Question

Design the pyramidal horn antenna which operates at WLAN point to point frequency. The horn antenna must have directivity of 22 and HPBW of 30 degree in E plane. Then, determine the power gain (in dB) and the HPBW in H plane of this antenna. Please give your opinion about this antenna.

Answer 1

Horn antenna is one type of aperture antenna. The radiation fields from aperture antenna can be determined from the knowledge of the fields over the aperture. The aperture fields become the sources of the radiated fields at large distances. Horn antennas are very popular at UHF and higher frequencies. Microwave horn antennas occur in a variety of shapes and sizes. There are different types like E plane H plane and EH or Pyramidal horn. Of these the simplest horn antenna is the pyramidal horn. It is fabricated by flaring a hollow pipe of rectangular or square cross section to a larger opening. It is robust, simple to construct, easy to excite and can provide high gain. Horn antennas have a wide impedance-bandwidth, implying that the input impedance is fairly constant over a wide frequency range. The bandwidth for practical horn antennas can be on the order of 20:1

A very long horn with small flare angle is required to obtain as uniform an aperture distribution as possible. For practical convenience horn should be as short as possible. Optimum horn antenna is a compromise between extremes that provides minimum beam width without excessive side lobe level.

For given length L as aperture and flare angle are increased Directivity increases and bandwidth decreases.However, if they become very large the phase shift between paths at edge and axis may become equivalent to 180 electrical degrees and field at aperture edge will be in phase opposition to field along axis. This results in reduced directivity and increased side lobe levels. Maximum directivity occurs at largest flare angles for which the phase shift does not exceed a certain value (usually 0.1 to 0.4λ). Optimum horn is preferred as it results in the shortest axial length for a specified gain.

The whole design can be actually reduced to the solution of a single fourth-order equation. For a horn to be realizable, the following must be true: RE= RH=RP

The expression for designing optimum horn dimensions is

 If not, change the approximation of A and repeat the above procedure till RE=RH is satisfied.

Using the above procedure an optimum horn is designed for the specifications (i) f= 9.5 GHz that is λ=3.15 cm (ii) Gain, G = 20dB or Gain numerical value is 100 (iii) a=2.286 cm (iv) b=1.016 dimensions of waveguide. The iterations are shown in table 1. The final dimensions obtained from the design iterations are A = 14.09cm; B=11.04cm; R1=20.95 cm; R2=19.33cm; RE=RH= 17.55 cm.

Testing:

Using X band reflex klystron powered microwave bench the impedance of horn antenna is found at 9.5 GHz. With the help of rotating mechanism the radiation intensity of horn as a function of Φ the azimuth angle and θ the elevation angle is observed and the half power beam widths ΦHP and θ HP in azimuth and elevation are noted. The Directivity of the antenna is calculated using the relation

With the same set up and orienting the antennas for maximum reception gain of the antenna is evaluated using 3 antenna method. For this the fabricated antenna A1 and two other horn antennas A2 and A3 are used. Three sets of power transmitted and power received are measured using antennas1, 2; 1, 3; and 2, 3 respectively. For free space communication link the Friis transmission formula is

where Pr = power received. Gt=gain of transmitting antenna, Gr = gain of receiving antenna, R= distance between transmitter and receiver and λ= wavelength of signal

If G1, G2 & G3 are the gains of the three antennas A1,A2, A3 used, then using Friss formula the following three simultaneous equations can be formed. A=G1+G2= 20 log (4πR/λ) + 10 log (Pr /Pt) {A2 tx, A1 rx} B= G2 +G3 = 20 log (4πR/λ) + 10 log (Pr /Pt): {A2 tx, A3 rx} C= G3+G1 =20 log (4πR/λ) + 10 log (Pr /Pt): {A3 tx, A1 rx} Substituting the corresponding values and solving the three simultaneous equations the gain of the fabricated antenna can be found from G1= (A+C-B)/2:………….….…... (7)

Thus in this method there is no need to have prior knowledge of gain of any antenna used. The efficiency η of antenna can be found using the relation Gain G=η x Directivity D ……………………… (8)

In these measurements the crystal input current is kept below 20μA so that it acts like a square law device and voltage or current measurements made at crystal output are proportional to input power. These parameters also can be estimated theoretically (a) The Half Power Beam Widths of horn antenna are: Optimum E-plane rectangular horn = 56/aEλ Optimum H-plane rectangular horn= 67/aHλ (b)The directivity of horn antenna can be estimated from ……

The results of the parameters calculated from experimental observations and estimated from theoretical expressions are presented in the next section and discussed.

RESULTS AND DISCUSSION

The radiation patterns in azimuth and elevation are given in figure 4

(i) Half Power Beam Width of the antenna from Pattern is ΦHP= 16o =0.279 radians; ΘHP=17o =0.2967 (ii) Directivity D = [4π/(ΦHP θHP)] where the half power beamwidths are taken in radians. Using the values found from pattern Directivity D= 151.8= 21.8dB (iii) For measurement of gain of fabricated antenna by three antenna method the readings obtained resulted in the following simultaneous equations: A = 34.92 ; B=35.94; C=32.08 Gain of antenna G= (A-C+B)/2 = 19.49dB From theoretical relations the estimated values are

(c) Gain from relation G (dB) = 8.1+10 log (AB/ λ2 ) G =20.05dB The practical value of gain is 19.49 dB which is very close to the specification of 20dB. Further there is very good agreement between the theoretically estimated and practically calculated values of half power beam width and gain. However it is observed that the antenna is susceptible for shape change due to small thickness of sheet used.