Документ взят из кэша поисковой машины. Адрес оригинального документа : http://www.atnf.csiro.au/whats_on/workshops/synthesis2001/material/antennas.pdf Дата изменения: Tue Jan 15 06:15:10 2002 Дата индексирования: Wed Dec 26 11:21:04 2007 Кодировка: IBM-866 Поисковые слова: столовая гора

Antennas
The primary elements of a synthesis array

The Antenna Structure

M. Kesteven AT NF 25/September/2001

* Backup structure * Reflector surface(s) shape accuracy construction * Two axis Mount

Antenna Design Kit
Basic shapes : conic sections Parabola Hyperbola Ellipse

Parabola:

wavefront

All ra ys parallel to the axis are concentrated to a point (the focus). All paths from a wavefront to the focus are of equal length.

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Hyperbola: Ellipse P A

B

Light converging towards B -> reflecting off the hyperbola: converges at A § For an arbitrary point P on the hyperbola, (AP н BP) = constant
Rays are reflected by the ellipse to the second focus And all these paths have the same distance

Correctly focussed antenna: Equi-length paths from axial wavefront to the receiver

в

б й й из ж ед г

б
Source at one focus.



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Beam width = Full Width at Half Power

Operational Characteristics (I)

Main Beam н the central lobe Sidelobes the secondary responses Note: logarithmic scale (dB) ATCA
@22 GHz

Operational Characteristics (II)
GAIN:

Antennas as primary elements in an aperture synthesis array 1. Beamwidth defines the field of view 2. Collecting Area defines Gain which defines sensitivity

How much energy can the antenna extract from a wavefront? Depends on: - antenna size - efficiency - wavefront orientation relative to boresight (beam pattern) 3. Large diameter = high sensitivity = small beamwidth

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To calculate the radiation pattern : Compute field in focal plane from surface currents excited by the incoming wavefront.

In detail: 1. Compute the phase of the currents on the surface н = 2 (path from wavefront) /

2. For each point in the focal plane, sum the contribution from each surface current. Need path from surface to focal plane for phase and (1/R) field reduction. 3. Compute the coupling of the electric field distribution to the feedhorn.

Notes. 1. These curves are generic as to (Focal Length/Diameter)

2. The plate scale (=displacement in focal plane for a given angle offset from boresight) is proportional to Focal Length Antennas with same (F/D) have same focal plane function; BUT the larger the antenna, the smaller the beamwidth. 3. The axis is expressed in wavelengths F/D=0.4 (eg, Parkes) F/D=2.0 (eg, ATCA) 4. The higher the frequency, the smaller the beamwidth

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Alternative view : the feed horn as a transmitter GAIN : The Problem. The coupling of the feed horn to the focal plane field is critical. It sets the efficiency of the antenna н typically 60%. The feed horn expects a particular field distribution (phase and amplitude) which a conic section reflector cannot deliver. This is the AT feed pattern, designed for a subreflector which subtends 28 deg. at the feed. н it is down 50% at 7 degrees from boresight

It is difficult to reconcile the goals: 1. Uniform illumination on reflector (broad beam) 2. Not wasting energy, with some radiation missing the subreflector (narrow beam).

Remedies: 1. Shaped reflectors to modify the focal plane distribution - equivalent to the Schmidt Corrector plate. 2. Focal plane array to construct a "super-feed" Shaped Reflectors The secondary is slightly conical in the central region to redistribute the energy flow from main reflector to the feed. - Uniform plane wave at the main reflector - Converging gaussian at the feed.

The main reflector has to be tweaked to retain the constant path length criterion.

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The main reflector deviates from a parabola by about 30 mm Sensitivity to subreflector mis-positioning
De viatio n in MR profile fro m p arab ola (tru e - p ara bola )
20

10

Surface devi ati on (m m )

0

- Lateral (in focal plane) null for ~ 0.5 displacement. largely recovered with a pointing correction -Axial null for ~ 1 displacement no operational fix.

-10

-20

-30

-40 0 2 4 6 Ra di a l di sta nce (m ) 8 10 12

Radial distance (m)

Additional losses : Blockage

The blockage paradox : The loss can scale as TWICE the area. A thought experiment: remove the outer ring of panels. - you lose first of all because the collecting area is reduced. - you lose a second time because the feed is designed for the original F/D. You could recover this second component with a new feed. The AT shaping performs a similar trick on the sub-reflector blockage.

1. Subreflector 2. Feed Legs (quadrupod on the AT antennas) plane wave shadows (radiation which does not reach the main reflector) spherical wave shadows (radiation blocked between the main and the sub-reflector)

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Surface Errors н Ruze formula Let be the rms surface error. This translates to an rms phase error of (4/). In computing the focal plane field, the sum will be reduced by the phase error н by ~cos (4/), and the power, by the square of this. More correctly :
G = G 0e
- ( 4 / )
2

Unwanted Stray Radiation The receiver signal = + + + astronomical signal 3 K background atmosphere scattered, stray radiation

Alternative approach н assess antenna as a transmitter. Reciprocity Theorem : transmit pattern = receive pattern Algorithm: 1. Launch wave from the feed (= expanding spherical wave) 2. Compute the phase and amplitude of the surface currents 3. Transform to the Aperture Plane 4. Compute the far-field wave due to the aperture plane fields

Shaped Reflector
§ Subreflector is slightly conical in the central region to redistribute the feed's radiation.

- More uniform illumination. - Reduce impact of central blockage - Improve efficiency

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Feeds are `compact' and `corrugated' horns The shaped reflectors redistribute the feed's radiation,
The inner profile is curved The inner surface has grooves

Cross-section of a horn

Step 2 : Compute the phase and amplitude distribution over the main reflector surface. Step 3 : Compute the far-field distribution

A( ) = V ( r )dr e
0

2

j 2

r cos( ) sin( )

d

A( ) =

J1 ( 2 ( R / ) ) 2 ( R / )

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§ Sidelobes

§ Surface Errors
G =e G0
- ( 4 / )
2

(Ruze)

§ Spillover

§ Backup structure § Reflector surface(s)
н Shape н Accuracy н Construction

§ Two-axis mount

й % !г ж же иг и из й б й й из же дг б "г д1 !1 2й ж з д г и 1 0г е )( '& ж% $# "е й !ий ж з д ж е иг з й ж д



н J1(x)/x --- 2% sidelobe н Aggravation due to blockage

The Antenna Structural Characteristics



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Antenna Design
§ Focussing
н Ray tracing н Equi-phase paths

Electromagnetic Characteristics
§ Gain
н Collecting Area : antenna aperture that intercepts an incoming wavefront. н Efficiency : the useful fraction of the aperture

§ Single Reflector ("Prime Focus")
н parabola

§ Dual-Reflectors
н Cassegrain (parabola+hyperbola) н Gregorian (parabola+ellipse)

§ Radiation Pattern
н Beamwidth н sidelobes

Focal Plane Considerations
§ Energy distribution in the focal plane § Feed Antenna + Receiver § Focal Plane Arrays
= radio photographic plate

Antenna - Design
§ Reciprocity
н Transmit pattern = Receive pattern

§ P ro c e d u re
н Feed pattern > (ray tracing) : field distribution in aperture plane > far field pattern

§ Aperture Plane as intermediate step

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