The K9AY Terminated Loop
A Compact Directional
Receiving Antenna

By Gary Breed, K9AY
(Source: QST, September 97, pp. 43-46)

Abstract: Describes a directional receiving antenna for low band use which is smaller than an EWE but provides equal performance. It is switchable through four quadrants.

Digest: From the earliest days of radio, low-band operators have valued directional receiving antennas, such as the Beverage, for their ability to reduce both QRM and QRN arriving from the back side. Beverages require lots of real estate, so the EWE array described by WA2WVL in QST Feb 95 was a great step forward since it could be erected on a modest size lot. Now, K9AY has gone farther and describes an antenna that can be erected in almost any back yard and provides performance equal to that of a EWE.

The terminated loop, simply stated, is a vertical loop of wire, grounded at the bottom center. The loop is fed at the bottom, through a 9:1 transformer connected between a ground stake and one side of the loop. The other end of the loop wire is connected to the same stake through a load resistor. The optimum value of the resistor is between about 390-ohms and 560-ohms, depending somewhat on ground resistance and also which band is in use. At the author's location, 390-ohms gives optimum front-to-back ratio on 160-meters, and 560-ohms is optimal for 80-meters. A value of 470 represents a compromise that is reasonably effective on both bands.

The shape of the loop is not critical. The author's loop is essentially a triangle with a 30-foot (9.1-m.) horizontal base near the ground and an apex supported atop a 25-foot (7.6-m.) pole. Two of these loops are erected using the same pole and ground stake, oriented 90-degrees from each other. A switch box containing relays is mounted on the ground stake and a four-position selector switch in the shack allows the operator to switch the feedline between either side of either antenna, and the load resistor to the corresponding opposite side, so as to select azimuth quadrants. The direction of maximum gain is in line with the plane of the loop and toward the side containing the feedline.

The size of the loop is also not critical. Larger ones capture greater signal voltage, but also more noise, so the signal-to-noise ratio is unchanged. If the loop is too small, a low-noise preamplifier may become necessary. The author's antenna provides ample signal strength on both the 160- and 80-meter bands without a preamplifier.

The author ends the article with a suggestion that he has not tried. The terminated loops are sufficiently small that it would be easy to build a four-square array of them. This would greatly increase F/B ratio and also narrow the forward lobe substantially. One would have to remember to switch the direction of each component whenever that of the whole array is switched.


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