Last week, I observed that with the USRP/DBS_RX combination terminated
at the input, there was still
a couple of spectral spurs when I set the system up to observe at the
neutral hydrogen line at
1420.405Mhz. This worried me a little, but I surmised that once I had
the LNA and another
line-amp on, their added noise would swamp any little spurs.
Here's a screenshot of my RA application with the feedhorn setup on a
1.25M dish, with a 15dBg/0.7dBNF LNA,
a 15dBg/3.0dNF line amplifier, and roughly 6dB of cable loss to the
DBS_RX front-end. The spectrum
is nice and quiet, which is just what you want, with perhaps the
occasional HI region passing through
the beam to de-flatten the spectrum :-)
I measured the response of the setup to a blackbody source at 315K this
morning, which produced a
massive (at least in RA terms) 2.25dB increase in the total power
reading. That's a ratio of
roughly 56 (17.4dB) between the average magnitude of the random
fluctuations seen in the total
power display, and that produced by a blackbody source at 310K. It
could stand a further increase
in front-end gain, I suspect.
But the main thing is I was very pleased to see the spurs go away, and
also to see how relatively
stable the system is. I'll be making some attempts to capture
transits of Cass. A--a standard
calibration source that produces a flux density of about 2000Jy at the
21cm wavelength.
If that is unambiguously successful (and in theory, it should be, but
in amateur observatories,
there's nearly-always a big gap between theory and practice), I'll let
the list know.
A couple of good messages to tak e away from this are that GnuRadio
forms a good basis for
amateur radio astronomy observing, and that USRP/DBS_RX combination
make this possible
with a bare minimum of hardware futzing required, and at a fraction of
the cost of
the only comparable amateur RA system on the market today.
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