New Zealand Radioscience Group

 

Formed in 1996 with the aims of:

Further background given below.  

 

RECENT NEWS:

4.5m radio-interferometer now in action at Emerald Hill.

  

The Emerald Hill 11GHz Interferometer

        Ian Gallagher -- NZRG -- August 2008

The initial beginnings of the Emerald Hill Ku-band interferometer came from the gift of some surplus satellite receiving
equipment. In my role as board member of the NZRG I received from Mr Mark Powley of VSAT Communications a variety
of satellite receiving equipment --- including three Ku band Low Noise Blocks with external synchronisation, and two
Scientific Atlanta down-converters. Having this equipment to hand got me thinking of what to do with it. 

After a bit of Internet research, I came across the work of Ken Tapping on an interferometer built from surplus satellite stock.
His system had been set up at 4GHz (C-Band) and used slightly different technology, but I decided to give it a go.

My plan was to build an interferometer at Ku Band (11GHz), making use of the LNBs and components of the down-converters.
Initial work focussed on building a single receiver chain sufficient to power the LNB and prove the hardware would work.
This soon progressed to bringing up the second receiver chain and linking the two together. The final needed items were
suitable antennas. I obtained two small microwave linking antennas (600mm diameter) that were being scrapped, and yes ---
I was on my way.  First fringes for the interferometer were received from a transit of the Sun through the system on Sep 02 2007.

OK -- so what is it?

An interferometer is basically a type of light-responsive instrument, but in this case the 'light' comes at radio wavelengths
and the sources are in the sky.  It comprises of two (or more) sets of receiving equipment linked together, receiving an incoming
signal wave. As the wavefronts from the astronomical target are essentially parallel, there is, in general, a (path) difference in the 
arrivals of these wavefronts at either feedhorn.  If the receiver systems were electrically identical this difference would depend solely
on the distance between the antennas and their inclination to the incoming front. The two received signals are combined and,
depending on the path difference, they may either support each other or cancel each other out.  The net 'interference effect' is
detected as a slowly varying low frequency voltage. As the Earth turns, the phase relationship changes, and so the combined
net signal changes magnitude and polarity. When plotted over time this gives rise to characteristic graphs of fringes as showm above. 
With our present set-up the timescale of interference fringes from an astronomical object are about one and a half minutes for the
Emerald Hill interferometer.

Right.  How was it built?

The basic interferometer consists of:
 1/ Two microwave antennas fixed to a common shaft to maintain rigidity 
2/ Two externally synchronised Low Noise Blocks (Norsat XC1000s LNB) synchronised to an external 10MHz master oscillator 
3/ Twin receive chains also synchronised to the same 10MHz master oscillator. The receivers consist of:
The down-converted signal (now at 100 to 180MHz) is amplified again in a second IF stage and fed to the final mixer.

4/ 10MHz ovenized Crystal Oscillator. This critical component keeps all of the receiver components in phase with each other,
and hence maintains the phase relationship between the two antennas.

5/ The final mixer. In this double balanced mixer the outputs of the two receivers are combined and the difference signal
(being the phase information) is output as a DC voltage (with a slowly varying AC component).

6/ A DC amplifier was added as the output from the mixer was very weak (a few mV) and hard to read accurately on my digital multimeter.
(This was built from a Dick Smith Electronics/Electronics Australia audio preamplifier kit, based on an LM833 dual op amp.)
The addition of the DC amplifier allowed me to pick up more sources, such as the Galactic Centre (see below).

Enhancements

Once I got the system to go, there have been as steady series of enhancements as the fever has taken hold, such as: 

The two new antennas were mounted at each end of a 4.2m shaft (50mm galvanised steel pipe from Building Recyclers) mounted in
‘modified’ engine hoist frames and attached to a wooden frame.

I had also been gifted an electric jackscrew and controller (Astro Pro Z500), typically used to rotate a C band TVRO dish. The jackscrew
was mounted onto the wooden frame and attached via a radial arm to the shaft. A certain amount of creative butchery inside the controller
has enabled me to interface it to a computer (using a Vellman K8055 kit). The completed array now looks as shown above and is under
manual/computer control. (The large insulators at the back of the antenna are actually counterweights).

The coax feeds from the LNBs to the receiver were lengths of second hand RG6 of dubious history. I had previously been given some
drums of RG11 75 ohm cable TV coax, so equipped the new antennas with 25m of RG11 each. Measured loss at 1 GHz was just 2.5 dB! 
Following the DC amplifier (made with one half of the LM833 kit) I used the other half to construct a level shifter to keep the DC output
positive for a sampling circuit to measure it. (The sampler was not keen on negative DC voltages).

Measurements with the DMM at this point proved disappointing. A sensitivity of about 18 janskies per milliVolt of DC output (using the Moon
as a reference) was being achieved, according to the formula:


Flux = [Boltzmanns Constant x Surface Temp x Angular Area]/[Wavelength squared]               (about 48 kJy for the Moon)


Crab Nebula, the Galactic Centre (Sagittarius A*) and Venus all gave of the order of 80 mV peak to peak fringes. The Moon was good for
about 1.2V peak to mean, or approx. 2.4V peak to peak.

This was a lot better than the original rig (about 38Jy/mV), but still below expectations. The final touch (so far) has been to put a high pass
filter and further stage of amplification in following the DC level shifter. The output (now very low frequency AC) is fed to a computer sound card
and analysed with “Spectrum Lab” -- a freeware program from Wolfgang Buescher, based on fast Fourier transforms. This has opened up a
whole new world!  I am getting so much data that there is a difficulty identifying real sources, but it is clear that the sensitivity is much improved
with many obvious fringes in the data

Parameters of the system 

The trace (from an Excel record) shows the transit of the Omega Nebula (M17 -- also called the Swan Nebula) giving the first clear
observation of this source in the microwave range in New Zealand. The transit is over to the left of the trace, what follows is
typical system noise.  Interferograms like this can be matched by calculating what a given model of the source would produce when
scanned through the pair of beams of the twin antennas.  From best-fitting the calculation to the data we derive an optimal model of the source.

To do list:  Parametrize a model for a given interferogram.

Previous news: 'Dark Ages of Universe' 

This proposal (directed to the RSNZ's Marsden Fund) arose in 2005 and involved the NZRG in discussions with a group of fundamental science 
researchers, in particular Drs Slava Kitaev (AUT) and Gregory Tsarevsky (ATCA & Sternberg Institute), from whom further information can be
sought.  The NZRG retains interest in such work, particularly through friendly support from the ATCA, who supplied some of the Group's initial
equipment at the CIT.   

Original base of the NZRG at former-CIT, Heretaunga, Upper Hutt

The 5 metre NZRG Antenna, with 4 and 12 GHz dual feedhorn attached, on the CIT campus.
The picture was taken during the visit of Prof. L. Mestel in November 1999, here shown
in the foreground with Mrs P. Budding

The New Zealand Lottery Grants Board (Lottery Science) was particularly helpful
in launching this project: providing the financial support to allow basic equipment to be
acquired and set up at the Central Institute of Technology in 1997-99.

In 1997 the Group became affiliated to the Royal Astronomical Society of New Zealand (see links below).

NZRG interests also relate to communications and TV technology, remote sensing and education.

Links

Other Information

Current Activity

Background Information

Research Context

Links

http://www.rasnz.org.nz/

http://astronomy.wellington.net.nz/

Back to top

NZRG Team: mailto:nzrg@outpost.co.nz

Tel: 0064 4 232 6388 ; Fax: 0064 4 232 6356

Acting Secretary: mailto:budding@xtra.co.nz

Tel: 0064 4 232 6388

Back to top

The NZRG's first aim was the setting up of a two-element (East-West) astronomical interferometer on the CIT campus
(see also Research Context). Two f/0.4 ex-satellite-tracking antennae and associated equipment were applied to this.
Equatorial mounts were imported from the disused radiohelioscope at Culgoora, as operated by CSIRO Radiophysics
Laboratories, Australia.

Though early experiments with the solar emission in C-band (4 GHz) using a ~250m baseline were unconvincing, the
experiment succeeded on a shorter (8m) baseline and has been written up (Thresher et al, 2002) (First results shown below).

 

Brief description of 'Mark I' antenna at the CIT

The LNBs (EchoStar model 950) are sensitive to a bandwdith of some 400 MHz centered at around 4 GHz. With technical
input from the company 4RF of Wellington, the local oscillator of one LNB was deactivated and replaced by an amplified
signal sourced from the local oscillator of the other, fully functional LNB. In this way, a single mixed conversion frequency
(around 5.1 GHz) was used to shift the amplifier outputs to a 400 MHz band centered at 1.1 GHz. These downshifted outputs
were fed into a passive IF combiner block. The common output was connected to a commercially available diode detector,
designed as an aid for positioning of antennas for TVRO signal reception.

The DC analog signal from the diode detector was modulated to 200 Hz and connected to the input of a computer soundcard.
The data was then displayed and logged using the downloadable program "Radio Sky-pipe"

The main 5m (West) antenna was built and first tested on bright astronomical objects and TV satellites in 1997. A local control
hut was constructed on the antenna's concrete plinth. 3-phase power was fed to a purpose-built switchboard in this hut. Position
encoders have been attached to the two axes and these display digitized angular positions currently on the switchboard facia.
These displays link into a PC-based position control unit in the hut.

Regular observations of the Sun (and, to some extent, the Moon) were carried out during the period 1998-2001. Comparisons of
this data with standard references has shown that the simple 5m dish with a commercial TV detection system can produce reliable
data on bright sources (see Dodson & Budding, 2000; Budding et al., 2000).

Back to top

At the Regional Meeting of the IAU in Sydney in 1990, there was a proposal for a relatively large sum to support a New Zealand-based
Very Long Baseline Interferometry (VLBI) "leg" to the Australia Telescope National Facility.

This proposal was well received scientifically, and has formed a key goal for the longer term, but this purpose requires the development
of an infrastructure of interest and support. This is a basic purpose that the NZRG has attempted to address.

The relatively large engineering requirements of radioastronomy seek appropriate experience. This has been sought from tertiary sector
organizations in the Wellington region. The Group was based at the former Central Institute of Technology (CIT) in Upper Hutt, and
included several former CIT members of staff on its board.

The 5m dishes and receivers came from the estate of a local amateur enthusiast (Ray Illingworth). Software for a control system for
directing the antennae to targets of interest was devised by 3rd year electronics students at former-CIT. Another former-CIT student
built a broad band, high-gain amplifier, based on the design given in William Lonc's guide book Radioastronomy Projects
(St Mary's University, Halifax, NS).

The New Zealand Radioscience Group has held regular meetings since its formation and details of progress during this time are recorded
in a minutes book.  This covers well the period 1996-2002, but the closure of CIT in 2002 necessarily impinged on the Group's activities. 
The unfortunate loss of our former chief on-site engineer - Tony Dodson -- in 2004 was another heavy blow to the Group (see http://newsletter.carterobservatory.org/Sept04/ ).  The underlying aims of the Group remain valid, such unhappy events notwithstanding.

Research context

Academic aims of the Group were outlined in the article of Budding & Roberts (1996). This was updated by Budding (1998), and another
summary of the Group's aims and background can be found in Dodson & Budding's RASNZ Annual Conference 2000 ("Spacetime 2000")
paper. After that there were determinations of the lunar subsurface temperature cycle (Budding et al., 2000) and observations of the
strong microwave enhancement due to solar active region 9393 (Wellington Astronomical Society Newsletter, April edition, 2001): this
work all based on the CIT facility. 

Another project, to carry out a Radiometric Study of Geostationary Broadcasting Satellites in the Longitude Sector 140 E to 140 W, was
mentioned in a published paper in Solar and Stellar Physics Through Eclipses ASP Conference Series, Vol. 370: Astronomical Society of the
Pacific, 2007., p.261.  This concept was reported more fully to the Minister of Research Science and Technology (Mr S Maharey) and some
of his parliamentary colleagues in 2006, as part of an ongoing discussion around the Group's original 'Expression of Interest' document
submitted to the MOE at the time of the closure of the CIT in 2001.  A few further words can be added about this.

Physicists have sometimes discussed anomalies in the conventional explanation of gravity (see e.g. New Scientist , Nov. 27, 2004). 
There may be measurable negative effects on the stability of satellites in geostationary orbits. A radiometric high accuracy positional
study would provide fundamental understanding and solutions to mitigate these effects. A radio frequency study to locate geostationary
broadcasting satellites by high-accuracy signal timing from three 5m-dishes at the proposed locations: 1. NZ International Campus in Upper
Hutt, 2. the Pohangina Valley Facility, and 3. Mt. Taranaki could be relevant. Resulting high-precision data would have relevance to
geophysical studies.  Various specific items of interest were mentioned in the original proposal, but, further investigations since 2006 have 
indicated other practical problems not discussed in the original proposal.  

Surveillance of radio-bright stellar sources was also seen as a useful future research activity that the Group could relate to (cf. e.g. Slee
et al., 1994).  The early CIT work showed how the solar rotation rate could be determined, and insights into the nature of the lunar surface
rocks gained, from simple broadband microwave single-dish measurements (Dodson & Budding, 2000). Direct inferences on the physical
properties of a few bright cosmic sources are also possible by considering longterm but repeated measures of flux densities. Similar work
to that envisaged by the Group in its early stages, apart from that discussed in Lonc's book, was documented, for example, in papers such
as Jiricka (1994), Storey et al., (1994) and Roberts et al. (1994). The MSc thesis of Gene Davidson (1994) also summarises useful relevant activities.

References

Budding, E. and Roberts, E., 1996, Southern Stars, v36, 275.

Budding, E. 1998, Southern Stars, v38, 62.

Budding, E., Dodson, A. and Trethowen, H., 2000, Southern Stars, v39, 36.

Dodson, A.W. and Budding, E., 2000, Southern Stars, v39, 21.

Davidson, G., 1994, M Sc Thesis, Waikato University.

Jiricka, K., 1992, Bull. Czech. Inst. Astron., v81, 1.

Lonc, W.P., 1996, Radio Astronomy Projects, Radio-Sky Publishing.

Roberts, J.A., Hajsaleh, J. and Benge, R., 1994, J. Roy. Astron. Soc. Canada, v88, 233.

Slee, O.B., Sadler, E.M. Roberts, J.E. and Ekers, R.D., 1994, Mon. Not. Roy Astron. Soc., v269, 928.

Storey, J.W.V., Ashley, M.C.B., Naray, M. and Lloyd, J.P., 1994, Amer. J. Phys., v62, 1077.

Thresher, W., Wheatley, M., and Budding, E., 2002, CIT Occasional Papers Ser.., No 28.

Last Revised: 6/12/2008