Small broadband loop

[richard]

I built this simple HF loop antenna and amplifier with parts from my junk box. The loop is intended for use with a portable receiver.

In my case the loop antenna was 1.8m of shielded audio cable, with the shield providing the conductor. The cable can then be stretched over the nearest support. A transformer wound on a miniature ferrite ring was used to help boost the pick up voltage and provide balance and isolation. Unfortunately I don’t know the type of ferrite ring used and whether these ferrites are still easily available. I found that without a centre tapped transformer EMI pick up from the any noise source connected to the system is increased significantly.

Given the size of the antenna and simplicity of the circuit I doubt the performance is as good as commercial designs. However the response across all bands is better than the whip on my Degen receiver. Other advantages are that the loop can be used to null out interference sources like TVs etc. and does not suffer the same EMI problems as the whip when the receiver is connected to a PC or power socket.

Richard

[richard]

Reception tests using an unbalanced loop antenna showed a significant drop in SNR occurred when the switched mode power supply on my laptop was operated. Even when I tried using a coupling transformer between the loop and amplifier, as shown in the previous circuit, there was still a small drop in SNR (around 1-2dB). This is maybe due to stray capacitance between the coupling coils or electric field pick up from the amplifier circuit itself which I hadn’t shielded.

To see if any further improvements in SNR could be obtained a balanced amplifier (shown in diagram) was constructed. The circuit was built using whatever components were available, suggestions for how it could be improved are appreciated. Again the antenna was just a single loop of wire coupled to the amplifier by two coils on a miniature ferrite ring. The balanced input stage consisted of a differential FET pair and constant current sink.

I made some tests on 3.995KHz, comparing the performance of original unbalance loop to the balance version shown. When the switched mode power supply was turned on reception using the unbalanced loop showed a steep drop in SNR, around 10dB. However, using the balanced circuit I could not discern any drop in SNR at all. The loop was only 2-3 meters away from the power supply and cables. So the tests show the benefits of using a balanced antenna.