HBA tile #75 flat-lining

Following the high winds of 08-Mar-2013, we carried out a series of remote system tests to verify that KAIRA is still okay and functioning correctly. The best way to do this is to examine the spectrum of every antenna for each polarisation. These are processed by the Receiver Units (RCUs) and there is one for each antenna/polarisation.

The first test we did was for the High-Band Array (HBA). The following is the collected set of spectra for the entire HBA. We expect a broad "Saana-shape" spectrum with occasional vertical spikes of radio-frequency interference (RFI).

As can be seen, there are a couple of weak channels which show non-typical spectra. The RCUs are 80 and 81, which correspond to tile #75 X-polarisation and Y-polarisation. This indicates that there is a problem with the entire tile, and further investigation is needed. Here is the location of the tile in the array:

Given that this is the leading edge against the northwest gales that we have had makes us very nervous.

An expedition to the site will be required.

Snow on the LBA field

Over the last couple of days, we've reported how snow affects the RF performance of the LBA aerials. Today we have some more photographs from #L33 and #L34.

Just a bit deeper here. Note how this aerial has its wires partly
covered, where the others in the background have their guys and
elastics still visible above the snow line. (Photo: D. McKay-Bukowski)

There was about 20 cm of ice along each
receiver wire. (Photo: D. McKay-Bukowski)

Another image of the iced up wire. The snow depth is
approx. 70-75 cm. The HBA array is visible in the
background. (Photo: D. McKay-Bukowski)

Digging down to the ground plane. Notice how the aerial
is leaning. This is due to the ice tension on the wires,
guys and elastics, causing distortion of the LBA aerial.

Snow affecting the RF-performance

Yesterday we showed a plot which indicated strange spectral responses of the LBA aerials #L33 and #L34. Today we show what is causing this strange effect...

Snow.

Each LBA aerial has four wires that descend from the top of the aerial down towards the corner pegs. This is the receiving wires. However, the lower part is just for support and is either nylon or elastic. As long as the snow is below that level, it does not affect the performance of the wires to receive the radio waves.

The wires reach down to approx. 65 cm above the ground. But, if the snow depth exceeds 65 cm, then this partial covering will start to affect the antenna. And this is precisely what happened to aerials #L33 and #L34.

The author inspecting #L34. The snow depth here is 70 cm.

Strange subband responses

Data Monday again. This time we're featuring some subband statistics from KAIRA's Low-Band Antenna (LBA) array.

There are 48 aerials and for each of these there are two polarisations. Thus, there are 96 signals coming back from the array. Each one has its own Receiver Unit (RCU). After filtering, the signals are digitised and split into 512 frequency channels, called subbands. 1-second average power for each subband for each RCU is calculated and recorded. These are the subband statistics and they can be used for form a spectrum for each RCU, to show its performance. We've occasionally shown these before.

However, last week we noticed something a bit odd...

Four of the RCUs were showing low signals. These were numbers 65, 66, 67 and 68. Here's a plot showing them. Also shown in the plot are RCUs 64 and 65 (which are behaving correctly).

KAIRA subband statistics 05-Feb-2014. RCUs 64 & 65
are behaving normally. The others are a bit odd.

The forest of vertical lines towards the left is radio-frequency interference (RFI) from shortwave radio. The two near the right are FM radio stations. We do expect a peak towards 55-60 MHz, which is the best response for the amplifiers and antenna design. However, notice how RCUs 67, 68 and 69 have their peak shifted to the left a bit. And RCU 66 has a double peak.

Very strange.

If we look up the LBA aerial numbers associated with these RCUs, we find that RCUs 66,67 are for LBA #L33. And 68,69 are for LBA #L34. And, if we check the array map, we find that these two aerials are on the southwestern edge of the LBA array.



Well, we've now done some investigations and have found the cause of the problem. But we'll save that for tomorrow. Until then, you can speculate for yourself.  :-)