HF loop antennas have blind spots

To the editor: I found Alan L. Smith’s recent article on loop antennas for marine SSB to be very interesting (In the loop Issue 131, July/Aug. 2003). The loop antenna he describes eliminates the need for a good ground system, thus fixing a common problem that plagues single-backstay antennas. However, there is one drawback to loop antennas that should be mentioned: Loop antennas aren’t as omnidirectional as backstay antennas, and they can have the radio equivalent of a small blind spot.

It’s true that all real-world antennas do not transmit or receive equally well in all directions. Even a simple backstay antenna is slightly directional; it can radiate and receive radio energy most strongly in directions perpendicular to the backstay and most weakly in directions parallel to the backstay. This pattern is a good one for a typical sailboat, though, because it sends most of the radio energy toward the horizon in all directions. It also avoids sending too much radio energy up into the sky at extremely high angles, or down into the sea off the stern; both of these directions are not useful for long-distance communication.

Loop antennas are also somewhat directional. They radiate and receive radio energy most weakly in directions perpendicular to the plane of the loop and most strongly in directions in the plane of the loop.

When mounted vertically, the way Smith describes, the result would be that signals to or from stations dead ahead or astern could be weaker than those to or from stations abeam. This effect would be more pronounced at lower frequencies (by my estimates roughly below 3 MHz, using Smith’s design).

In the past, I have built loop antennas on land for ham-radio use. However, I have not been able to build or test Smith’s marine SSB loop. Therefore, I’m interested in knowing if anyone who has built and used the loop has observed any directionality in their antenna(s). One could test for this by tuning into a distant broadcast (preferably from a station far beyond the horizon and below 3 MHz), and then sailing in a slow circle while noting if the signal strength changes as one’s heading changes. In particular, one should watch for a dip in the signal strength as the distant station crosses the bow or stern.

Finally, I should point out that sometimes a radio blind spot caused by an antenna can be put to good use. Small loop antennas are used in some radio direction finding (RDF) systems. Those typically use a small loop that is rotated until a received signal’s strength dips to a minimum. The minimum is used instead of the maximum partly because the loop causes a sharp dip in signal strength for a narrow range of angles, but it has a much wider peak. When the signal-strength minimum is found, one knows that the transmitter is on a line perpendicular to the plane of the loop antenna. (A good introduction to small loop antennas’ characteristics and RDF can be found at www.dxing.com/tnotes/tnote09.pdf.)

As with many design choices, the choice of which marine SSB antenna to use is always a compromise. If one does not have access to a good ground system aboard and can work with some mild directionality in the antenna, then the loop antenna Mr. Smith describes is certainly a good choice for marine SSB use.

Chris Hefele is a sailor, electrical engineer and amateur radio operator. He lives in Hoboken, N.J.

By Ocean Navigator