Loops for Meter DX

VE2CV has been experimenting with and modeling low-frequency antennas for many years. Jack recounts his extensive practical experience using different antennas on the lower bands.

By John S. Beirose, VE2CV

A simple yet effective antenna for the 80 and 160-meter bands is the half-wave dipole, although other antennas are used, particularly by those interested in working DX. Propagation conditions during nighttime and evening hours support communication on both long and short paths. The station antenna or antennas ideally should radiate equally well at both low and high radiation angles, also referred to as elevation angles ity) measured from the Earth's surface.

For communication with a distant station, you must listen to the signal against a background of noise and interference, particularly interference from strong, near-vertical-incidence skywave (NVIS) signals (\j/ > 60°). In addition, local man-made noise and atmospheric noise from nearby thunderstorms is almost always present. Horizontal polarization is preferred over vertical polarization from the ground reinforcement point of view, unless you are fortunate to have seawater in front of the antenna in the

ARRL Technical Adviser 17 Tadoussac Drive Aylmer, QC J9J 1G1 Canada desired direction^). But practical dipole heights for the 80-meter band are too low to achieve an effective low angle of radiation (y < 15°). This is even more of a problem for the 160-meter band.

Since I became a radio amateur, I have been interested in devising efficient antennas that resolve some of these difficulties. While I'm not a DXer, from my QTH in Aylmer, QC, I like to "check into" distant 80-meter nets, such as the North West Ontario Net or the Newfoundland Phone Net. For almost a decade, I've been a control station for the Trans-Canada Pow-Wow Club, a group of amateurs that meets on-the-air daily during winter and equinox months, beginning at midnight local Eastern time on 3750 kHz. I am the control station for the Sunday morning club meetings.

Pow-Wow Club members and other amateurs attracted by the activity on the frequency call in from across Canada, coast-to-coast, as well as from the UK. Participation in the Club is intended to inspire members to improve their stations' capabilities to a point where they can be heard from coast-to-coast. Linear amplifiers are acquired or constructed and antennas are put up, modified and put up again—antennas that radiate as well as possible at both low and high elevation angles. Beverage antennas are used by some to improve the received signal-to-noise. I use three simple antennas:

• a dipole—for many years 15-meters high, but presently at 26 meters, the height of the trees,

• a ground-plane type of delta loop, an antenna that I devised and dubbed a half-loop1,

• a compact loop, an AMA-11, a 1.7-meter diameter loop used primarily for receiving2.

These antennas have very different radiation patterns, and their different responses to noise and interference is a distinct advantage. The ability to switch quickly between two (or more) antennas is important, since this provides some ability to optimize reception in the face of variable propagation conditions, noise and interference.

Half-Loop Antennas

For a decade and a half I have employed loops in various configurations, but the loop antenna I've used for the longest time is the half-delta loop. This vertically polarized loop has a perimeter of only half an electrical wavelength, since the other half of this ground-mounted loop is its image in the ground plane. An article by A1 Christman, N3AL (ex-KB8I), and follow-on studies by me (using the Numerical Electromagnetics Code, NEC-2) show that the radiation efficiency for a monopole with only three or four resonant elevated radials is equivalent to that for a grounded vertical monopole with 120 buried radials.3-4 The result of this work inspired me to consider lifting my half loop off the ground, using radials to simulate connection to ground.

The Effect of Real Ground

First, we need a brief discussion of the effect of finitely conducting ground on antenna performance. After that we will consider the ground-mounted half-wave loop in detail. In the case studies that follow, I created radiation patterns using EZNEC, developed by Roy Lewallen, W7EL.5 EZNEC is a menu-driven version of NEC2, the Numerical Electromagnetics Code. Unless otherwise stated, the frequency is 3.75 MHz and the patterns have been calculated for resonant loops. When I refer to "principal-plane patterns," this means the azimuthal pattern at the elevation angle of maximum gain, and the elevation pattern for the azimuth angle of maximum gain.

Both directly beneath and in front of an antenna in the direction of propagation, lossy ground affects the antenna's impedance, the current on the antenna and its radiation pattern. If ground conductivity is poor, it is better to use a horizontally polarized antenna. The gain of a horizontally polarized antenna, particularly at low launch angles, is less affected by the finite conductivity of the ground in front of the antenna.

Ground reinforcement of vertically polarized waves results in a pattern maximum at the angle of launch (\|/max), where the direct and the ground-reflected rays are in phase. This occurs at low launch angles for "perfect" ground. But when the ground conductivity is finite, the phase of the ground-reflected ray changes rather abruptly from an in-phase to an out-of-phase condition at a particular launch angle called the Brewster angle. For launch angles below the Brewster angle, antenna performance is seriously degraded.

Over seawater this happens at launch angles that are a fraction of a degree above the horizon, and the formation of 1 Notes appear on page 16.

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