The picture is in two parts. At the bottom is the view across our back garden a frosty morning. At the top is the same view taken at night using 11GHz 'light' and it shows the arc of the geostationary satellites across our sky. Here's the story of the image:
It came about from conversation with my son, Kevin, where we wondered what the world would look like in frequencies of electromagnetic radiation beyond visible light. IR and UV pictures were easy to find on the net but we wondered about the TV transmitter 20 miles away. It's spraying 400-800MHz across the landscape and we wondered whether, if you had a very directional antenna, you could produce an image that would show even the nearby hills being illuminated by this 'light'.
Then I realised I had a very directional antenna - a satellite TV dish. It's a little camping satellite TV kit with a 40-cm dish and it includes a LNB (low noise block - at the dish's focus) and a receiver. We bought a simple signal finder meter and wafted the dish around the garden. Though it has a conventional meter display, it does not directly measure the signal level. Rather, it is a bistable device which emits a tone on one side of a threshold level and sends the needle to the opposite end of the scale, However, the level of that threshold can be set by a knob and I realised this could be a way to gain signal level measurements for low cost. Our garden trials were showing profound differences between, say the lawn, and the path running across it. Since the TV sats transmit around 11GHz, we would get a 'photograph' in that 'light'.
My son's a drummer and he used some of his hardware to fashion a rig with which he could pan and tilt the dish. We fashioned protractors from CDs with labels stuck on them sporting scales made on a drawing package on the computer. That gave us azimuth and altitude angles. We made another small protractor for the signal meter, bored a tiny hole through its knob and passed a sewing needle through to give degrees of rotation. Our plan then was to point the dish in a known direction, adjust the meter to read '5'; the middle of its scale (and therefore in the middle of its threshold) then take the angle of the knob's adjustment as a measure of the signal level. The meter's frequency range is 930 MHz to 2.4 GHz. The LNB is downconverting the 11GHz into a frequency in that range. I don't know how that translates into the bandwidth of the system as a whole.
We set up and started at 20:30 one evening to take measurements for a 66 x 30 pixel image - 1,980 samples in all! It took 6 hours (!) of Kevin out in freezing temperatures pointing the dish, and me in the relative warmth adjusting the meter and entering angles into a spreadsheet. We were shattered by the end; Kevin using his stamina as a hillwalker to stay with it despite the cold. As we worked, I could perceive variations in the levels that looked interesting.
The next day, we used Paint in Windows to directly transfer the angle measurements into grey values in a 66 x 30 pixel image. Another 2 hours! Once finished, the levels were inverted and stretched in Photoshop and the image enlarged to make the final 'photograph'. I was delighted to see the arc of the geostationary satellites. The bright splodges are us looking right down the boresight of the TV sats that serve this area. I'm guessing the dimmer splodges are leakage glow from other TV sats or other downlinks at a similar frequency but aimed at other parts of the world. I was really pleased to see the outline of the trees and a hint of the path across the lawn. Also, there is an obvious reflection from the powerful sats off our wet wooden deck.
Now if someone could provide a computer-controlled pan/tilt platform, wideband signal measuring gear that will directly attach to a computer and software to directly translate level measurements to pixel values, we'd be laughing! I'm told that the Cassini probe was made to do a very similar thing by tediously rocking the probe backwards and forwards to use its large antenna to image in the Saturnian system in 13GHz.
Thanks for reading.
David Woods
