Horn and Reflector Antennas Questions and Answers

Q: What is an E-plane sectoral horn?

An E-plane sectoral horn opens out in the direction of the electric field and keeps the guide dimension in the other plane, so it narrows the beam in the E-plane only. The H-plane version does the opposite, and flaring both gives the pyramidal horn. An unflared guide is simply an open ended waveguide. Sectoral horns are used where a fan beam is wanted, narrow in one plane and broad in the other, which suits many radar and landing aid applications directly.

Q: What is a pyramidal horn?

The pyramidal horn flares in both the E and H planes, giving a rectangular mouth and a pencil beam. It is the commonest laboratory and reference horn and the usual gain standard. Circular cross section gives the conical horn, corrugations give the hybrid mode horn, and dual polarised feeds are a separate refinement that may be applied to several of these shapes. The geometry is simple enough that its gain can be computed accurately from its dimensions, which is why it became the standard.

Q: What is a conical horn?

A conical horn is the circular guide equivalent of the pyramidal horn, flaring a round guide out to a round mouth, and it is the natural feed for a circular aperture such as a paraboloid. Its pattern is still not perfectly symmetric in the plain form, which is why corrugated and dual mode versions were developed. A horn that tapers inward would be a transition rather than a radiator, and a continuously curved flare describes some specialised profiles rather than the basic conical shape.

Q: What is an optimum horn?

An optimum horn has the flare and length chosen so that the gain from the larger aperture just balances the loss from the quadratic phase error, which fixes the aperture size for a given length. Making it longer for the same mouth would reduce the phase error but is often impractical. Perfect matching, equal beamwidths and zero phase error are all desirable and are pursued by other means. Optimality here is a property of proportions rather than of performance in any absolute sense.

Q: Why is the aperture efficiency of an optimum pyramidal horn only about 50 per cent?

The dominant waveguide mode supplies a half sine taper in one plane, and the flare leaves a quarter to half wavelength of phase error at the rim, and the two together cost about half the aperture. Neither absorption nor reflection accounts for anything like that much, and a horn radiates forward. Corrugated and dual mode horns improve on the figure by attacking the amplitude and phase distributions directly, which is what makes them worth their extra manufacturing cost.

Q: What is a corrugated horn?

Quarter wave grooves in the walls present a high impedance to the axial current, allowing a balanced hybrid mode whose aperture field is nearly rotationally symmetric and falls to zero at the rim. The result is a pattern almost identical in every plane, very low cross polarisation and very low side lobes. Stiffening ribs, impedance steps and absorber linings are all real constructions but none produces the mode that gives the corrugated horn its properties.

Q: What is the main benefit of a corrugated feed horn on a reflector antenna?

Because the pattern is the same in every plane and the field falls smoothly to zero at the rim, the reflector is illuminated evenly around its circumference and very little cross polar energy is generated. That is what earth stations and radio telescopes need. Its gain is comparable to a plain horn of the same size, dual band operation requires further design, and it is usually longer rather than shorter. Matching the symmetry of the feed to the symmetry of the reflector is the point.

Direction for [ Question No: 1 To 1 ] :

Read the passage and answer the questions that follows based on your

understanding of passage : An antenna is generally a metallic object, often a wire or collection of wires, used to convert high frequency current into electromagnetic waves and vice-versa. Antenna serve either or both of the following two functions, generation or the collection of electromagnetic energy. In a transmitting system, a radio-frequency signal is developed, amplified, modulated and applied to the antenna. The R-F currents flowing through the antenna produce electromagnetic waves which radiate into the atmosphere. In a receiving system, electromagnetic waves "cutting" through the antenna induce alternating currents for use by the receiver. Efficient operation also requires that the receiving antenna be of the same polarization as the transmitting antenna. Polarization is the direction of the electric field and is therefore, the same as the antennas physical configuration. Thus vertical antenna will transmit vertical polarized light, any antenna having a physical length that is one-half wavelength of the applied frequency is called a Hertz antenna. Hertz antenna are predominantly used with frequencies above 2 MHz. Usually, at frequencies below 2 MHz, a Marconi type of antenna is used. The Marconi antenna is usually a quarter-wave grounded antenna or any odd multiple of a quarter wavelength. The Yagi-Uda antenna consists of a drivers element and two or more parasitic elements. Yagi-Uda has two parasitic elements, a reflector and a director. This Yagi-Uda antenna provides about 7 dB of power gain with respect to a halfwavelength dipole reference. More complex antennas may be circularly polarized both vertically and horizontally polarized waves are radiated with equal power in both. If the powers are unequal the antenna is said to be elliptically polarized. A horn is an ideal antenna for terminating a waveguide and may be conical, rectangular or sectorial. Wide band antennas either when the transmissions are wide band or when working of narrow channels over a wide frequency range is the major application. Helical antenna, which consists of a loosely wound helix backed up by a metal ground plane. Loop antennas are often used for direction finding. Loops have many shapes and generally consists of a single turn of wire. Discone is a ground plane antenna evolved from the vertical dipole and having a very similar radiation pattern.

Which of the following antennas is best exerted from a waveguide ?

A.Biconical
B.Folded dipole
C.Horn
D.Discone

Exam Year(s)

UGC NET - 2014,

Answer: C

The horn is the antenna best excited from a waveguide. The passage says a horn is an ideal antenna for terminating a waveguide and may be conical, rectangular or sectorial, because its flared mouth matches the guide to free space.

The biconical, folded dipole and discone antennas are wire or surface structures fed by a coaxial line or twin line, so they are not naturally driven from a waveguide.

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What is an E-plane sectoral horn?

A.A horn flared only in the plane of the electric field
B.A horn flared only in the plane containing the magnetic field vector
C.A horn flared equally in both planes to give a symmetric beam pattern
D.A horn with no flare at all, being simply an open ended rectangular guide

Answer: A

An E-plane sectoral horn opens out in the direction of the electric field and keeps the guide dimension in the other plane, so it narrows the beam in the E-plane only. The H-plane version does the opposite, and flaring both gives the pyramidal horn. An unflared guide is simply an open ended waveguide. Sectoral horns are used where a fan beam is wanted, narrow in one plane and broad in the other, which suits many radar and landing aid applications directly.

Discuss in Forum

What is a pyramidal horn?

A.A horn fed from two waveguides to produce two orthogonal polarisations
B.A rectangular horn flared in both principal planes
C.A horn of circular cross section flared out to a round mouth aperture
D.A horn whose walls are corrugated to give a rotationally symmetric pattern

Answer: B

The pyramidal horn flares in both the E and H planes, giving a rectangular mouth and a pencil beam. It is the commonest laboratory and reference horn and the usual gain standard. Circular cross section gives the conical horn, corrugations give the hybrid mode horn, and dual polarised feeds are a separate refinement that may be applied to several of these shapes. The geometry is simple enough that its gain can be computed accurately from its dimensions, which is why it became the standard.

Discuss in Forum

What is a conical horn?

A.A horn tapering inwards towards the aperture to concentrate the field
B.A horn whose flare angle changes continuously along its length in a curve
C.A circular waveguide flared out to a round aperture
D.A rectangular horn whose mouth has been rounded off for mechanical reasons

Answer: C

A conical horn is the circular guide equivalent of the pyramidal horn, flaring a round guide out to a round mouth, and it is the natural feed for a circular aperture such as a paraboloid. Its pattern is still not perfectly symmetric in the plain form, which is why corrugated and dual mode versions were developed. A horn that tapers inward would be a transition rather than a radiator, and a continuously curved flare describes some specialised profiles rather than the basic conical shape.

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Why does the gain of a horn not increase indefinitely with flare angle?

A.The metal walls absorb more power as the horn becomes physically larger
B.The waveguide feeding the horn cannot supply enough power to a large mouth
C.The higher order modes generated at the throat carry the extra energy away
D.The quadratic phase error across the aperture grows with the flare

Answer: D

Opening the flare enlarges the aperture, which raises gain, but the path from the apex to the rim grows longer than to the centre, and that phase error eventually costs more than the extra area gains. The optimum sits where the edge phase error is about a quarter to half a wavelength. Wall loss is negligible, the guide supplies whatever power is fed to it, and higher order modes are a secondary effect. Balancing an area gain against a phase loss is the whole of horn proportioning.

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What is an optimum horn?

A.One proportioned for maximum gain at a given axial length
B.One matched perfectly to its waveguide across the whole operating band
C.One whose E-plane and H-plane beamwidths have been made exactly equal
D.One in which the aperture phase error has been eliminated altogether

Answer: A

An optimum horn has the flare and length chosen so that the gain from the larger aperture just balances the loss from the quadratic phase error, which fixes the aperture size for a given length. Making it longer for the same mouth would reduce the phase error but is often impractical. Perfect matching, equal beamwidths and zero phase error are all desirable and are pursued by other means. Optimality here is a property of proportions rather than of performance in any absolute sense.

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A pyramidal horn has an aperture of \( 10\lambda \times 7\lambda \) and 51 per cent efficiency. What is its gain?

A.About 16 dBi, which is typical of a small sectoral horn
B.About 26.5 dBi
C.About 20 dBi
D.About 31 dBi

Answer: B

Applying the aperture gain relation with the area expressed in square wavelengths:

\[ G = 4\pi \times 70 \times 0.51 \approx 449 \]

which is about 26.5 dBi. The efficiency of about half is characteristic of an optimum pyramidal horn and comes from the combination of the guide's amplitude taper and the residual phase error. Knowing that figure lets a horn's gain be estimated from its mouth dimensions alone, which is often all the information available.

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Why is the aperture efficiency of an optimum pyramidal horn only about 50 per cent?

A.Half the power is reflected back into the waveguide at the throat junction
B.The horn radiates half its power backwards from the open end of the guide
C.The amplitude taper and the residual phase error both reduce it
D.The metal walls of the horn absorb about half of the transmitted power

Answer: C

The dominant waveguide mode supplies a half sine taper in one plane, and the flare leaves a quarter to half wavelength of phase error at the rim, and the two together cost about half the aperture. Neither absorption nor reflection accounts for anything like that much, and a horn radiates forward. Corrugated and dual mode horns improve on the figure by attacking the amplitude and phase distributions directly, which is what makes them worth their extra manufacturing cost.

Discuss in Forum

What is a corrugated horn?

A.A horn whose walls are ribbed to give it mechanical stiffness at low weight
B.A horn with a series of steps that broaden its impedance bandwidth
C.A horn lined with absorbing material to suppress its side lobe radiation
D.A horn with grooves in its walls supporting a hybrid mode

Answer: D

Quarter wave grooves in the walls present a high impedance to the axial current, allowing a balanced hybrid mode whose aperture field is nearly rotationally symmetric and falls to zero at the rim. The result is a pattern almost identical in every plane, very low cross polarisation and very low side lobes. Stiffening ribs, impedance steps and absorber linings are all real constructions but none produces the mode that gives the corrugated horn its properties.

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10.

What is the main benefit of a corrugated feed horn on a reflector antenna?

A.Symmetric illumination with very low cross polarisation
B.A much higher gain than a plain conical horn of the same mouth diameter
C.The ability to operate over two widely separated bands simultaneously
D.A physically shorter feed structure and so less blockage of the aperture

Answer: A

Because the pattern is the same in every plane and the field falls smoothly to zero at the rim, the reflector is illuminated evenly around its circumference and very little cross polar energy is generated. That is what earth stations and radio telescopes need. Its gain is comparable to a plain horn of the same size, dual band operation requires further design, and it is usually longer rather than shorter. Matching the symmetry of the feed to the symmetry of the reflector is the point.

Discuss in Forum

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Horn and Reflector Antennas Questions and Answers

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