LBA bandpass spectra

We're now in the midst of some intensive commissioning of KAIRA. Some of us have been working on the High-Band Antenna array and others on the Low-Band Antenna array. Today's report is about this LBA work. One of the things we need to do as part of the commissioning is confirm that all the low-noise amplifiers are generating the correct signal outputs. For each aerial there are two outputs: one for each polarisation. The bandpass spectrum for each of them should look the same (a bandpass spectrum is a plot showing the intensity received as a function of frequency, for the range of frequencies that the system can measure). In the case of a good working antenna, we should see something like this:

The lowest part (0-10 MHz) is filtered out. Then there are various short-wave radio frequency interference. The main part of the band rises up and then tapers off at the top end (going into the FM radio band).

However, we've noted that some channels are a bit weak:

And some channels are quite low across the main part of the band.

Before we can operate the entire LBA array, we will need to resolve these problems and ensure that all components are working correctly. This sort of trouble-shooting and tuning is a standard part of commissioning these sorts of systems.

LBA amplifier unit

Today we have a photograph of the top of one of the Low-Band Antenna (LBA) aerials. This is the so-called "LNA cap". The black disc at the top of the post in the photograph contains the two Low-Noise Amplifier (LNA) units which increase the strength of the very weak electrical signal that is created from the reception of the radio waves. The LNA cap is potted with resin to keep the unit water proof. The output from the LNAs goes into coaxial cables that run down the grey post. The red band on the post is just the label... in this case "01" (aerial L01 in the sequence L00-L47). There is also a bar code on the label for easy scanning and identification of the aerial.

The LNA cap of LBA aerial L01. Photo: D. McKay-Bukowski.

First pulsar observation at KAIRA

PSR J0332+5434 observed in single station mode with 31 subbands. The dedispersed signal is shown on the left hand side, while the original incoming signal is shown on the top right hand side. The lower left plot shows the power averaged across all 31 subbands after dedispersion. 

Today we tried something different. Instead of looking at our local atmospheric plasma, we looked at interstellar plasma, this time with the help of a pulsar. In this case, PSR J0332+5434, which is the brightest pulsar in the northern hemisphere.

Pulsars are highly magnetized neutron stars that spin very fast. This spinning movement causes broadband radio emissions due to acceleration of charged particles in the magnetic field of the pulsar. Due to the fact that the interstellar space contains charged particles (although typically less than one electron per cubic centimeter), the broadband pulse arrives to us at different delays for different frequencies. This is a familiar phenomena for radio waves travelling in our own local ionosphere, although the scales are much larger. In order to improve signal statistics, this dispersion has to be measured and also corrected for before averaging over wider bandwidths.

Using the LBA as a solar radio emission receiver

A five minute dynamic spectrum recorded using a single LOFAR LBA antenna element. Apart from the HF radio band, the most prominent features are several type 3 solar radio bursts and the FMCW over-the-horizon radar signal transmitted from Cyprus.

We have now started comissioning the LBA antennas. Although there still are several problems, we have managed to get over half of the antennas up and running. Here is a plot from one of the first spectral measurements done using this new antenna field. Here is a spectrum recorded using a single LBA antenna element over a 100 MHz band with one second time resolution. We happened to catch several solar radio bursts. We also regularly see the FMCW over-the-horizon radar located in Cyprus, so we are confident that the system is working as expected.

Cloud formation

Just a nice photograph to finish the week. This cloud formation was spotted over Pikku Malla. (Photograph by A. Jutila.)

Special lid

In order to service the HBA tiles, it is necessary to be able to reach into them. Normally, LOFAR stations are built directly on the ground. This means you can open up a tile, step into it and then work on the parts.

For KAIRA, this is not an option. The reason is that there is insufficient support under each tile to allow a person to stand inside it.

As a result, we need to use a special tile lid that we can place on an open cell. From there, we can get in sufficiently close to the parts to work on them, without actually needing to step into the tile itself. Just for the record, here are some photographs of the special tile lid.

Top view. (Photo: A. Jutila)

And the underside. (Photo: A. Jutila)

The reindeer problem

At KAIRA, we have a reindeer fence to keep the larger animals from getting in amongst the aerials. Apart from the RF-implications and possible damage to our experiments, we also want to prevent the animals from being hurt, should they get caught in any wires or frames. And we know that they do like visiting, given half a chance!

As we have reported before, a fence has been placed around the entire area. It is a wire mesh, some 2 metres high.

A small section of the reindeer fence. (Photo: A. Jutila)

At the site entrance, there is an even larger gate. Although we haven't featured this since the foundations went in, we can assure you that it is in place and working.

And stay out! A mother and calf are thwarted by the main gate. (Photo: A. Jutila)

So, given all this anti-reindeer security, we remain perplexed as to how the creatures continue to get into the KAIRA area.

Cute is no excuse... you are still on the LBA field! (Photo: A. Jutila)

And it is not just the little ones. Even large reindeer are getting in as well. Given the number of times we have checked the gate, perimeter and all, the only logical conclusion that we can draw is that they are flying in.

But flying reindeer? Really?