DK6ED DX-Loop

Magnetic loop with improved radiation pattern for optimum DX reception in distorted locations.

Basic ideas

Reception can be realized by either analyzing the electric or the magnetic field radiated by any source. This is important to know because in most cases local noise sources are radiating the electric field by the mains or any electrical network. Only after a distance of more than one wavelength the distorting energy will be evenly distributed on the electric and the magnetic field. This is why magnetic loops are recommended for use in distorted areas.

But, every loop is made from a defined length of wire, picking up the electric field of the noise source. So the circumference of the loop must be limited. Because of this limitation, magnetic loops are not sensitive enough for convincing DX reception on low frequencies. By introducing a current amplifier, suppressing the voltage picked up, it is possible to extend the circumference of any loop. This is the first advantage of the new proposed design. Another point is the fact that magnetic loops are having two minima and two maxima. To suppress a local noise source only a single minimum is needed. On the other hand the maximum should be as broad as possible for all-round reception. This is exactly the pattern of radio direction finding (RDF) antennas. But these systems are very small using resonant loops. So a new design has to be introduced for the antenna also.

New antenna

The new antenna system consists of a large loop made from coax and a sense antenna.

This results in a very broad horizontal pattern over a wide frequency range:

The vertical pattern:

This is the way to reduce local interference.

New preamplifier

The signal of the loop is fed to a newly designed current amplifier, suppressing any voltage input. It consists of a combination of a common source and a common drain stage. Common source circuits shift the phase of the signal, common drain circuits do not. When combining these two stages, the voltage signal will be extinguished. But due to the different current gain of the stages, the amplifier still offers a convenient gain. For the sense antenna another drain stage will be implemented. Its output is fed into the center lead of the coax loop.

The amplifier is connected through a choked coax line to the receiver. Choking the line is most important to avoid any strew in distorting the pattern from the backside. 

Results:

My main interest is DXing on the low amateur radio bands 80 / 160 m on my small lot. Experimenting with receive antennas led to 310 DXCC on 80 m and 272 DXCC on 160m. In my location the new antenna system improves the signal-/noise ratio significantly.

To show the results, a transmit vertical antenna is compared to the proposed loop system. The loop is installed at a distance of 30 ft. from the local noise source. The receiver bandwidth is set to 4.7 KHz. The following diagrams show the difference in SNR receiving the signals of a beacon on 80 m.

This is the signal on the transmit vertical:

This is the same signal on the receive loop:

This leads to an improvement of SNR of more than 20 dB.

Even on the very small bandwidth mode FT8 improvements up to 10 dB were experienced.

Tests were made with even larger loops covering an area of 400 ft². These systems are more sensitive but still all local noise was suppressed.