Home built Shielded magnetic loop for LW to HF

Useful information regarding antennas for SDR products.
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Home built Shielded magnetic loop for LW to HF

Post by DaveB » Sat Jun 02, 2018 3:45 pm

With zero VHF activity up until early May I decided to attempt making a magnetic loop antenna for MW-HF. The one that appeared to be most likely to work was a shielded magnetic loop. A design published by PA0FRI was the starting point and it used a standard MMIC chip which happens to be available as a ready made 30dB 0.05-2GHz (or similar variants) preamp on E-bay for under £5 (and a long wait for the slow boat from China!)

The end result was a loop of 1.2 m diameter made from 10 mm soft copper tubing with the receiving loop made of heavy duty speaker wire. A 1:1 binocular ferrite transformer connects the loop to the amplifier, which is built into a small diecast box to which the copper tubing is fixed via 1/2 inch bsp adapters.

Mounted around 2m above the ground I have received on three consecutive nights in early May Transatlantic MW signals. It works well on the short wave bands with signals on 80m/40m amateur bands for the first time in I don't know how long. It is a work in progress as it doesn't have a permanent home yet and is set to one side until the VHF Sporadic E seasons finishes. I've assembled the needed components to produce a balanced low noise pre-amp using discrete components to see if this improves performance which I will do in the autumn.

For me the main things are it costs a fraction of a commercial one such as the Wellbrook and it gives me a nice doable construction project with lots of scope for tinkering around.

What initially completely threw me was that I didn't know (or forgot) that a shielded magnetic lop antenna has the nulls at right angles to the plane of the loop. The penny having dropped - I found I was able to choose between Algeria or RTE on LW 252 kHz when the loop was suitably oriented and confirm it had good nulls.

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