Friday, 21 November 2014

Radio Sculpture

A sculpture is a physical representation of an idea, a concept or even a feeling. In many ways, anything we can build becomes a sculpture of our thoughts.

Scott Mitchell is a Sculpture Artist and has many works to his name. As an entrant in the Melbourne Prize for Urban Sculpture 2014, Scott set his mind on a sculpture that would divert our attention to the sky. As a propositional work (for a larger version in Melbourne's Royal Park) he decided to build a working Radio Telescope.

Scott approached the Astronomical Society of Victoria and came along to one of the Radio Astronomy section monthly meetings. We spent several hours helping him understand some of the concepts, the science and the electronics behind Radio Astronomy. We also went over the History and the Pioneers of Radio Astronomy. Eventually Scott decided a Horn Antenna would be most appropriate and sculpturally pleasing.
The Horn Antenna has an opening at the wide end to allow radio waves to enter and funnels the waves down to a receiver element at the narrow end, usually just a short wire that is tuned to the particular frequency of interest. Then it connects to an amplifier to greatly increase the signal level. In Scotts design a receiver normally plugged into a computer to pick up Radio and TV signals is used to receive signals on the Hydrogen Line at 1.420 GHz.
The Hydrogen Line is the frequency where neutral Hydrogen atoms emit radio waves and allow us to see into the Cosmos. Other atoms also emit radio waves at different frequencies but Hydrogen is by far the most abundant element and therefore is the easiest to detect. Scott has named his sculpture 'The Listener' and has thoughtfully provided a pair of headphones as part of the display so that visitors can listen in on the static from the Stars much as Ellie Arroway did in the brilliant movie 'Contact'. The display also has a chart recorder under a perspex cover that draws lines representing the level of radio noise being received. From what I saw, the chart paper is replaced every morning at 10am which will give a permanent record of the two weeks that the display is available for viewing.
I visited the sculpture at Federation Square and took a few pictures. I even chatted with quite a few people that came to find out what it was. Being the friendly and knowledgeable fellow that I am I was more than pleased to tell them. I guess I was happy that Radio Astronomy was on display.


Thursday, 5 June 2014

Steering a Dish Antenna

Motor Drives

Firstly, before I write too much more, I should apologize for taking so long to get another article on this blog. I have a new radio astronomy project thats taking a lot of time but is proving interesting. I'll try writing about that shortly.

In the meantime I've worked out a cheap and readily available method of azimuth drive on my dish antenna. Last year I was looking for a way to effectively mount and turn a TVRO type mesh dish and use it as a point-able Radio Astronomy antenna. Thanks to the amazing developments in 3D printing technology there are some very useful devices available through Ebay.

I decided that I needed a cheap and usable stepper motor to control the azimuth (from 0 to 360 degrees). With careful attention to mounting hardware in order to reduce friction and the amount of torque needed I think I have a reasonably practical method of control.

The stepper motor I'm now using is a Nema17, 2 phase, 1.8 degree step device with a 5mm shaft. This is fitted with a 12 tooth aluminium timing pulley and drives a timing belt of two meters in length. The pulley and belt are available on Ebay as part of a kit for position control on a 3D printer.

The parabolic dish mounting is a circular disk 600mm in diameter around which the timing belt is wrapped and fixed at either end. Some slack in the belt is needed to go around the timing pulley.

In the picture above, taken from a random seller on Ebay, the timing belt is a rubber belt but reinforced with fibreglass to give it a lot of strength. The belt wont stretch therefore wont lose tension once fitted to the azimuth drive table.

The best part about this is the ability (I hope) for the Arduino and associated stepper controller shield to drive the stepper motor (rated at 2 Amps) without too much effort. I have read a lot of reports where the stepper shield H-Bridge L-298 overheats very quickly so a stick-on heat sink and reduction in voltage may be needed to reduce heat.

The other important part of this will be a brake system. When power on the stepper motor is off, the motor turns easily and will result in the dish rotating in a breeze. I think the best option will be a relay controlled device that will hold the azimuth table while the stepper shield is powered off.

The next post will hopefully show the stepper motor, toothed pulley and belt with the azimuth table all mounted together.