out-of-band signals: this winter, we could point to any direction in the sky and we received
a very low power level (about 50 cts), in stark contrast to other times - such as in the
first days of operation in summer 2009 - when the empty sky gave a reading of 800 cts.
We identified a nearby (about 400m distance) tower with cell-phone transmitters
causes the preamplifier to saturate, because in is original version has no selective filter
in front of the amplifying device (a MMIC). Thus, the preamplifier operates with low or
no gain, creating a weak receiver output. We did not find any clear pattern
for the occurrence of the very weak power levels, and so we simply tried to observe as often
as possible and at various times and days.
The plot below shows an experiment we ran overnight: During about 10 hours, we were observing
the Galactic Anticentre l=180° b=0°. The left panel shows that during that time the received
power level fluctuated rather strongly between about 50 and 700 cts, so between complete saturation
and fairly decent reception. If one compares the apparent power of the galactic feature -
properly baseline-subtracted, of course - with the power level of the nearby flat continuum
(in the spectrum shown above one sess the flat region to the right of the galactic feature),
one finds a very close correlation, depicted in the right hand panel below, each small circle
representing one of the 3650 individual spectra:
Because we know that the galactic emission does not change, the 'feature power' is a direct
measure of the preamplifier gain. The strict correlation of the continuum power tells us
that its level is also a good measure of the gain. Therefore, we can indeed use the total
average power (which is displayed by the software) as a good indicator of how much the
out-of-band interference is saturating the preamplifier. This permitted us to give these
recommendations:
- if the power indicated was below about 200, there was very little chance to get
any useable observations. If it was less than about 50, we gave up any hope! We
had a coffee break, but we kept an eye on the software, until the situation
sometimes got better
- if the power was more than about 200 - and if there were no strong in-band signals
which would raise the power, of course - we could obtain decent results. If the
power level
also stayed reasonably constant, we could even interpret the data quantitatively!
Of course, we fought back, by building narrow-band filters to protect the receiver from the
off-band interference, and after locating another problem in a leakage of the braid of a small
coaxial cable, we no longer speak of interference ... Below shows the 2 resonator filter during
initial tuning (before closed completely), and a screen shot of one of the swept measurements
of the passband (about 150 MHz across the screen, the sharp dip near the centre marks 1400 MHz).
A variety of filters was built and tested. Initially, system temperatures of only 1000 K and
700 K could be reached, but this was already sufficient to execute complete surveys of the Milky
Way, and getting reliable and interesting data. Although the filters were carefully tuned with
50 Ohm impedance equipment, their performance at the telescope left much to be desired.
Obviously, the actual impedances of the preamplifier and the probe at the antenna's focus were
not close to 50 Ohm. After various trials, a procedure was developed that permitted to tune
reliably a filter to optimum response. This needs quite a bit of work, adjusting carefully the
resonators, inspecting at each step the results at the computer, and finally measuring the system
temperature. (One also benefits from good exercise, as one needs to walk many times the 120 m
between telescope and computer :-)
Eventually, a filter configuration was found that gives a sufficiently low system temperature
over a broad frequency range, which also coincides well with the maximum overall frequency
response, indicating successful power and noise matching. The narrow feature at 1420.5 MHz is
merely due to the presence of galactic emission in the spectrum of the "empty sky".