ridge guide

44 www.rfdesign.com May 2004

TX RX Technology

New broadband EMC double-ridgeguide horn antennaBroadband antennas, due to the large frequency bands required by standards,are the work horse of electromagnetic compatibility testing. Traditionally theantenna parameter of interest to the EMC engineer was the antenna factor. Theadvent of higher frequency testing has brought the development of a newdouble-ridge guide horn design.

By Vicente Rodriguez

adiated immunity or susceptibility and radiated emissions areR the two main types of radiated EMC measurements. Antennasare used in radiated EMC testing to sense and generate fields.International and national standards have defined the test distance,the antenna to be used, and the location of the equipment [1].

For years the EMC engineer paid little attention to the pattern ofthe antenna being used. The engineer would have an idea of thedirection of the main beam and would point the antenna to theequipment under test (EUT) so that this fell under the main beam.Originally most standards called for the use of half-wavelength di-poles for frequencies 80 MHz and higher and for short dipoles forfrequencies below 80 MHz. However, to reduce test time, broadbandantennas such as biconical dipoles and log periodic dipole arraysbegan to be accepted.

Figure 1. The original model of the new DRGH.

Figure 2. The feed cavityof the new DRGH forEMC applications. Figure 4. One of the three prototypes of the new antenna.

Figure 3. The final geometry of the new DRGH for EMC testing.


The use of broadband antennasreduced the test time because thetechnician did not have to stop thetest and adjust or change the dipoleantenna for the next short band offrequencies. As the use of broad-band antennas extended, standardswere changed to allow for the useof broadband antennas as long asthe measurements performed withthese antennas could be related tothe half-wave dipole. Other stan-dards went further and defined whichbroadband antennas must be used.The latest version of the MilitaryStandard Mil-Std 461E stated theuse of broadband DRGH as the an-tenna of choice for frequenciesabove 200 MHz [2].

One of the antennas required bythis military standard was a DRGHfor the 1 GHz to 18 GHz range.This broadband horn has been anaccepted antenna in EMC for morethan 40 years. In February 2003, apaper was published [3] that showedthe numerical analysis of a tradi-tional 1 to 18 GHz DRGH com-monly used in EMC measurements.The authors pointed out deficienciesin the pattern that in their view ren-dered the antennas’ use in EMC ap-plications as questionable.

These revelations were not a sur-prise to most users, especially thoseusing the antenna for susceptibility.In susceptibility or immunity testingthe antenna must generate a uniformfield over a given vertical plane.These users knew of the problemsfor the traditional 1 to 18 GHz an-tenna to effectively illuminate theuniformity plane.

An improved design of the1-18 GHz DRGH maintains a singleradiation lobe for the entire frequencyrange. As in the February 2003 pa-per, the entire horn was modeled,including the coaxial feed in Micro-wave Studio. The geometry used inthe numerical model was then ex-ported to Solidworks. A mechanicalmodel was then generated, and pro-totypes of the antenna were manu-factured and tested and the resultscompared with the model predic-tions. The improvement was basedon applying different ideas to thehorn and making sure that the propa-gation of higher order modes wassuppressed.

Numerical analysisThe antenna is modeled as a

perfect electrical conductor (PEC)Figure 7. Radiation pattern at the two principal planes at 10GHz. computed and measured.

Figure 6. Comparison of directive gain between prototypes and prediction.

Figure 5. The predicted VSWR compared with the 3 prototype antennas.


structure fed by a coaxial line with 50 Ohmimpedance. Because of the realistic feed, onlya single plane of symmetry can be used inanalyzing the antenna. A perfect magneticconductor (PMC) symmetry plane is used sothat it is only needed to solve half of thegeometry. Figure 1 shows the geometry gen-erated in the numerical model.

The starting point of the design was tomodel the traditional DRGH design as it wasdone in the 2003 paper. From this analysisseveral modifications were adopted. The firstmodification to the new antenna was to re-duce the size to push to higher frequenciesthe split pattern problem. Additionally thefeed cavity was redesigned. Figure 2 showsthe new feed cavity showing a structure de-signed to suppress higher order modes. Also,the curvature of the ridges was changed toachieve better matching at the aperture of thehorn.

The results from the analysis showed thatthe side-bars were increasing the gain at thelow frequencies. Furthermore, a study of thefields in the antenna showed that the dielec-tric supports for these bars were having adetrimental effect on the main beam at thehigher end of the range. The final design wasimplemented without any sides. Mechanically,no additional support was needed for the topand bottom plates. Figure 3 shows the finalgeometry.

Numerical results andmeasurements

The results from the numerical model thatwere important to the design goals were stud-ied in detail. These parameters of importancewere: the directivity or directive gain, theVSWR, and the radiation pattern quality.The desired objective was to get an antennawith similar performance to the traditionaldesign but with better pattern behavior. Thedesign shown in Figure 3 was exported toSolidworks. Using Solidworks, the mechani-cal engineer designed a way to manufacturethe antenna. Once the mechanical design wasfinalized, three prototypes were manufac-tured. Figure 4 shows one of the three proto-types.

The VSWR, gain and pattern of the proto-types were measured. Figure 5 shows theVSWR of the model compared to the threeprototypes.

The results show good correlation betweenmodel and measurement only at frequenciesbelow 13 GHz. Above 13 GHz, a deviationexists—probably due to not having enoughunknowns at the higher end. Due to memoryconstraints on the model, 15 cells per wave-

length were the maximum allowed at thehighest frequency of interest.

The gain was measured following the SAEARP-958-C [4]. Figure 6 shows the com-parison between measurement and predic-tion.

Again a good correlation exists betweenmodel and actual measurement. The higher

gain of the prediction can be explained by thelosses in the aluminum body of the antennaand also effects due to small gaps betweenthe parts that make up the antenna.

The radiation pattern was computed atfrequencies every 1 GHz between 1 and 18GHz. Additional frequency steps were com-puted at 18.5 and 19 GHz and every 0.25GHz between 16 GHz and 18 GHz. Fullthree-dimensional patterns were measured inan anechoic chamber. Figure 7 shows thetwo principal planes of the pattern at 10 GHzboth computed and measured.

Figure7 shows good agreement betweenthe predicted pattern and the measured re-sults with the exception of a slight shift in thepattern. But it is less than 5 degrees.

Comparison with traditionalDRGH antenna

As can be seen from Figure 8, the gainfrom 1 GHz to 3 GHz is much lower for thenew when compared with the traditional de-sign. However, one must recall that at thosefrequencies it is still possible to find goodpower to price ratios for amplifiers. Addi-tionally, note that from 8 GHz to 18 GHz,

the gain for the new design is fairly flat withno more than 2 dB of variation. This fre-quency range is where the advantages of thenew over the traditional design are clearlyseen.

It is true that the traditional design ap-pears to have a higher gain from 15 to16.5 GHz, but this result is due to a narrow

beam such that the antenna is unable to illu-minate the entire EUT. At 18 GHz the notchin the traditional design pattern causes thegain to drop about 6 dB below the newdesign gain.

While it is understood that amplifier poweris an issue for EMC engineers, it must bepointed out that this antenna has the ability togenerate fairly uniform field planes through-out the required frequency range. Also, un-like the traditional design and since a widerbeam is obtained at 1 GHz, it is possible tobring the antenna closer to the EUT and stillilluminate the entire object with the requiredfield. This is potentially useful in smalleranechoic chamber dimensions such as 3meters.

Radiation pattern is the key issue on thenew design. It has a superior pattern behav-ior than the traditional horn. Overall, theEMC engineer must realize the advantage ofhaving a good pattern behavior for the wholeband even if the tradeoff is lower gain for 12percent of the operational band of the an-tenna. Additionally the better pattern behav-ior makes the antenna suitable for applica-tions other than EMC. The traditional design

Radiation pattern is the key issue on the new design. It has asuperior pattern behavior than the traditional horn.

Figure 8. Comparison of traditional and new design


The result is an antenna that is bettersuited than the traditional design for EMC

and other applications.pattern behavior was not suitable for theantenna to be used as a source for reflectorsor in anechoic chambers for antenna patternmeasurement. The new design’s more con-stant pattern makes the horn suitable for theseother applications.

ConclusionsThe results both measured and predicted

show that the new design is comparable ingain and antenna factor (AF) to the tradi-tional horn. The lack of side structures hasdecreased the low-end gain when comparedwith the traditional design. Also the opensides have caused the beamwidth to be largerat the low end than the traditional design.However for most of the 1 to 18 GHz band,the performance is similar and the better pat-tern behavior has translated into a more stablegain and AF for the high end of operation.

Even more important is that the new de-sign has a better pattern. The main beamdoes not split into four separate lobes at any

ABOUT THE AUTHORVicente Rodríguez-Pereyra attended

the University of Mississippi where heobtained his B.S.E.E., MSEE andPh.D. with an emphasis on electromag-netic theory. In June 2000 Rodriguezjoined EMC Test Systems (now ETS-Lindgren) as an RF and electromag-netics engineer.

Rodriguez’s interests are numericalmethods in electromagnetics especiallywhen applied to antenna design andanalysis. Since his association with ETS-Lindgren, Rodriguez’s interest hasspread to the use of these numericaltechniques in designing EMC and RF/MW absorber. Rodriguez is the authorof more than 20 publications includingjournal and conference papers as wellas book chapters. He holds a patent forhybrid absorber design and has a patentpending for a new dual-ridge horn an-tenna design for EMC applications.Rodriguez is a member of the IEEEand several of its technical societiesincluding the MTT and the EMC soci-eties.

He may be reached [email protected].

Circle 39 or visit freeproductinfo.net/rfd

frequency of operation. The result is an an-tenna that is better suited than the traditionaldesign for EMC and other applications. RFD

References1. D Morgan, A Handbook for EMC test-

ing and Measurement. Peter Peregrinus Ltd(on behalf of the IEE): London, UK 1994.

2. MIL-STD-461-E “Requirements for theControl of Electromagnetic Interference Char-acteristics of Subsystems and Equipment”Department Of Defense, August 1999.

3. C Burns, P. Leuchtmann, R. Vahldieck,“Analysis and Simulation of a 1-18GHzBroadband Double-Ridge Horn Antenna,”IEEE Transactions on Electromagnetic Com-patibility, Vol 45, No 1, pp 55-60, Feb 2003.

4. Society of Automotive Engineers (SAE)Surface Vehicle Electromagnetic Compat-ibility (EMC) Standards Manual. Society ofAutomotive Engineers, Inc: Warrendale, PA,1999.

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