history

For me it all began in the early 1970's after reading Frank Hyde's book "Radio

Astronomy for Amateurs". Before this I expected one must have some rather

gigantic antenna in order to receive deep space radio noise.

After a couple of years of thinking about such a project, but not actually building

anything as yet, I was given a copy of J.R.Smith's publication of a practical radio

telescope (B.A.Soc.of G.B.). This particular design was for 178MHz, used small

trough parabolas in phase switched interferometer mode. This was new to me

and it took a while appreciate the considerable advantages of this mode. He also

showed several other modes of operation, like beam switching, all using the same

circuit ,but switched in different configurations. In short, I was hooked!!

Being reasonably familiar with electronic design and construction, I set out to

build my first radio telescope. 245MHz was the frequency I decided on, just well

clear of the VHF-TV band. The "LNAs" were equipped with MPF121 MOSFETs,

probably 3bB N.F. (or maybe worse!). Phase switching was done right at the

receiver input. One of the major hurdles was the need for a chart recorder .

I had to build one myself, a not inconsiderable mechanical and electronic work

effort . It did work quite well, but fortunately later I was given a decommissioned

larger instrument,which was easy to convert into a reliable chart recorder.

During the subsequent years I got interested in EME (moonbounce) communication.

It took several years to "get it all together". I built a 6m fully steerable parabolic dish

which was auto-track processor controlled to keep pointing at the moon. The trans-

ceiving equipment was in part commercial, the high power 432MHz transmitter and

receive LNAs were homebrewed. In many respects EME was almost opposite to ARA. To get the most from the tiny reflected signals the bandwidth was reduced

to a little as 40Hz. Due to the huge path loss in EME mode ( -262dB), the 6m dish

was not much more than minimum requirement. In that time I first started to use

the new generation of low noise receive devices, GaAsFETs. They made a very

impressive improvement in EME receiption, and I realised this would also make a

great front end for ARA !!

Finally, by 1990 I returned to ARA. First I operated with my "cut-down" quad helix

array; one 15turn helix at either end of a 23m E-W baseline, op. on 408MHz. Also

two 5el. 142MHz Yagis were added and for nearly two years solar drift scans were

done simultaneously on 408 and 142MHz. This was "supplemented" by optical

solar astronomy as well, to compare sun spot appearances with flare events.

Many flares were recorded, several x-flares as well, the 142MHz frequency being

far superior to 408MHz for that purpose.

It was obvious that the telescope was confusion limited, rather than sensitivity

limited. In an effort to sharpen up the beam, the frequency was changed to

625MHz. Alas, this was not successful, because I found it impossible to con-

struct a suitable collinear feed strip which kept the beam exactly central.

The larger number of individual elements invariably spread the beam out due

to small additive phase errors.

Because of this I then decided to scrap the trough parabolas and build two

5metre "stressed" parabolic dishes. These turned out to be far superior to any

other antenna type I had previously ever used in ARA.

Like the trough parabolas, the dishes were fixed in R.A. (E -W baseline) and

manually settable in declination. For a while I operated the dishes with a dual

frequency helical feed system, for 625 and 1400 MHz.

A typical chart recording from that system is shown below :

Dual frequency recording of

two X-class solar flares .

Top trace is 142 MHz

Bottom trace 408 MHz

Note the two events visible in the

408MHz trace associated with the X-6 flare. They still come up

as fairly large fringes, although

the Sun is already out of the main antenna lobe ! One can

only guess how large the fringes

would have recorded,had the Sun

still been in the main lobe.

Date : 27th Oct.1991

By March 1992 two 3.6 x 4.2m trough parabolas were constructed and installed

along the same E-W baseline. The linear feed elements were made from alumi-

nium strip, operating frequency 375....385MHz. There was too much RFI closer

to and above 400MHz. Numerous experiments with modifying the feed system

were done, in order to minimize direct frontal pick-up rather than via the reflec-

tor surface. It culminated in fitting subreflector assemblies out from the feed

elements. This paid off , and many recordings of deep space sources were

made until late 1995.

A higher resolution interferometer telescope can be used for some interesting

observations. Below is a chart recording showing both, the Sun and Tau-A

radio source. The beamwidth in R.A. is actually smaller than the Sun's diameter

and therefore the Sun is partially resolved (in R.A.). The amplitude of the solar

interferometer fringes is therefore much reduced, in fact if the baseline spacing

was any wider, the fringes would tend to get suppressed altogether. If the base-

line spacing was narrowed, the amplitude of the solar fringes would greatly in-

crease. The actual ratio of solar to Tau-A fringe amplitudes is roughly 200:1 .