Saturday, February 19, 2011

Off-center-fed nonresonant sloper antenna


Nonresonant off-center-fed sloper (OCFS) antenna  consists of a wire radiator that must be
longer than 3λ/2 at the lowest frequency of operation. The feedpoint is elevated at least λ/4 above ground at the lowest operating frequency. The antenna is fed with 75-Ω coaxial cable. The shield of the coax is connected to a  λ/4 resonant radial (counterpoise ground). There should be at least one radial (more is better) per band of operation.

The far end of the radiator element is sloped to ground, where it is terminated in a 270-Ω noninductive resistor. The resistor should be able to dissipate up to one- third of the power level applied by the transmitter.

The G5RV multiband dipole


Figure  shows the popular G5RV antenna. Although not without some problems, this antenna is very popular. It can be used either as a horizontal dipole, a sloper, or an inverted-vee antenna (which is how I used it). The dipole elements are each 51 ft long. The feedline can be either 300- or 450-Ω twin lead. For 300-Ω cases, use 29 ft of line, and for 450-Ω line, use 34 ft. One end of the parallel transmission line is connected to the antenna, and the other end is connected to a length of 50-Ω coaxial  cable. Although most articles on the G5RV claim that any length of 50-Ω line will work, J. M. Haerle (HF Antenna Systems: The Easy Way) recommends that the  50-Ω segment should be at least 65 ft long.

True longwire antennas



Figure  shows the true resonant longwire antenna. It is a horizontal antenna, and if properly installed, it is not simply attached to a convenient support (as is true with the random length antenna). Rather, the longwire is installed horizontally like a dipole. The ends are supported (dipole-like) from standard end insulators and rope. The feedpoint of the longwire is one end, so we expect to see a voltage antinode where the feeder is attached. For this reason we do not use coaxial cable, but rather either parallel transmission line (also sometimes called open-air line or some such name), or 450-Ω twin lead. The transmission line is excited from any of several types of balanced antenna tuning unit (see Fig.). Alternatively, a standard antenna tuning unit (designed for coaxial cable) can be used if a 4:1 balun transformer is used between the output of the tuner and the input of the feedline.

Demiquad single element one wavelength quad antenna.


The demiquad is a single-element  one wavelength quad antenna. The length of the antenna is, like the cubical quad beam antenna , one wavelength. Figure shows a type of demi-quad based on the tee-cross type of mast.

The impedance-matching section is a quarter-wavelength piece of 75-Ω coaxial cable (RG-58/U or RF-11/U). The length of the matching section is determined from:


L is the overall length, in feet
FMHz is the frequency, in megahertz
V is the velocity factor of the coaxial cable (typically 0.66, 0.70, or 0.80)

Tubing coaxial vertical VHF antenna


The sleeve is a piece of copper or brass tubing (pipe) about 1 in in diameter. An end cap is fitted over the end and sweat-soldered into place. The solder is not intended to add mechanical strength, but rather to prevent weathering from destroying the electrical contact between the two pieces. An SO-239 coaxial connector is mounted on the end cap. The coax is connected to the SO-239 inside the pipe, which means making the connection before mounting the end cap.

The radiator element is a small piece of tubing (or brazing rod) soldered to the center conductor of a PL-259 coaxial connector. An insulator is used to prevent the rod from shorting to the outer shell of the PL-259. (Note: an insulator salvaged from the smaller variety of banana plug can be shaved a small amount witha fine file and made to fit inside the PL-259. Alternatively, the radiator element can be soldered to a banana plug. The normal-size banana plug happens to fit into the female center conductor of the SO-239.

sloping dipole or slipole antenna


The sloping dipole  is popular with those operators who need a low angle of radiation, and are not overburdened with a large amount of land to install the antenna. This antenna is also called the sloper and the slipole in various texts. The author prefers the term “slipole,” in order to distinguish this antenna from a sloping vertical of the same name. Whatever it is called, however, it is a half-wavelength dipole that is built with one end at the top of a support, and the other end close to the  ground, and being fed in the center by coaxial cable. Some of the same comments as obtained for the inverted-vee antenna also apply to the sloping dipole,

Some operators like to arrange four sloping dipoles from the same mast such that they point in different directions around the compass  A single four- position coaxial cable switch will allow switching a directional beam around the compass to favor various places in the world.

Inverted-vee dipole half-wavelength antenna


The inverted-vee dipole is a half-wavelength antenna fed in the center like a dipole.By the rigorous definition, the inverted-vee is merely a variation on the dipoletheme. But in this form of antenna (Fig. 6-7), the center is elevated as high as possible from the earth’s surface, but the ends droop to very close to the surface. Angle a can be almost anything convenient, provided that a > 90 degrees; typically, most inverted-vee antennas use an angle of about 120 degrees. Although essentially a compensation antenna for use when the dipole is not practical, many operators believe that it is essentially a better performer on 40 and 80 m in cases where the dipole cannot be mounted at a half-wavelength (64 ft or so).By sloping the antenna elements down from the horizontal to an angle (as shownin Fig. 6-7), the resonant frequency is effectively lowered. Thus, the antenna will need to be shorter for any given frequency than a dipole.

There is no absolutely rig-orous equation for calculation of the overall length of the antenna elements. Although the concept of “absolute” length does not hold for regular dipoles, it is even less viable for the inverted-vee. There is, however, a rule of thumb that can be fol-lowed for a starting point: Make the antenna about 6 percent shorter than a dipole for the same frequency. The initial cut of the antenna element lengths (each quarterwavelength) is

After this length is determined, the actual length is found from the same cut-and-try method used to tune the dipole in the previous section. Bending the elements downward also changes the feedpoint impedance of the antenna and narrows its bandwidth. Thus, some adjustment in these departments is in order. You might want to use an impedance matching scheme at the feedpoint, or an antenna tuner at the transmitter.220

Six-element 2-meter Yagi beam antenna


Figure 18-8 shows the construction details for a six-element 2-meter Yagi beam antenna. This antenna is built using a 2 X 2-in wooden boom and elements made of either brass or copper rod. Threaded brass rod is particularly useful, but not strictly necessary. The job of securing the elements (other than the driven element) is easier when threaded rod is used, because it allows a pair of hex nuts, one on either side of the  2 x 2-in boom, to be used to secure the element. Non threaded elements can be secured with RTV sealing a press-fit. Alternatively, tie wires (see inset to Fig ) can be used to secure the rods. A hole is drilled through the 2 x 2 to admit the rod or tubing. The element is secured by wrapping a tie wire around the rod on either side of the 2 x 2, and then soldering it in place. The tie wire is no. 14 to no. 10 solid wire.

Mounting of the antenna is accomplished by using a mast secured to the boom with an appropriate clamp. One alternative is to use an end-flange clamp, such as is sometimes used to support pole lamps, etc. The mast should be attached to the boom at the center of gravity, which is also known as the balance point. If you try to balance the antenna in one hand unsupported, there is one (and only one) point at which it is balanced (and won’t fall). Attach the mast hardware at, or near, this point in order to prevent normal gravitational torques from tearing the mounting apart.

The antenna is fed with coaxial cable at the center of the driven element. Ordinarily, either a matching section of coax, or a gamma match, will be needed because the effect of parasitic elements on the driven element feedpoint impedanceis to reduce it.