Progress In Electromagnetics Research Symposium Proceedings, Moscow, Russia, August 19–23, 2012 239

Bended Rectangular Slotted Waveguide Antenna

Dana Baz Radwan1, Ali Harmouch2, and Mustapha Ziade2

1American University of Science & Technology, Beirut, Lebanon2Faculty of Engineering, Lebanese University, Lebanon

Abstract— Rectangular slotted waveguide antennas have been widely used in many areasof telecommunications from plane antennas to wireless internet applications due to their highdirectivity, compactness and feasibility. One of the major drawbacks of such antennas is thatthe side lobe level is significant since the slots are placed on a finite conducting plate whichconsequently leads to a decrease in the overall directivity of such radiators. In this paper, abended slotted rectangular waveguide antenna is presented. The antenna provides a narrowbeamwidth in addition to a significant decrease in the Sidelobe level. FEKO Simulations andexperimental tests performed on bended slotted waveguide antenna have shown a significantimprovement in the directional characteristics for about 4 dB without any increase in the overallantenna’s dimensions.

1. INTRODUCTION

Spreading information over large or small areas is now human’s concern all over the world. The mostimportant means implemented in the information is the wireless communication system. Wirelesscommunication continues to present challenges to designers, in particular in the area of slottedwaveguide antennas design. Slotted Waveguides Antennas, SWA are one of the competent networkapplications due to their compactness, their electrical and their mechanical characteristics thatmake them easy to work with, and flexible to changes that might be made on them. They canwork in different ways with respect to their frequencies of operation, sizes, and their number ofslots. These waveguides are typically evaluated according to their radiation pattern in the E and Hfield which relate to their directivity [1, 6]. The major drawback of rectangular slotted waveguideantennas is their reduced directivity; in addition their side lobe levels are usually higher due to thefinite dimension of the waveguide conductive surface over which the slots are placed.

Antenna terminals are one of the basic components of wireless communication system. Throughdifferent antennas’ terminals information is transmitted and received. These antennas radiate theelectromagnetic wave towards a designated receiver with maximum radiation efficiency; such aprocess requires the designed antenna to have advanced electrical and mechanical characteristicssuch as high directivity, radiation efficiency, high wind resistance and preferably low cost andweight.

The above mentioned requirements are to be taken into consideration, since most of the alreadyexisting antennas in the wireless market either are comprised of all these characteristics or promoteone characteristic over another; a fact that limits the usage of these antennas as radiating elementsin wireless networks. For instance, corner reflector and other types of directive antennas havebeen used as transmitting elements of wireless networks for many years since they provide highenough directivity (up to 10 dB) over the frequency band; however, the finite size of the conductingreflector compromises the antenna’s wind resistance and also results in a high level of side lobeswhich negatively affect antennas’ performance. Parabolic reflector antennas are also used for thesame purpose and have almost the same drawbacks, except for their very high directivity [3, 4].

2. SLOTTED WAVEGUIDE ANTENNAS

Slotted waveguide antennas have also been used in the market in many different applications as anantenna for aircrafts due to their compactness, which fits well with the aerodynamic characteristicsof flying objects. [x] Recently these antennas have also become a better choice as transmittingelements in network applications since they offer extra high directivity, compactness, high windresistance and consequently excellent reliability. The most commonly used shapes of slotted waveg-uide antennas are rectangular and circular configurations. The circular one is widely used formilitary applications when designing missiles, submarines and space shuttles because of its suit-able geometry. Rectangular waveguides are hollow-pipes that consist of conducting walls, filledby dielectric material within which the electromagnetic wave propagates. The upper wall of theguide consists of a finite number of slots that are arranged together in a specific way to provide a

240 PIERS Proceedings, Moscow, Russia, August 19–23, 2012

desirable radiation from the slots. The number, shape, size and position of the slots indicate theperformance of this antenna; therefore these characteristics are selected according to the specificsituation under consideration [1, 2].

The major drawback of rectangular waveguide antennas is that the conducting wall on whichthe slots are placed is finite, which causes the side lobe levels to increase and consequently reducingdirectivity. The reduction of the antenna’s directivity causes interference problems with neighboringantennas. In spite of all the aforementioned disadvantages, waveguide antennas are the mostsuitable for wireless applications among others [8, 11, 12].

The aim of this study is to enhance the electrical and mechanical attributes of the already exist-ing waveguide antennas by simply changing its form to a new bended form that electromagneticallyprovides an in-phase addition of waves radiated by the slots in the far-field. Such a modification isexpected to result in an increase in the directional properties, and simultaneously, in a decrease inthe side lobe levels.

The increase in the directivity makes the antenna capable of transmitting the same electromag-netic energy for longer distances and consequently enhancing their area of coverage. In addition,the decrease of the side lobe levels makes the antenna less vulnerable to noise sources produced byadjacent radio stations operating in a close area or at a close carrier frequency [6]. The bendedrectangular waveguide antenna offers a broadside radiation pattern over the frequency band whichis normal to the plane containing the slots. The angle of bending which was selected theoretically,within the interval of 2◦ to 15◦, shows a significant change in the radiation pattern. Simulationand experimental results have shown that an angle bending of 3◦, provides an increase of 3 dB inthe directivity compared to the regular slotted waveguide antenna. This increase in the directivityis accompanied by a decrease of the side lobe level which makes the designed antenna competitivein the antenna market.

3. ANTENNA DESIGN

3.1. The Straight Waveguide

In this part, we show the different parts of the antenna design, including the description of theFEKO program components that we used. First we performed our calculations using free spaceinside the waveguide in order to determine its dimensions for a 2.4GHz center frequency. The maingoal is to achieve a gain of 15 dB. The calculations used in this paper are as follows [1]:

In order to calculate the wavelength:

λ = v/f =(3× 109

)/

(2.4× 109

)= 0.125m

d = λ/2 = 0.125/2 = 0.0625ma = d = 0.0625mb = 0.7× a = 0.7× 0.0625 m = 0.0437m

where: a is the height of the waveguide and b is its width.The experiment’s desired directivity was 15 dB, which allowed the number of slots N to be

calculated.N = (D × λ)/(2× d) =

(100.5 × 0.125

)/(2× 0.0625) = 31.622

Finally, the Length of the waveguide was determined as follow:

N × d = 31.622× 0.0625 = 1.97m

According to the above calculations we had a waveguide with wavelength of 0.125m, width of0.0625m, height of 0.0437m, 31 slots, and a length of 1.97m which is obviously considered asa long antenna. In order to reduce the dimensions of this antenna, we considered changing thesubstance inside the waveguide and substitute it with a higher permittivity material such as glass.Glass has an effective permittivity, εeff ≈ 6.

The new calculations became:

v = c/√

(εµ) = 3× 109/√

6 = 1.224× 109

Progress In Electromagnetics Research Symposium Proceedings, Moscow, Russia, August 19–23, 2012 241

In order to calculate the wavelength v was used:

λ = v/f = 1.224× 109/2.4× 109 = 0.051md = λ/2 = 0.051/2 = 0.0255ma = d = 0.0255b = 0.7× a = 0.7× 0.0255 = 0.01785m

With a desired directivity of 15 dB, this allowed for the number of slots to be again calculated:

N = (d× λ)/(2× d) = 1015 × 0.051/2× 00.0255 = 31.622

The length of the waveguide is then calculated:

N × d = 31.622× 0.0255 = 0.806 m

According to the new calculations we had wavelength of 0.05 m, width of 0.0255m, height of0.01785m, 31 slots (Length of each slot is λ/2 = 0.0255 and the width is λ/16 = 0.003187), and alength of 0.806m. These results are much better considering the size of the antenna, our dimensionsdecreased by more than one half of the previous dimensions.

After using these dimensions and simulating the designed antenna at the operating frequencyon FEKO we obtained the radiation pattern in both planes from which the directivity has beencalculated and summarized in the table below.

3.2. The Bended Waveguide

After reaching a good size for the antenna, we started to search for ways to get a better performancein increasing directivity and decreasing side lobes. The idea was by bending the antenna at differentdegrees and checking whether bending gives better results or not. Starting our experiment we chose2 degrees, doing the same procedure that we did in the first experiment by putting the model onFEKO and simulating it getting new results concerning wave pattern, directivity, and side lobes.As results we get much better results see Table 1. Having performance made us proceed withour choices in order to get the best performance that we can get between the different bendings.Moreover, we took the 3, 4, 5, 8, 10, and 15 degrees, simulate every object alone and observe itsresult see Table 1.

As you can see in the table above concerning the different bendings, the results showed improve-ment on the straight waveguide. The directivity improved then started degrading as we increasedthe bending angle. We stopped our trials when we saw that the result is decreasing to the firstresult that is the straight waveguide. We show the results of the best design with 3◦ bending. Weobtained the best directivity, wave pattern, and by that the smaller side lobes is the three degreesbending Figure 2. And later on this is the bending that we used in our practical experiments. As aconclusion bending the waveguide by 3◦, gave us a better results as by this tiny bending we madethe slots have similar distance from destination in order to direct the wave in a better way.

Figure 1: Rectangularstraight slotted waveguide.

Figure 2: 3-D pattern of thethree degrees bended waveg-uide antenna.

Figure 3: 3-D image for the horizontal, verti-cal and overall patterns of −3 degrees bend-ing.

242 PIERS Proceedings, Moscow, Russia, August 19–23, 2012

Table 1: Straight and bended waveguides.

Waveguide Vertical pattern, degrees Horizontal pattern, degrees Directivity, dBStraight 40 90 10.5642 degrees 45 56.5 12.0753 degrees 44 55 12.2894 degrees 46 63 11.0755 degrees 45 56 12.1138 degrees 53 61 11.03110 degrees 57 62 10.64515 degrees 55 65 10.595

Table 2: Straight and opposite bended waveguide.

Waveguide Vertical pattern, degrees Horizontal pattern, degrees Directivity, dBStraight 40 90 10.564−2 degrees 45.5 56 12.065−3 degrees 46 55 12.096−5 degrees 46 56 12.018

3.3. Opposite Bended Waveguide

As the third application that we wanted to apply here and since our waveguide is meant to be usedon airplanes and as we all know that the bottom of the airplane takes a belly shape, but as wewant to put the waveguide in it, the slots should be directed to the bottom toward earth, so in theshape that we have, we will face a problem. However, we reached to an idea to bend the waveguidein the opposite way and check if we will lose our wave or if we have a slight difference.

Looking at the previous results we chose the best three results in order to use them for the newexperiment. We took the 2, 3, and 5, and recalculated them when bent in the opposite direction sowe get −2, −3, and −5 and simulate them on FEKO. The results are shown below in the Table 2.

As a conclusion, waveguides bent at three degrees and minus three degrees gave the best resultsfor the two applications, with the best directivity and the smallest side lobes. The new bended formelectromagnetically provides an in-phase addition of waves radiated by the slots in the far-field.The increase in the directivity makes the antenna able to transmit electromagnetic energy for longdistances and therefore enhances its ability to provide a bigger area of coverage. In addition, thedecrease of the side lobe level makes the antenna less vulnerable to noise and interference.

Figure 3 shows the FEKO results obtained on the minus three degrees bent waveguide slottedantenna.

4. EXPERIMENTAL RESULTS

Three copper waveguides, one straight, one bended to the inside and a one bended to the outsidewere built in the dimensions calculated and simulated earlier.

Two of the waveguides were bended; one to the inside and the other to the outside at an angle of3 degrees. After the coaxial cable was fixed in its proper place, the waveguide was filled with silicon.As previously mentioned, the waveguide was filled with silicon because it was then impossible, usingthe tools available, to fill the waveguide with solid glass or melted glass, as this procedure requiredadvanced manufacturing tools. Also for the bent antennas it was impossible to use solid glass,which is why silicon was used since silicon quickly changes to a solid substance after it is melted.At this point the manufacturing step was completed; the next step was to perform the testing.

Figure 4 shows the test setup that we used to test the antenna at 2.4 GHz. We used a transmitterand a spectrum analyzer. The measured antenna was positioned as a receiver. Blue absorbentsponges containing carbon were positioned around the antenna to eliminate unwanted reflections.

The first consideration was that since the waveguides were filled with silicone instead of glass,the relative permittivity would be different from that of the initial calculation. If the relativepermittivity is now lower then according to the calculation a higher frequency would be needed,

Progress In Electromagnetics Research Symposium Proceedings, Moscow, Russia, August 19–23, 2012 243

Cable

Spectrum analyzer

TransmitterAntenna

Figure 4: Test setup.

Figure 5: Antenna patterns of each antenna plotted separately.

as by using the previous frequency (2.4 GHz) with silicon (which is expected to have a lowerrelative permittivity) the waveguide would be excited below the cutoff frequency, which is impossibleand would thus, never yield good results. The relative permittivity for silicone was researched,and silicone was found to have different types with different relative permittivity. Furthermorewaveguides follow the reciprocity theory, which states that as the frequency varies, the wave patternwill remain constant relative to itself, although its size will change [1].

Waveguides are excited according to the cutoff frequency, so as long as they are excited above thecutoff frequency then the wave propagation and the differences between one antenna and anothercan be seen. This is the point of this experiment, which is designed to verify if the bent waveguidewill have a better performance than a straight waveguide. As long as the performance can bemeasured then a comparison can be made.

Measurements were taken using software called “Cassy” which reads the wave propagationevery half degree. The trial was started by giving an order to the software with all the necessaryconditions, at which point the automatic rotator began to rotate and the software began recordingthe results. This was repeated for every antenna and graphs were created and shown in the figuresbelow. The bent waveguide gave the best results; the side lobes of the bent waveguide had thelowest values. The values for directivity for each antenna were plotted separately in Figure 5.

As mentioned before the results showed the wave propagation for every half degree, so in orderto have a clear graph the results of every two, four and ten degrees were taken. Normalizationwas applied for every group of results (ranging from 0 to 360 degrees), where every ten degreeswave propagation showed the clearest figures for the three antennas, in addition to highlighting thedifferences among them.

The graphs of the radiation pattern were superimposed on the same figure to give a clear viewof the three waves and how the antenna bent to the inside had the best results, than the one bentto the outside, and last the straight waveguide. This is what was expected during the simulationphase. The waveguide which was bent to the inside had the best results among the three antennasFigure 6 and Figure 7 give a clear explanation of these results. Here the results were normalized inorder to allow an easy comparison of the performance of the best antenna. All the results on everyangle were normalized to 1 in such that the highest level is equal one and all the other points were

244 PIERS Proceedings, Moscow, Russia, August 19–23, 2012

Figure 6: Comparative graphs for the three anten-nas.

Figure 7: Directivity comparison.

below one; by this the main lobe can be clearly seen as well as the side lobes on both sides. Theangles are shown from −200 to 200 because on the rotator this is how the angles are distributed(from 0 to 180◦ and from 0 to −180◦. Since the values are taken in V , the following formula wasused in order to normalize:

VdB = 20 log V log10 V = VdB/20 ⇒ V = 10VdB/20

This formula gave a set of values of V , then the maximum value was taken and all the values weredivided by this number to achieve normalization. This formula was repeated on all three sets inorder to have the three peaks on one graph, so Figure 6 shows the pattern of each antenna at everyangle for a clear comparison. This procedure was performed not only to show where the side lobesbegan and ended, but especially to show that the directivity increases for the bended waveguides.The simulation indicated that the directivity increases as the angle of the wave propagation de-creases. This was also demonstrated through the practical results. Figure 7 depicts the angle vs.the radiated energy from the designed antenna. The blue graph shows the pattern of the waveguidewhich was bent inward by three degrees, the green graph shows the wave propagation waveguidewhich was bent outward by minus three degrees, and the red graph shows the wave propagation ofthe straight waveguide. The graph depicts a point for comparison (black line) with lines indicatingthe directivity of each angle for each waveguide (blue, green, red lines). The figure shows that theblue graph has the smallest half-power beamwidth, then the green graph, and then the red graph.The rule of directivity used in this article is calculated according to [1, 6]:

D = 41000/θE × θH

According to the figure above the half power beamwidth of the antenna bent inward by threedegrees (blue) is the smallest which means the directivity is the highest. In addition the side lobesare the lowest also in the blue graph, which means the side lobes decreased. This was a betterresult than in the simulation, where the side lobes did not decrease to that extent, meaning thatthe practical experiment gave better results concerning the side lobes.

5. CONCLUSION

Waveguides are very wide range antennas and commonly used in the market. They are flexible touse and work on a wide range of frequencies. Due to the reciprocity theory, changes are repeatedwhen the frequency is varied. Waveguide antennas are used in many applications such as for theinternet, airplanes, and military devices.

In this work, we combined waveguide theory and dish parabolic theory to bend the antenna atvarious angles, and determine the optimum bend that provides best directivity and lower sidelobes.This bend represent a situation where the distances from the slots to the receiver are nearly equal.

Moreover FEKO simulations found that using a 3 degree angle gave the best directivity, reaching3 dB, the simulation results were supported by actual measurements at 2.4GHz. The reduction ofthe waveguide dimensions were achieved by filling the waveguide with silicone.

Finally we demonstrated that bending the waveguide by three degrees inward or three degreesoutwards provides higher directivity without affecting negatively the cost of the antenna system.

Progress In Electromagnetics Research Symposium Proceedings, Moscow, Russia, August 19–23, 2012 245

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