Message-ID: <16613.200104251159@gemini>
From: "James Moritz" <j.r.moritz@herts.ac.uk>
Organization: University of Hertfordshire
To: rsgb_lf_group@blacksheep.org
Date: Wed, 25 Apr 2001 13:00:03 +0000
MIME-Version: 1.0
Content-Type: text/plain; charset=US-ASCII; format=flowed
Content-Transfer-Encoding: 8bit
Subject: LF: Re: Loop preamps
Precedence: bulk
Reply-To: rsgb_lf_group@blacksheep.org
Sender: <majordom@post.thorcom.com>

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