Basic VHF Radio Telescope
A few years back I bought some large VHF Broadside Antennas at a stock-take sale. So during one weekend, I thought I'd just weld up a stand and see if I could use them as a simple radio telescope.
Using a WinRadio WR-3700e Wide-band receiver and small preamp, I scanned the VHF spectrum within the antenna's designed frequency range for a radio quiet area where I could conduct a meridian drift scan of the sky.
After a week of scanning and logging, the quietest frequency identified was 231.58Mhz which was reasonably clear either side by about 1Mhz. The scans where conducted at a receive sensitivity less than 1uV, the bandwidth was 17Khz and was logged at 1khz steps, AGC (Automatic Gain Control) was disabled.
The purple line indicates the maximum signal strength detected over the 24 hours and similar the green the average and orange minimum. Note that over the 24 hour period, some RFI interference occurred at different times of the day. Nevertheless this could be reduced using a band-pass filter at a latter stage.
The audio of the receiver was connected to a computer using the sound card and was logged over 24 hours from 7pm to 7pm using the the famous amateur radio astronomer software Radio-SkyPipe and the results so far are not to bad for a first attempt.
The antenna was aligned approximately by eye to line up with the sun as it passed over, if there are any extraterrestrial objects it is difficult to verify, as no planning at all was attempted.
The next trace was recorded from 9pm to 9pm where the antenna was tilted directly vertical to zenith.
The last is an example of the noise generated by heat in the equipment without an antenna over a similar 24 hour period. Note that the noise floor has been amplified significantly.
The two antenna where stacked to offer a 32 element collinear phased array and should in theory offers a gain of around 20dbi and a beam width of about 18 degrees. Although the gain may seem low compared with microwave radio telescopes the all important aperture size is over 4 metres square. Also note that galactic noise is very bright compared to microwave in the VHF spectrum.
Observations at 173.5Mhz
Along with the 232Mhz band experiment, I've also identified another band inside the frequency range of my VHF collinear array antenna which is also relatively radio quiet at around 173.5Mhz. However, in this series of observations I will try to set up my scans in such a way that I can also identify where in the sky the antenna is pointed and what would cause the rise and fall of signals recorded.
To do this I have first set-up the antenna using a compass to orient it in a North-South direction. This is so that the antenna can be tilted in ark from North to Zenith to South in order to conduct a meridian drift scan in different parts of the sky, as the earth rotates. In my case I used a simple Protractor to measure Zenith at 90 degrees directly up.
Then using a Level and a Protractor I have first pointed the antenna slightly North at an elevation of 80 degrees North and conducted a 24 hour scan using a software data-logger called Radio-SkyPipe. The result is the following trace in Red.
The time scale on these graphs begin in Adelaide Time February 2010 at 4pm for 24 hours.
I then repeated the scan, beginning at the same time on the next day, but with the antenna re-pointed at an elevation of 80 degrees South recording a different trace seen here below in Blue. In preparation I for these scans also run several in order to rule out terrestrial interference and although the the peak noise spikes seen in these graph changes, the same overall shape of the 24 hour scan is consistent.
Then using a program called Radio Eyes which is similar to a optical planetarium program I was then able to plot my location and time; along with the direction, elevation and beam width of the antenna. The program then provides a prediction of radio signals that the antenna could expect to receive at any specific time or date. The follow is a image is of the sky at radio frequency and has been edited to demonstrate the approximate source locations are in the above scans. Traced as it falls in relation to each.
Red being an elevation of 80 degrees North, and Blue being an elevation of 80 degrees South.
