This issue's contribution to the “Prac-tical Projects” series describes the suc-cessful development of a patch an-tenna for the ISM band at 5.8GHz. Itcan replace the stalk antenna of thestandard ISM module in commercialuse.

1.Scope of the project

A patch antenna for the ISM band at5.8GHz was developed for monitoringvideos 2 storeys away in the new Tet-tnanger Electronic Museum. Since noholes could be bored and no slots couldbe cut out for cables in the historicmuseum building, we simply used thewall of the house opposite as the reflec-tor for the 5.8GHz signal, the patchantenna was aligned with it. To give theresult away in advance, it functioned aswell as we had hoped.

2.Parameters for the antenna

Look for the frequency diagram for the5.8GHz ISM band (Industrial ScientificMedicine Band) on the Internet, and youwill find the following information:

The 5.8GHz ISM band contains 16 chan-nels at 9MHz intervals. Channel 1 has acentre frequency of 5732MHz, and chan-nel 16 has a frequency of 5867MHz.The antenna thus has to display a selfresonant frequency of 5800MHz and abandwidth of approximately 140MHz(2.4%). Let the input resistance be 50Ω,with a semi-rigid cable having a solderedSMA plug. The circuit board should bemade from Rogers R04003, which is avery stable material from the mechanicalpoint of view and is easy to machine. Itshould have the following data:

• εr = 3.38

• Printed circuit board thickness = 32MIL = 0.813mm

• Dieleectric loss factor = 0.001

• Copper coating = 35 micrometresA λ/4 line transforms the antenna radia-tion resistance to the required 50Ω.Thecircuit board size should be 50mm x50mm and so we also need a short 50Ωmicrostrip from the transformation line tothe cable connection on the circuit board.The finished product can be seen in Fig.1 and makes the practical format selectedclear.

Gunthard Kraus, DG8GB

Practical Project: A patchantenna for 5.8GHz

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3.Development process

3.1. Design procedureThe basic principles for the procedurecan be found in an article on the subjectfrom VHF Communications [1], [2]. Butfirst, using the Internet, you shoulddownload the parameters of the 5.8GHzISM band:16 channels at intervals of 9MHz. Chan-nel 1 has a centre frequency of5732MHz, Channel 16 has a centre fre-quency of 5867MHz.The antenna should thus have a naturalfrequency of 5800MHz and a bandwidthof approximately 140MHz. This gives a

relative bandwidth of 2.4%. So we did alittle experimenting with the “Patch16”program from the Internet, and producedan initial design with the following char-acteristics:The centre frequency is exactly5800MHz, while the bandwidth has de-liberately been somewhat increased andestablished at 2.9%.All pre-set or calculated characteristics ofthe antenna can be seen in Figs. 2 and 3.The values that are required for thesubsequent work using PUFF must firstbe converted from inches into millime-tres:

• Patch width = 20.32mm

• Patch length = 13.39mm

• Total radiation resistance = 142.4Ω,which gives 284.8Ω on each patchedge.

Now we can use a text editor to open thesetup file of PUFF2.1, to enter theR04003 material and circuit board data.Then PUFF is started up (work with theprotected mode version by loading puff-p.exe) and the patch is modelled as alarge width, lossy transmission line. Thetwo radiation resistances positioned onthe two patch edges and the centrefrequency is set at 5.8GHz.And this is what we do next:

• First the exclamation mark is omittedafter the entry “Tl” in field F3, andwe experiment with the characteristicimpedance of the line until (once the

Fig 1: The finished patch antenna for5.8GHz.

Fig 2: The input parameters for"Patch 16".

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equals sign has been entered) a widthof w = 20.32mm is set.

• Now the exclamation mark isreplaced, and we vary the electricallength of the line until theconfiguration is as near as possible toresonance. This situation can easilybe recognised, since then the phaseangle of S11 is exactly zero degrees.But please select a swept frequencyrange as small as possible, and alsotry to obtain the highest possibleresolution regarding the amplituderange for |S11|.

The process is clarified in Fig. 4, whichimmediately supplies the required data:For a patch width of 20.32mm, we need amicrostrip line with a characteristic im-pedance of 7.43Ω at low frequencies. Amechanical length of 14.36mm thengives exactly λ/2 as the electrical lengthat 5.8GHz.Note:If you're surprised by the big differencebetween this length value of 14.36mmand the “Patch16” suggestion of L =13.39mm, let me clear up the mystery.

Fig 3: The simulation results for the patch antenna.

Fig 4:Simulation ofthe patch antennausing PUFF.

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First the patch has to be shortened by theopen-end extension on both sides. Wellcome back to this subject later, but wecan give the result in advance: it is0.41mm on each side. So now we haveonly 14.36mm - 2 x 0.41mm = 13.53mm.And if we then pay heed to a propositiondiscovered by chance in the specialistliterature: ”....the difference between thepatch resonance and the electrical lengthfor the corresponding λ/2 microstrip isabout 1%...”, then we get 0.99 x13.53mm = 13.40mm.Now, with the help of a λ/4 transforma-tion line, we have to bring the inputresistance of the configuration to pre-cisely 50Ω. In Fig. 5, this has alreadyhappened, and the required line data are:

• Line length = 8.18mm

• Line width = 0.765mmIf we now also provide the configurationwith a 50Ω feed (required length about11mm. up to edge of board), then wehave Fig. 6. From this, we can determinethe width of the feed (once again, afterremoving the exclamation mark after“Tl” and pressing the equals sign....) at w= 1.89mm.

3.2. The first circuit boardFirst, we must determine the open-endextensions for the microstrip line. Thequickest way is still to use the appropri-ate diagram from the PUFF manual. Fig.7 shows the necessary procedure and alsosupplies the raw data required:For a patch with Z = 7.4Ω, we need 51%of the board thickness of 0.813mm =0.42mmFor the 50Ω feed, we need 45% of theboard thickness of 0.813mm = 0.37mmSince the transformation line displays thehighest characteristic impedance and thusthe smallest width, it must be extendedby the following amounts at both ends(the correction formulae can be found onthe same page in the PUFF manual):

• For the patch side, we obtain:

• For the feed side, we obtain:

So for the transformation line, we finallyneed a length of 8.18mm + 0.41mm +

Fig 5: Using a λλλλ/4transmission linethe inputresistance ismatched to 50ΩΩΩΩ.

mmwwL 41.042.0

32.2077.0142.0

121 =⋅

−=⋅

−=∆

mmwwL 22.037.0

89.177.0137.0

121 =⋅

−=⋅

−=∆

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0.22mm = 8.81mm, and its width is0.77mm.The best way to work on the patch is touse the length supplied by the “Patch16”program, i.e. 13.39mm, with a patchwidth of 20.32mm.In the printed circuit board CAD pro-gram, the patch is now centred on theselected circuit board, with the dimen-sions 50mm x 50mm, the transformation

line is added, and finally the feed line isconnected, with a width of 1.89mm, upto the board edge. Please look at Fig. 1again. Everything can be recognised veryeasily there.Now the semi-rigid cable, with an SMAplug, must be connected to the circuitboard with the minimum of electricalirregularities. The necessary informationfor solving this problem can be found inFig. 8:

Fig 6: Matching to50ΩΩΩΩ.

Fig 7: Determiningthe open endextension using thegraph published inthe PUFF manual.

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• First, the cable is sawn off and theend is carefully filed flat at an angle.Naturally, you must not forget to trimit.

• Using a fine saw (e. g. a jig saw),make a cut parallel to the cable,precisely following the innerconductor and a few millimetreslong.

• By means of a second, careful cutperpendicular to the cable, we nowexpose the inner conductorcompletely and remove the internalTeflon insulation.

• The circuit board is pushed into this

cut and the inner conductor issoldered to the 50Ω microstrip feedline. Carefully solder the remainderof the cable sheathing to theunderside.

3.3. Evaluation of test results and newdesignAn investigation of the antenna using anHP8410 network analyser, HP5245L mi-crowave counter and an HP5257 transferoscillator, gave the resonance at5690MHz, with a value of |S11| = -16dB.A more precise examination, using apolar display, showed that the inputresistance here exceeds the system resist-ance of 50Ω and consequently the radia-

Fig 8: Details showing how to fit the semi rigid feeder.

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tion resistance must have a higher value.These findings were immediately con-verted into a PUFF simulation, and itbecame clear that in reality a resistanceof 407.5Ω should be assumed on eachpatch edge (Fig. 9).We should have the patch length requiredfor this frequency of 5890MHz displayedimmediately in the F3 parts list itamounts to L = 14.65mm. Now in fieldF4 we simply change the design fre-

quency to the required 5800MHz andthus once again simulate the patch reso-nance at this frequency. We can see fromFig. 10 that for this the length must bereduced to 14.36mm consequently, short-ened by 0.29mm! Thus, in the printedcircuit board CAD system, the lengthused for the initial design of 13.39mm isreduced to

13.39mm 0.29mm = 13.1mm

Fig 9: PUFFsimulation showingthe patch edgeresistance of407.5ΩΩΩΩ and |S11| of-16dB.

Fig 10: Changingthe designfrequency to5800MHz gives alength of 14.36mm.

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The patch width, naturally, remains at20.32mm.All that remains is the final changecorrecting the λ/4 transformation line toobtain better values for the matching ofthe measured - 16dB. This is also a veryeasy matter for PUFF, and the result canbe seen in Fig. 11. The new valuesrequired are:Length = 8.29mm and width = 0.52mm.Naturally, the required open-end exten-sion for each side must be added to this.For the patch connection, the value of0.41mm used before is still valid, how-ever something is altered on the feedside:

The transformation line therefore has awidth of 0.52mm and a length of 8.29mm+ 0.41mm + 0.27mm = 8.97mm.

3.4. Test readings on second prototypeSee Fig. 12 there is nothing more to say,and nothing more was done.

4.What do modern EMsimulators say about this?

Two free EM simulation programs areavailable, “Mstrip40” and “Sonnet Lite”,that have already been presented and/orused in projects in VHF Communica-tions. Since SONNET is currently run-ning an intensive publicity campaign

Fig 11: Correctingthe matchingtransformer.

mmL 27.037.089.152.01 =⋅

−=∆

Fig 12: A good test result with theisantenna.

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concerning the improvements in its new-est version, it was used on the radiatingpatch, in order both to re-check theresonance frequency and to investigatethe matter of the much higher radiationresistance. Using an online menu made itmuch easier to use this new version ofthe program, and indeed it was childsplay provided you pay heed to the rulesof the game as regards simulating suchantenna structures in the SONNETmanual. You can check up on them in thecorresponding article [3]. In addition,you also need the additional license(obtainable free of charge) for extendingthe maximum usable PC workingmemory to 16 megabytes, in order tocarry out the simulation successfully.In Fig. 13, we see the SONNET editorscreen with the selected box and celldimensions, together with the new“Quick Start Guide”. Owing to the re-strictions on the Lite version, the trans-formation line is simply omitted and

replaced by a 50Ω power feed taken rightup to the box wall. The actual radiationresistance of the antenna can then bedetermined easily and directly from thereflection factor determined in this way.This is how to do it:The simulation result can be seen in Fig.14, with a resonance of 6.03GHz and|S11| = -4.4dB. This gives a reflectionfactor of

Because we are using a 50Ω feed, wenow have to “cut back the circuit length”in our minds in the Smith diagram untilwe arrive at resistances exceeding 50Ωon the real axis. Then, in accordancewith the following relationship, we havethe total resistance on the patch edge:

Fig 13: Using the Sonnet-Lite simulator for the patch antenna.

60.020

4.410 =−=dBdBr

Ω=Ω⋅−+=⋅

−+= 20050

6.016.01

11 ZrrR

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Consequently, each radiating edge is af-fected by a radiation resistance of 400Ω.Compared with the measured value of407.5Ω, this is an extremely satisfactoryresult, and thus confirms the validity ofthe measurement. The resonance fre-quency was simply predicted too high,with an error of

Well yes, thats the way with all EMsimulators, to be sure (for a comparisonsimulation with the cell data used forSONNET used in the “mstrip40” pro-gram gives precisely this resonance fre-quency, but with rather larger discrepan-cies of approximately 10 to 15% in theradiation resistance).

5.Final observations

Nowadays, the most modern design aidsare available, even to private developerswithout an expensive industrial scale testrig, and they will scarcely strain theirbudgets. Those willing to spend a little

more time and to work their brains a bitharder (and to use measuring the latestgeneration instruments, or the generationbefore), can also create developmentsyielding data which need not fear com-parison with professional products. Theauthor hopes to have made some smallcontribution towards this with this articleand to have encouraged people to bebrave enough to carry out some criticalexperiments.

6.Literature references

[1]: Modern patch antenna design, Part 1;Gunthard Kraus, DG8GB; VHF Commu-nications 1/2001, Pages 49 - 63[2]: Modern patch antenna design, Part 2;Gunthard Kraus, DG8GB; VHF Commu-nications 2/2001, Pages 66 - 86[3]: Section 4.3 on Page 155

Fig 14: Simulationresults fromSonnet-Lite.

%96.3%1005800

58006030 =⋅−MHz

MHzMHz

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