandwidth further increase the number of resonant cavities in the structure which leads to other wide banding techniques such as design with stacked patches, slots on ground plane. Also making the s11 flat all through the band helps in having uniform gain all over the band. Hence along with the presently designed structure if additional resonant cavities are added it leads to significant improvement in the band width.

References:

1) Antenna theory analysis and design—C A Balanis 2) Microstrip antenna design handbook—Ramesh Garg,Prakash Bhartia,

Inder Bahl, Apisak Ittipiboon. 3) Broadband Microstrip antennas—Girish Kumar, K.P Ray 4) Analysis of an aperture Coupled Microstrip Antenna-- Peter.L.Sullivan,

Daniel H.Schaubert 5) Large Bandwidth Aperture Coupled Microstrip antenna—F Croq, A

Papiernik 6) Broadband T-shaped microstrip-fed U-slot coupled patch antenna—

Y.W.Jang. 7) A Review of Aperture Coupled Microstrip antennas —Prof.D.M Pozar.