[Lf] Loop preamps]

[Andre' Kesteloot]

James Moritz wrote:

> 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