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Simulation and measurement results of a 0.7-7 GHz double-ridged guide horn (DRGH) antenna with coaxial input feed section is presented.

A suitable taper for the ridges in the horn is designed and its impedance variations along the horn are shown.

A new structure for the electrical field probe in the feed section is introduced by which a shift down of lower frequency to 0.7 GHz is achieved.

A sensitivity analysis is done on the parameters of the proposed structure.

The proposed antenna has been fabricated and measurement results show good agreement with the simulation.

03 Horn antennas are widely used in various areas such as EMC testing,

standard gain measurement, satellite tracking systems, reflectors feeding and radars.

The ridges in waveguides or horns create capacitance effects which decrease the cut-off frequency of the dominant propagating mode (TE10) and increase the single-mode bandwidth.

First, a double-ridged waveguide is designed with a desired cut-off frequency and characteristic impedance.

Second, a taper is proposed for the ridges in the horn.

Third, a new structure is proposed for the transition between the coaxial cable and double-ridged waveguide.

04A. Double-ridged waveguide analysis and design

DRGs are widely used in antenna and microwave systems since they exhibit low cut-off frequency, broad frequency band, low characteristic impedance, and capability of matching to coaxial cable.

Calculating cut-off wavelength of TEn0 modes:

Odd modes

Even modes

05Approximation of DRG Characteristic impedance for TE10 mode at infinite frequency:

Characteristic impedance at other frequencies is calculated by:

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06By choosing around 50 Ω, the matching between coaxial cable and DRG over a wide band can be achieved.

A DRG is designed to have cut-off frequency of 0.5 GHz and characteristic impedance around 50 Ω.

07B. Pyramidal horn analysis and design

To effectively transmit the wave from DRG to free space, a horn is utilized as the matching section. This section should provide a transition from DRG impedance (50 Ω) to that of the free space (377 Ω).

If the variable impedance Z(z) is used as the matching section between Z0 and ZL, the reflection coefficient is given by the theory of small reflections as follows:

Reflection coefficient for exponential taper

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If the horn is divided into n sections, we will have n DRGs. Characteristic impedance of each section can be calculated.

It is not required that impedance ends smoothly with free space impedance since Klopfenestein taper, which has been proved to be the optimum choice for the lowest reflection, has steps at the ends of thetapered section.

It is not required that impedance ends smoothly with free space impedance since Klopfenestein taper, which has been proved to be the optimum choice for the lowest reflection, has steps at the ends of thetapered section.

09C. Coaxial to DRG transition design

The goal in this section is to design a transition between TEM mode in the coaxial cable and TE mode in the DRG to obtain low levels of VSWR over the antenna bandwidth.

Wedges are positioned between the flares. This leads to a cavity at the end of the waveguide. This cavity can provide low levels of VSWR in the cable to waveguide transition structure.

Infinity impedance should be seen by the probe from the cavity side. The infinity impedance is paralleled with the waveguide impedance and results a total impedance of 50 Ω seen by the probe.

10 Adaptor structure a) front view, b) side

view without the waveguide, c) cross section perspective view and d) cross section side view.

The hole through which the probe crosses has a conical form. This form of the hole acts as an impedance taper which matches the probe to the coaxial connector.

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11Return loss sensitivity to a) probe diameter, b) probe diameter in the ridge and c) upper diameter of the cone.

(a) (b) (c)

12The DRGH antenna is assembled by putting together the designed parts.

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E-plane radiation pattern H-plane radiation pattern

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Simulated and measured antenna gain

15CONCLUSION

A 700 MHz – 7 GHz DRGH antenna for EMC testing was presented.

At first, a conventional double-ridged waveguide with 0.5 GHz cut-off frequency and characteristic impedance about 50 Ω was designed.

By using partial reflections theory, a suitable taper for the ridges inside the horn was provided.

Then, a new structure in the feed section introduced in order to shift down the lower frequency to 700 MHz and improve the return loss over the operating frequency bandwidth.

Finally, the optimized parameters of the antenna were obtained.

Thank you for your attention