Receive Loop Preamplifier

I have used a receive loop aerial for several years for LF reception. If the loop is big enough, then the signal picked up may be sufficient to overcome the noise in the receiver, and a preamplifier is not really necessary. My loop is marginal at 1.2m and 16 turns. It is mabe from 25 pair telephone cable so the Q is not very high. After some experiments I decided that i could more easily read very weak staions with the pre-amp. At this time I was couling the loop to the reciever with a single turn coupling loop. This is not a very good match to 50 ohms, and I felt that perhaps I was loosing some sensitivity. I decided to use a simplification of the amplifier described by Tony Preedy G3LNP in the Radcom.

My version is not balanced and there is no need for an output balun. It is possible that an isolating transformer may be an advatage for removing local interference, as signals received on the braid of the coax are injected into the pre-amp. If this happens is will make the usual sharp nulls of the loop very shallow, and will almost swamp the directional nature of the loop. A 50 ohm transformer can easily be wound in bifilar fashion on a toroid of high permiability, the inductance of a single unloaded winding should be at least 180uH. A further advantage of this circuit is that it does not place any 'damping' components across the loop. You will notice there is not protection against transmit RF power. I suggest that the power is removed from the amp during TX and that a small relay is made to short out the gate source of the JFET. I have used a 2N3819 for the JFET, but it is possible a J310 would have better characteristics with regard to intermodulation, but would need a different drain resistor. The JFET as in Tony's circuit is used with no biassing, and because of the spread in device parameters it maybe the case that your JFET will draw more current than mine. The drain resistor should be altered to ensure that the standing voltage on the JFET drain is half the supply voltage. The npn bipolar transistor can be anything suitable I have used an unmarked plastic device, but a BC108/9 would suffice.

N.B The 2N2819 drain resistor may need to be adjusted to allow for the spread in threshold volges of the FET. It should be selected so that the drain voltage is about half the supply rail voltage.

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