Dear Laurie, LF group,
When I was working on tuned loops at University of Herts, due to
various constraints we used packaged Mini-Circuits preamps with
performance similar to the MAR types. We found that the noise
performance was more than adequate, and limited by the thermal
noise of the loop itself or external atmospheric noise, but as others
have noted the IM performance was not great. The biggest
problems we had were parasitic oscillations - even the relatively
low-frequency MAR devices in the range have substantial gain into
the microwave region - and without measures being taken,
vigorous oscillation at about 1GHz would occur. This was quite
difficult to detect if the receiver attached to the output did not go up
to 1GHz, and didn't do a lot for the overall performance.
As Andy says, the noise power (from preamp, loop or wherever) is
proportional to the bandwidth, so reducing the bandwidth reduces
the noise in proportion, which is the main benefit of QRSS. As far
as improving the signal to noise ratio with a loop goes, Increasing
the area of the loop and increasing the Q are the two possibilities -
both these increase the signal power available to the preamp. The
inductive reactance of untuned loops behaves as an attenuator in
one way or another, so SNR is generally lower. With tuned loops
of reasonable size at LF, it usually isn't hard to hear down to the
external noise. A high Q loop acts as a preselector, which will help
to reduce intermodulation, although it is less convenient in having
to be re-tuned within the band.
My own current RX antennas are single-turn loops with sides of
about 2m or so. These connect to the shack by coax via a balun
(which does not seem to make a great difference), where there is
series inductance and parallel capacitance to tune the thing, and
an OPA604 op-amp as a preamp. This is not a very efficient set-up
from the noise point of view, but sensitivity is limited by
atmospheric noise, and intermodulation does not seem to be an
issue, in spite of being very close to MF broadcast transmitters.
Cheers, Jim Moritz
73 de M0BMU
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