Matthew will hopefully pull information/add links when the 'official' OMT ( One Mile Telescope ) page is created.
Members of our group
Main One Mile Telescope Web Page
Discussion 1 - Modification to the aerial
Discussion 2 - Connection of aerials to LNAs
Discussion 3 - Mechanical modification to the 408MHz dish feed
It was proposed that a second dipole be added to the 408MHz aerial 90 degrees rotated ( to make a cross ). The new elements were to be 1/4 wave back. However this would have the effect of mis-matching the new elements because of its proximity to the rear of the reflector, and the illumination pattern would be narrower than the front element.
Bernie and I propose to add the new element next to the existing elements ( still rotated by 90 degrees ) and delay the signal through a 1/4 wave of semi-rigid coax.
The 1420MHz aerial will be untouched.
Below is a diagram showing the two 408MHz dipoles at 90 degrees, combined to give circular polarisation. The clockwise and counter-clockwise components can then be filtered at 408 and 432 MHz before amplification. The need for this filtering is debatable, and we could risk intermodulation/blocking problems and work without filters. The aerial response is flat over the 408/432 frequency range. The combiner should be flat over this range ( to be tested ).
There are two coax feeds from the dish to the control, room. We can duplex the 1420Mhz feed into the 408MHz feed as shown below :
The requirement for the 1296/1420MHz amplifier and the 408/432MHz amplifiers are similar, there should be very little filter loss, but the amplifiers should be protected from our-of-band large signals ( cellular, paging, radar ). The block diagram below shows the general proposed layout.
As the input impedance to the filters when off-frequency will be high, the combining should be ( almost ) loss-less. It will not need a 6dB splitter. All the modules will be in different boxes, connected by SMA connectors.
The focus box mount
The full dish ( for those that haven't been to site )
A view of the control room
