Wednesday, December 16, 2009
Monday is antenna day. After breakfast, Thorsten dug a test hole for our first antenna, so about 50 feet from the station. This was partly to see how the digging goes; it will also be a site to transmit signals to the other antennae. He ended up with a roughly foot wide slot, about 6 feet deep, and 7 feet wide at the top. He needed to cut steps into one end to get to the bottom. It does the job; as you can see above, the top of the antenna is a few inches below the snow surface. We had initially wanted to dig the antennae in deeper, but this is close to the limit of our digging technology. Going deeper would require larger holes in all 3 dimensions, and far more effort.
I spent some time figuring out exactly where to put the station antennae and stomping down the ground to firm up the snow. This is more complex than we realized - between the equipment boxes that must be buried (later), the guy ropes for the station (we certainly don’t want to compromise these), within the length of the antenna cables, the choices are limited. Eventually, we decided that the East-West antenna pair must be offset – not ideal, but the best that we can do.
After lunch, I took a turn digging an antenna hole – the first for the station antennae. I do finish it, but it’s the bulk of my afternoon.
We also take measurements of antennae properties, with an antenna a few feet in the air, sitting on the snow surface, and in the hole. The simplest parameter to measure is something called the standing wave ratio; a measurement of it’s impedance (complex resistance) at different frequencies. The goal is to understand how the antenna behavior changes in ice, compared to air. We do this with a nifty gadget, an Agilent FieldFox network analyzer. Twenty years ago, a measurement like this required a decent laboratory; now we can lift the analyzer with one hand. Judging from the brochure pictures, the FieldFox is designed for use servicing cell phone towers, but it works well for us also. It is a real kick resting it on a wood plank laid on the snow to make measurements. Between that and our oscilloscope (also made by Agilent), power supply, etc, we have a reasonably well equipped lab.
Toward evening, Thorsten started working on the transmit-receive measurements, whereby we send a signal from one antenna to a second one; we expect to observe an initial ‘direct’ pulse, followed by a 2nd pulse when the signal reflects off the ice-water interface. Of course, the 2nd pulse should be much later, and also much smaller. The photo below shows our receiver laboratory, with low-noise RF preamp and oscilloscope.
Finally, this is Martha sitting at our dining room table. As in most new high-end housing, our kitchen opens to a breakfast nook, which in our case also serves as our formal dining room. If you look closely at the floor, you'll see why it's not always a good idea to choose the low bidder to install hardwood floors.
Posted by Spencer Klein at 5:08 PM