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Some general antenna considerations for LF-SKA, and why the "magic-carpet"
concept could be a "dingo".



Bruce MacA Thomas


11 February 1999



Introduction


In this discussion document, the antenna principles underpinning some of
the concepts discussed in Ref 1 for the LF-SKA are extended to illustrate
the potential difficulty of achieving constant collecting area, and hence
the practical realisation of the "magic-carpet" concept. Applications to
the "randomised" antenna-element approach to array-station design, and the
trade-offs that need to be considered, are briefly discussed.


Some Basic principles (see also Ref 1):


For LF-SKA, the astronomers are calling for:

a) Constant collecting area (106 m2)

b) Constant sky viewing angle (100њ)

c) Wideband coverage (10:1), say 150-1500 MHz

d) Dual polarisation

It has been generally assumed that these desires can be achieved through
the use of some form of planar-array (or "magic-carpet"). Because antenna
engineers see such a concept as out-of-the-ordinary, a compromise solution
which could satisfy (b), (c) and (d), but not (a) was proposed to stimulate
thought and discussion (See Ref 1). In this reference, a particular element
performance (gain 10dB, beamwidth (HPBW) 55њ) was chosen to illustrate the
compromise between instantaneous sky-coverage and number of antenna
elements, together with the mean sidelobe level for an array-station when
the elements are arranged in a "randomised" fashion. The characteristics of
the antenna element used in this first study has now been extended by
considering antenna elements of different gain, and hence HPBW; see Table
1. (Note: Ae is the effective collecting area of a single element). The
highest-gain element corresponds to the one used in the first study. This
element and the one with an HPBW of 75њ would need to be mechanically
steered to meet the specification (b). The lowest gain element having a
symmetric pattern (HPBW 100њ) would not need to be steered, and would be a
candidate for the "magic-carpet". (How one realises such an antenna with a
10:1 bandwidth and dual-polarisation is another question).



3. The "Magic-carpet" concept:


This leads us to the "magic-carpet" concept, which is interpreted to mean a
planar surface, which has a constant absorbing collecting area, constant
(primary element) sky viewing angle and wideband coverage, ie it meets all
the astronomers' desires (a) to (d) above.

Let us consider an element, which meets the 100њ HPBW criterion across the
10:1 frequency band. To be "carpet-like" the elements should be spaced
approximately (/2 at the maximum frequency. This alone will put severe
restrictions on the element size and its overall performance. As the
frequency decreases, the element spacing (in wavelengths) reduces, and all
the horrific effects that antenna array engineers quietly warn us about
come into play. Incidentally, these deleterious effects actually probably
help to maintain constant collecting area (albeit with very low
efficiency!).

I think this illustrates that the concept of the "magic-carpet" could well
be a "dingo".



4. Design factors for a "randomised" element configuration for the Array-
station


If we revert to using "randomised isolated" elements for the array-station
(see also Ref.1) Table 1 can be used as input into a design optimisation
study which considers the following key factors:

. Trade-off between number of elements per array station and the
instantaneous sky-coverage, including costs for antennas, LNA etc, time-
delaying system, and any associated mechanical drives;

. Consideration of the impact on cost and performance of breaking an array-
station into a number of "concentrators".

5. Conclusion


It would appear that 3 out of the 4 astronomers' desires could be met by
using "randomised" widely spaced elements. The elements should have
constant beamwidth (and hence gain) across the 10:1 band. This then raises
a very critical question for the astronomy community to ponder on: What is
the collecting area versus frequency characteristics, which would be the
most desirable compromise for all useful astronomy observations. If a
relatively large collecting area is required at the higher frequencies, it
may be desirable to add additional high-frequency elements interspersed
with the elements covering the complete bandwidth, for example.


6. Reference


Bruce MacA Thomas, "An evolutionary approach to the development of the
"Square Kilometre Array", and related generalised antenna layouts and
concepts, particularly for a low-frequency facility covering the
approximate frequency range 150 - 1500 MHz", ATNF Technical Document
39.3/087, 22 January 1999.