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e-MERLIN

e-MERLIN is a major project to up-grade the MERLIN array of radio telescopes, extending its frequency coverage and greatly enhancing its sensitivity. The concept of e-MERLIN implies a major change to the infrastructure of MERLIN. A considerable redesign and rebuild will be required. We are responsible for the development of the following subsystems.

  • Antenna feeds
  • Polarisers
  • Low noise amplifiers
  • Filters
  • Front-end frequency converters
  • Remote diagnostics

L band (1.33 – 1.73 GHz)

e-MERLIN will see the implementation of deployable horn/lens feeds to allow L band operation at the secondary focus, thus providing for remote controlled frequency changeover within 1 minute. A scale model feed system has been built and tested (see photo) on the lab indoor test range and on the Pickmere radio telescope, at 22 GHz.

A scale model of the feed system
Fig.1 A scale model of the feed system.

C band

MERLIN’s C band receivers will be upgraded from the current frequency coverage of 4.5 to 5.2 GHz, to cover the band 4 to 8 GHz. This requires five new feed designs (Lovell, Cambridge, MkII/Defford, Pickmere/Darnhall/Knockin), a wide band polariser system, and new broad band LNAs.

Su Yan leaves us to return to NAOC in Beijing on 17th April 2003. Her six-month visit has been a very productive one, during which she has completed a design for a prototype C band (4 – 8 GHz) ortho-mode transducer, designed C band feed horns for the Lovell, MkII and Defford (these have been constructed and tested successfully), and completed most of the theoretical design work on cassegrain feed horns for the E-systems and Cambridge telescopes. Yan’s work has been extremely valuable to the e-MERLIN project, and we very much hope that she will continue her collaboration with JBO in the future. When she returns to China the first priority is to complete her PhD, and she will then move on to design feeds and receivers for the Miyun 50 metre telescope project.

C band Feed and polariser designs for e-MERLIN

e-MERLIN requires the development of four new feed designs to cover the 4 to 8 GHz band, for the Lovell Telescope, MkII/Defford, Cambridge, and the E-Systems Telescopes (Pickmere, Darnhall, Knockin). The need, in each case, for good return loss and constant beamwidth across an octave band presents a set of difficult challenges. Ansoft’s High Frequency Structure Simulator (HFSS) has been used to model designs. Work on the cassegrain telescope feeds is in progress. A prototype MkII/Defford feed (a wide flare-angle scalar horn) has been manufactured and tested, both in the laboratory and on the MkII, and on the Defford telescope (Fig.2).

The MKII/Defford feed prototype and its radiation pattern measurements and simulations. The overall diameter of the feed is approximately 150mm.
Fig.2 The MKII/Defford feed prototype and its radiation pattern measurements and simulations. The overall diameter of the feed is approximately 150mm.

The Lovell Telescope feed is a less conventional design, using a dielectric loaded circular horn with closely spaced chokes. A prototype Lovell feed has been manufactured and measured in the laboratory (see Fig.3).

The Lovell feed prototype and its radiation pattern, measurements and simulations
Fig.3 The Lovell feed prototype and its radiation pattern measurements and simulations.

In order to detect circular polarization at C band we will use a combination of an Ortho-Mode Transducer (OMT) and a 90 degree 3dB hybrid coupler. Quad-ridge OMTs have been used extensively in the radio astronomy community to give wide-band polarization discrimination, and we currently use this type of OMT for MERLIN at L band. The new C band design has been modelled in HFSS, partly based on work by James and Skinner at CSIRO. (See Fig.4) Agilent’s Advanced Design System (ADS) has also been used in the design of the coaxial matching sections. Manufacturing of the prototype OMT, with very good simulated performance, will shortly commence.

Fig.4 The OMT model in HFSS
Fig.4 The OMT model in HFSS

The important part of the e-system feed (Pickmere, Darnall, Knockin) has been modeled in HFSS and got an excellent impedanmce match. Because the feed dimension is very big, to guarantee the design is successful, it is planned to test one part of the feed (mode converter & transmission section) first. Engineering drawings have been produced.

Our 90 degree hybrid is being designed by the University of Birmingham’s Department of Electronic and Electrical Engineering. It will be constructed using high temperature superconducting materials, thereby ensuring that it contributes virtually no noise to the receiver.

The critical parts of the E-system feeds (Pickmere, Darnall, Knockin), including taper and mode converter, have been manufactured, as shown in Fig.1. They were tested in the lab and gave good return loss results as expected (see Fig.2) considering the effect from the taper. The complete E-system feeds are in manufacture and will be finished in March.

Part of the e-system feed.
Fig. 5 Part of the e-system feed.

The measured return loss of part of the e-system feed
Fig. 6 The measured return loss of part of the e-system feed.

In order to detect circular polarization at C band we will use a combination of an Ortho-Mode Transducer (OMT) and a 90 degree 3dB hybrid coupler. Quad-ridge OMTs have been used extensively in the radio astronomy community to give wide-band polarization discrimination, and we currently use this type of OMT for MERLIN at L band. The prototype C band OMT has been completed (see Fig.4). Because it is difficult to make such a complicated structure, one fin is offset slightly through the wave guide and the measured results are not as good as we expected. We need to do some fine-tuning. When we cut the end of 4 fins by 1.2mm with 45 degree slope, its return loss improved by 2 dB (see Fig.5).

A picture of the OMT at C-band
Fig.7 A picture of the OMT at C-band.
The measured results of the OMT, 2 ports
Fig.8 The measured results of the OMT, 2 ports.

Because the Cambridge feed dimension is very big, to guarantee the design is successful, one quarter part of the Cambridge feed has been modeled in HFSS and we obtained a constant beam width over an octave bandwidth. Fig. 5 is the simulated model in HFSS.

The simulated model of the Cambridge feed in HFSS
Fig. 9 The simulated model of the Cambridge feed in HFSS.

Current Activities May 2005

A material for the L-band lenses is being tested by the Electrical Engineering department. Cryostat contruction continues and the completed ones are being tested.

Last updated Thu Jun 16 10:07:29 2005