Practical Hot Guy Wire Antennas for Ham Radio

Do you use your tower as a vertical antenna for the low bands? Would you like to? Either way\ "light up" your towers metallic guys, too, for gain and directivity

What is a hot-guy wire (HGW) antenna? It is a vertical antenna designed to use tuned guy wires as parasitic elements to create a directional radiation pattern. That is, the guy wires increase the radiation from a tower in the desired direction when you transmit and reduce QRM and QRN from other directions when you receive.

Tuning an existing guy wire is an attractive concept for the 160 and 80-meter bands, because the hot-guy-wire antenna is very compact for the amount of gain it provides. You can modify an existing tower in a weekend

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to produce three decibels of gain over the omnidirectional case by tuning a single guy wire as a reflector. This is equivalent to doubling your transmitter power in the direction of maximum gain. If you must use guy wires anyway, why not put them to better use? This article will cover towers both with and without base insulators, with one and two parasitic elements and consider the effects of various ground conductivities. I have built several of these antennas for commercial medium-wave broadcasters, and they do indeed work as advertised. The "sloper" antenna is a type of HGW antenna, but it is not as easy to adjust as the designs presented in this article.

For simplicity when comparing the various radiation patterns in this article, unless otherwise noted, all wires are one-half inch in diameter, the relative dielectric constant of the earth is 15 and the analysis software is EZNEC2. However, when discussing impedance, we are better advised to use wire dimensions closer to typical actual values. For example, the tower can be modeled as a 12-inch-diameter wire. I sometimes use parallel wires in modeling and in practice in order to improve bandwidth and distribute current stresses. This technique also helps to avoid the occasional problems thatNEC2 has with unequal-diameter wires. However, one must take care not to place parallel wires too close together, because NEC2 doesn't like that either. In fact, when modeling radial ground wires, you will get bogus results if you use too many wires, because of their close proximity to each other near the tower base.

It is instructive to understand why we prefer to use vertical towers as radiators at longer wavelengths. The simple fact is that a conventional horizontal dipole designed for the 80-meter band would have to be mounted very high above ground to produce radiation that isn't essentially straight up. A dipole mounted too close (electrically) to ground just doesn't radiate much of its signal at the low departure angles needed for DX. Fig 1 shows the unfortunate elevation pattern off the sides of a 3750 kHz X/2 horizontal dipole erected Xl4 above lossy earth. You would need two 70-foot towers spaced about 135 feet apart to accomplish this, if you include the dipole sag in your calculations. When you include the guy wires in your real-estate estimate, you would need a backyard at least 185 feet wide It is just not practical to build a horizontal dipole at this frequency that has the necessary height above ground (would 300 feet be enough?) to produce a decent radiation pattern, when you consider the alternative of a vertical monopole. Of course, a rotator is simply out of the question.

If you already have a tower that you use only for mechanical support of an antenna, chances are your ground sys tem consists of a single rod or pipe driven into the dirt. You need to improve this by laying some wires on the ground, extending radially outward from the base of your tower. If you use insulated ground wires, the effect is mainly to improve impedance stability when it rains. Of course, the wires should be anchored at the ends so they don't migrate in unpredictable directions over time. As a minimum you should have eight wires, each at least X/8 long. If you have a self-supporting tower, you will need to add guy wires to act as parasitic elements. If you already have guy wires, you will probably need to change the existing insulator positions, and possibly the guy-wire anchor positions as well. It might be simpler just to add more wires for use strictly as parasitic elements and not as mechanical supports for the tower. The choice really depends on your specific situation.

A practical base-insulated X/4 (66 feet = 90°) tower with eight 39-foot (electrical length = 54°) radial ground wires mounted four inches above earth produces a better signal at 10° elevation than the horizontal-dipole example. For a ground conductivity of 5 mS/m, Fig 2 shows that we have a gain of -2.3 dBi from the monopole antenna 10° above the horizon, compared to -3.6 dBi for the dipole in Fig 1. On the ground, the 1000 W field from the tower is 48 mV/m at one mile, but from the dipole, it is only 1 mV/m, a vast difference. If the ground conductivity were an excellent 30 mS/m, the dipole performance would be about the same, but the tower sky wave would increase to 1.3 dBi, and its 1000 W ground-wave field would become 143 mV/m at a mile. The point is that both close-in and skip signals are superior with the tower. This would be even more apparent for a X/4 tower operating in the 160 meter band, where the effects of lossy earth are not as severe.1

A note about clBi—this is decibels relative to an isotropic antenna, which radiates 108 mV/m at a mile in all directions for an antenna input power of 1000 W. So, in the 10° sky-wave example above, -2.3 dBi at a mile is 108 xl0"2-3/2° = 83 mV/m. This is what you would expect about 900 feet above the ground a mile distant from the antenna (10° elevation).

By the way, the loss-less ground-wave field from this particular tower would be about 195 mV/m at a mile,

1 Notes appear on page 57

80 Meter Half-Wave Dipole Freq = 3.75 MHz

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