Stuff that makes you go "hmmmm"

I’ve been working on a system for observing cosmic radio noise, over a
broad beamwidth, at the low-end of VHF (around 30-40MHz), using
a USRP2 and a BASIC_RX. The application measures RMS power over a
small bandwidth (between 100KHz and 750KHz, depending on
user input).

Yesterday, I discovered that I at some point blew up one of more of my
mini-circuits ZFL-500 amplifiers, so while I’m waiting for
replacements, I thought I’d do a quick experiment.

I plugged the crossed-dipole antenna directly into the USRP2, with a
5th-order Butterworth low-pass filter with a corner frequency of
48MHz between the antenna and the USRP2. I fully expected to get
nothing when I plugged in the antenna, or perhaps a minor
increase in output. But, quite the contrary, plugging in the antenna
produced a roughly 15dB increase in detected power across a
250KHz selected bandwidth.

So, I thought that the BASIC_RX had zero gain, but I’m surprised that
the A/D in the USRP2 is sensitive enough to plug directly into
a low-VHF antenna without any gain.

Does this match other peoples observations of the Basic_RX? Have people
been able to use the Basic_RX as an HF/low-VHF receiver with
no gain at all?

Marcus L.
Principal Investigator
Shirleys Bay Radio Astronomy Consortium

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