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.  :-)