Design and Implementation of a Pillbox Antenna for an Airborne ...

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H−plane Power Patternsradiation intensity,dBi−5−10−15−20−25−30−35−40−45−50−55co−polcross−polmeasured ************4.1 degrees−3 dB−20 −15 −10 −5 0 5 10 15 20theta−d,degreesFigure 10: Measured E-plane pattern and cross-pol performanceof Pillbox (W.r.t peak gain)Figure 11: Measured H-plane Pattern and Cross-polPerformance of Pillbox (w.r.t. peak gain)beamwidth was measured and found to be 24 ± 0.5 ◦in comparison to the predicted value of 25 ◦ . The firstsidelobe level was found to be approximately -11 dBcompared to a predicted value of -13 dB. The antennaoffers good cross-polarization rejection in the E-plane(levels less than -30 dBm) within 20 ◦ of the beam peak.The peak cross-polarization level was limited to about-22 dBm relative to the peak of the main beam in theE-plane. The noise floor of the receiver was approximately-80 dBm (-60 dB relative to the peak of the mainbeam co-polarized level).3.3.2 H-plane PatternFigure 11 shows the measured H-plane pattern and thecross polarization performance. The first sidelobe levelwas found to be approximately -24 dB in comparison tothe MATLAB prediction of -23 dB. The measured H-plane 3 dB beamwidth was found to be 4.1 ◦ in comparisonto the predicted value of 3.8 ◦ . This is justified sincea wider beam implies lower sidelobes due to the taperimposed on the reflector edges. A maximum crosspolarizationlevel of -28 dBm occurred −3 ◦ off the peakof the main beam. The cross-polarization rejection wasgenerally quite good within 20 ◦ of the beam peak (lessthan -30 dBm relative to the peak of the main beam).The noise floor of the receiver was approximately -77dBm (about 50 dB below the peak of the co-polarizedpattern).3.3.3 Power GainWe calculated the power gain using equation 21 whichis known as the Friis equation [2][8]:G t dBwhere( ) ( )4πR Pr= 20 log 10 + 10 logλ 10 −P tG r dB (21)G t dB = gain of transmitting antenna [dB]G r dB = gain of receiving antenna [dB]R =λ =antenna separation [m]operating wavelength of antenna [m]P r = received power [W]P t = transmitted power [W]We use the Friis equation for a far-field distance of 20m with a transmit power equal to 20 dBm. We accountfor all the losses in the measurement system prior to andafter taking measurements before calculating the powergain of the antenna. The losses are shown in the lossbudget Table 1.The power gain was calculated to be 24 ± 0.5 dBicompared to a directivity of 25.6 dBi. Where the totaluncertainty is the sum of the uncertainties associatedwith G r and P r . The other quantities in equation21 above were measured accurately before and after theexperiments and they stayed the same.
Table 1: Losses in Measurement SystemLoss of transmitter (before and after) 1 dBInsertion loss of transmitting cableInsertion loss of receiving cable4 Conclusions2 dB2 dBThis paper described the design, implementation andtesting of the pillbox antenna for SASARII. The antennais designed according to the specifications givenat the beginning of the paper.The antenna simulations in MATLAB gave the predictedE and H-plane patterns of the feed and the pillbox.The beamwidth of the antenna and the directivitywere computed from the radiation patterns.The fully fabricated antenna was tested and the resultsof the tests suggested that the antenna performedsatisfactorily and within the specifications. In summarythe measured azimuth beamwidth was found to be 4.1 ◦and the elevation beamwidth was found to be 24 ◦ . Thepredicted E and H-plane beamwidths were 3.8 ◦ and 25 ◦respectively. The power gain was measured and foundto be 24 ± 0.5dBi. The cross-polarization rejection wasgenerally quite good within the main beam for boththe E and H-plane measurements; X-pol measurementswere less than -30 dB relative to the pattern peaks.The slightly wider H-plane measured beamwidthcould have been due to improper focussing of the feed.The feed exhibits both lateral and axial movement as itis moved back and forth in trying to locate the phasecentre. A remedy to this might be to find a way of makingthe axial and lateral movements independent whilstattempting to find the phase centre. There is also a discrepancybetween the general shape of the predictedand the measured H-plane patterns. Both Figure 8 andFigure 11 have nulls at ±10 ◦ , ±15 ◦ ±20 ◦ but the measuredpattern has a 10 dB beamwidth of 8 ◦ while in thepredicted pattern it is 12 ◦ . This discrepancy could bedue to a quadratic phase error at the aperture of the pillboxor an error in programming Equation 10 to obtainthe predicted radiation pattern.AcknowledgementsThe authors would like to thank colleagues in theRRSG, the project sponsors and the examiner of S.Gambahaya’s dissertation, for all their contributions towardsthe research. Special thanks to Reuben Govenderfor supplying the CAD for the antenna.AAppendicesA.1 Method for Determining H-plane 3dB BeamwidthThis page explains the method for calculating the 3 dBbeamwidth of the antenna in the H-plane to an accuracyof 0.1 ◦ . The user requirements stated a desired azimuthbeamwidth of 3.8 ◦ , however since the measurementswere carried out manually there was no protractor availablethat could measure to an accuracy of 0.1 ◦ . Themethod described below might seem crude but it wasquite effective in measuring the H-plane beamwidth.The experiment was performed twice and consistentlygave the same results.A.2 DescriptionWe suggest a method based on the small angle approximation:WhereSRθS = Rθ= arc length=distance between dots= far-field distance= Small angular incrementThis method is justified since θ is very small and Ris large. See figure 12. The figure simply illustrates theconcept and is not at all to scale.A laser pointer was attached to the top plate of theantenna just above the aperture. We scanned throughpeak power to 3 dB points and then bisected the subtendedangle to find the peak. Having located the boresightwe marked it on a chart which was placed in thebackground of the receiver at the same height as the receiver.The laser beam was used to accurately mark offthe position of the peak on the background chart (seefigure 12). With the peak position as reference, the totalsubtended angle to the 3 dB points was measuredby shining the laser to those points and recording thedistance between the points.The distance between the dots corresponding to anangle of 0.1 ◦ is given by:
Page 1 and 2: Design and Implementation of a Pill
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Page 7: was found to be approximately 25
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