Starting again from ‘scratch’ much had to be reviewed and relearned. The Internet was of course a great source of knowledge and contacts. As I have gained so much from people – Amateurs & Professionals who publish their material on the Web I decided to make the effort and put some of my own observations into the public domain. I hope you find something of interest in these pages.
Amstrad 11Ghz TVRO 60cm Antenna
Work on building this Radio Astronomy equipment began in Summer 2005.
Having ceased to work for five days a week there was now time to begin to fulfil a long held ambition since school days to observe the skies at Radio Frequencies. The first efforts were made with a 60 cm offset reflector and an LNB that had been used for receiving analogue satellite TV. A simple screw driven elevation arm was attached to the dish and the whole mounted on a 3600 azimuth bearing with a brake. The IF @ ~ 1GHz was fed to a receiver via 75W satellite grade coaxial cable. The typical spectrum is shown below – many analogue transmissions interspersed with digital signal from ‘Hotbird.
Choosing a frequency between the TV transmissions it was possible to process the signal from the receiver and observe a transit of the Sun through the antenna beam. This detection was made on 2/6/05 and showed that it was possible to detect a non terrestrial object without much difficulty.
First Detection of Sun @ 11Ghz
It soon became clear however that there was little signal from the Milky Way at these frequencies, partly due to the lack of synchrotron emissions and the limited spatial integration afforded by the few degrees beamwidth of the antenna.
The obvious next step was to operate at a lower frequency and use an antenna with a broader beam.
Thus the first low frequency experiments were begun with a high gain Yagi on 408MHz. This was the simplest and lowest cost way of building a basic receiving system thought to be capable of detecting the Milky Way
A suitable design for a Yagi was found on the Society of Amateur Radio Astronomers Web Site
A 408 MHz Quagi Receiving Antenna for Radio Astronomy by Hal Braschwitz and Jim Carroll . Jim Carroll demonstrated a prototype of this antenna at the 1990 SARA Conference. It was a version of an antenna Wayne Overbeck described in the April, 1977 issue of QST Magazine and was called a "Quagi Antenna."
The Quagi antenna shown above was mounted on a wooden Az-El mount and supported on a non-conducting tripod to reduce the effect of mutual coupling to nearby conductors and thus minimizing any distortions of the predicted gain and beam pattern.
The 50 W coax used to connect the active element to the head amplifier was fitted with an inductive clamp balun. The total impedance to the out of balance sheath currents was around 500 W.
Early measurements of the antenna frequency response using a Spectrum Analyser + Tracking generator and an electrically small monopole transmitting antenna some 20m across an open field from the Quagi showed that the antenna had a good symmetrical response with the –3dB Bandwidth of ~40MHz.
Frequency Response of SARA Quagi Antenna
-3dB Bandwidth of SARA Quagi
The polar response of the Quagi was measured sometime later on a more sophisticate ‘Antenna Range’ and was found to be ~300.. At this stage only simple MMIC block amplifiers were being used as head amps with typical gains of 18 to 22 dB and Noise figures of > 3dB. The next step was to purchase a very low noise amplifier and try to detect the Milky Way. The LNA chosen was the Radio Astronomy Supplies 406MHz WD5AGO design with an 18dB Gain and a 0.37dB Noise Figure.
Several other LNAs were available and details are obtainable on the SARA Web Site all priced at around $150US. Several large manufactures in Europe and the US also produce suitable components, however they tend to be built to higher commercial / industrial standards and have precision machined metal enclosures – but at a rather higher price.
With the LNA and the Quagi antenna it was now possible to attempt to detect the Milky Way. The equipment was set up and the antenna set to an elevation angle of ~500. The antenna was manually rotated from North to South passing through the region from Gemini to Orion. The whole set up and conduct of the measurement was very ad hoc. However after several azimuth sweeps had been carried out it was clear that a consistent response was being obtained. The presence of a person close to the antenna to rotate it had a significant impact - but never-the -less the signal showed a clear broad peak between Gemini and Orion. This is the emission from the Galactic Plane looking outward toward the edge. Even though the whole affair was rather crude and unreliable, the thrill of a first detection is something to be experienced.
Being in the UK of course, this called for an immediate cup of tea!
The records are shown below : all hand written and the data plotted out using a simple pen recorder.
This was the beginning of activity in Radio Astronomy at this site in Wales in 2005. Much has been done since using a variety of antennas, dish feeds and interferometers at the frequencies of 408MHz, 1420MHz, 4GHz and 11 GHz.
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