Antennas for 3.5 and 7 MHz
Multiband antennas constructed as described in Chapter Six obviously will be useful on 3.5 and 7 MHz, and, in fact, the end-fed and center-fed antennas shown in Chapter Six are quite widely used for 3.5- and 7-MHz operation. The center-fed system is better because it is inherently balanced on both bands and there is less chance for feeder radiation and rf feedback troubles, but either system will give a good account of itself. On these frequencies the height of the antenna is not too important, and anything over 35 feet will work well for average operation. This chapter is concerned principally with antennas designed for use on one band only.
An untuned or "flat" feed line is a logical choice on any band, because the losses are low, but it generally limits the use of the antenna to one band. Where only single-band operation is wanted,
the half-wave antenna fed with untuned line is one of the most popular systems on the 3.5- and 7-MHz bands. If the antenna is a single-wire affair, its impedance is in the vicinity of 60 ohms. The most logical way to feed the antenna is with 72 - ohm Twin-Lead or 50- or 72-ohm coaxial line. The heavy-duty Twin-Lead and the coaxial line present support problems, but these can be overcome by using a small auxiliary pole to take the weight of the line. The line should come away from the antenna at right angles, and it can be of any length.
A "folded dipole" shows an impedance of 300 ohms, and so it can be fed directly with any length of 300-ohm TV line. The line should come away from the antenna at as close to a right angle as possible. The folded dipole can be made of ordinary wire spaced by light-weight wooden or plastic spacers, 4 or 6 inches long, or a piece of 300 - ohm TV line can be used for the folded dipole.
A folded dipole can be fed with a 600-ohm open-wire line with only a 2 - to - 1 SWR, but a
Fig. 8-1 - Half-wavelength antennas for single-band operation. The multiwire types shown in В, C and D offer a better match to the feeder over a somewhat wider range of frequencies but otherwise the performances are identical. The feeder should run away from the antenna at a right angle for as great a distance as possible. In the coupling circuits shown, tuned circuits should resonate to the operating frequency. In the series-tuned circuits of A, B , and C, high L and low C are recommended, and in D the inductance and capacitance should be similar to the output- amplifier tank, with the feeders tapped across at least 1/2 the coil. The tapped-coil matching circuit or the Transmatch, both s hown in Chapter Six, can be substituted in each case.
nearly perfect match can be otained with 600-ohm open line and a three-wire dipole.
The three types of half-wavelength antennas just discussed are shown in Fig. 8-1. One advantage of the two- and three-wire antennas over the single wire is that they offer a better match over a band. This is particularly important if full coverage of the 3.5-MHz band is contemplated.
While there are many other methods of matching lines to half-wavelength antennas, the three mentioned are the most practical ones. It is possible, for example, to use a quarter-wavelength transformer of 150-ohm Twin-Lead to match a single-wire half-wavelength antenna to 300-ohm feed line. But if 300-ohm feed line is to be used, a folded dipole offers an excellent match without the necessity for a matching section.
The formula shown above each antenna in Fig. 8-1 can be used to compute the length at any frequency, or the length can be obtained directly from the charts in Fig. 8-2.
Fig. 8-2 - The above charts can be used to determine the length of a half-wave antenna of wire.
The halves of a dipole may be sloped to form an inverted V, as shown in Fig. 8-3. This has the advantages of requiring only a single high support and less horizontal space. K7GCO and others have also reported that the dipole in this form is more effective than a horizontal antenna, especially for frequencies of 7 MHz and lower.
Sloping of the wires results in a decrease in the resonant frequency and a decrease in feed-point impedance and bandwidth as the angle between the two wires is decreased. Thus, for the same frequency, the length of the dipole must be decreased somewhat. The angle at the apex is not critical, although it should probably be made no smaller than 90 degrees. Because of the lower impedance, a 50-ohm line should be used, and the usual procedure is to adjust the angle for lowest SWR while keeping the dipole resonant by adjustment of length. Bandwidth may be increased by using multiconductor elements, such as the cage configuration.
Fig. 8-3 - The inverted-V dipole. The length and apex angle should be adjusted as described in the text.
For 3.5-MHz work, the vertical can be a quarter wavelength long (if one can get the height), or it can be something less than this and "top-loaded." The bottom of the antenna has only to clear the ground by inches. Probably the cheapest construction of a quarter-wavelength vertical involves running copper or aluminum wire alongside a wooden mast. A metal tower can also be used as a radiator. If the tower is grounded, the antenna can be "shunt-fed," as shown in В of Fig. 8-4. The "gamma" matching system described in Chapter Three may also be used. A good ground system is helpful in feeding a quarter-wavelength vertical antenna, and the ground can be either a convenient water-pipe system or a number of radial wires extending out from the base of the antenna for about a quarter wavelength.
The Ground Plane
The size of a ground-plane antenna makes it a little impractical for 3.5-MHz work, but one can be used at 7 MHz to good advantage, particularly for DX work. This type of antenna can be placed higher above ground than an ordinary vertical
without decreasing the low-angle radiation. The vertical member can be a length of self-supporting tubing at the top of a short mast, and the radials can be lengths of wire used also to support the mast. The radials do not have to be exactly horizontal, as shown in Fig. 8-5.
The ground-plane antenna can be fed directly with 50-ohm cable, although the resulting SWR on the line will not be as low as it will if the antenna is designed with a stub matching section, as described in Chapter Three. However, the additional loss caused by an SWR as high as 2 to 1 will be inappreciable even in cable runs of several hundred feet when the frequency is as low as 7 MHz.
Two or more vertical antennas spaced a half wavelength apart can be operated as a single antenna system to obtain additional gain and a directional pattern.
Fig. 8-5 - A ground-plane antenna is effective for DX work on 7 MHz, Although its base can be any height above ground, losses in the ground underneath will be reduced by keeping the bottom of the antenna and the ground plane as high above ground as possible. Feeding the antenna directly with 50-ohm coaxial cable will result in a low standing-wave ratio. The length of the vertical radiator can be computed from the formula, or it can be obtained from Fig. 8-2 by using just one half the length indicated in the chart. The radial wires are 2.5% longer. For example, at 7.1 MHz, the radiator is 65' 11 "/2 « 33'; the radials are 1.025 X 33 = 33' 10".
Fig. 8-4 - Vertical antennas are effective for 3.5- or 7-MHz work. The quarter-wavelength antenna shown at A is fed directly with 50-ohm coaxial line, and the resulting standing-wave ratio is usually less than 1.5 to 1, depending on the ground resistance. If a grounded antenna is used as at B, the antenna can be shunt-fed with either 50- or 75-ohm coaxial line. The tap for best match and the value of С will have to be found by experiment; the line running up the side of the antenna should be spaced from 6 to 12 inches from the antenna. The length (height) of the antenna can be computed from the formula, or it can be obtained from Fig. 8-2 by using just one half the length indicated in the chart. For example, at 3.6 MHz, the length is 13072 = 65'.
The following design for 40-meter phased verticals is contributed by Gary Elliott, KH6HCM/W7UXP. An 80-meter version can be constructed by proper scaling. There are practical ways that verticals for 40 meters can be combined, end-fire and broadside. In the broadside configuration, the two verticals are fed in phase, producing a figure-eight pattern that is broadside to the plane of the verticals. In an end-fire
Fig. 8-6 - Pattern for two 1/4-λ. verticals spaced one-half wavelength apart fed 180 degrees out of phase. The arrow represents the axis of the elements.
arrangement, the two verticals are fed out of phase, and a figure-eight pattern is obtained that is in line with the two antennas, Fig. 8-6. However, an end-fire pair of verticals can be fed 90 degrees out of phase and spaced a quarter wavelength apart, and the resulting pattern will be unidirectional. The direction of maximum radiation is in line with the two verticals, and in the direction of the vertical receiving the lagging excitation; see Fig. 8-7.
Physically, each vertical is constructed of telescoping aluminum tubing that starts off at 1-1/2-inch dia and tapers down to 1/4-inch dia at the top. The length of each vertical is 32 feet. Each vertical is supported on two standoff insulators set on a 2 by 4, 6 feet long and strapped to a fence. An alternative method of mounting would be a 2 by 4 about 8 feet long and set about 2 feet in the ground.
Originally each vertical element was 32 feet, 6 inches long, 234/f (MHz). After one vertical was mounted on the 2 X 4 it was raised into position and the resonant frequency was checked with an antenna noise bridge. It was found that the vertical
Fig. 8-7 - Pattern for two 1 /4-Х verticals spaced 1/4 wavelength apart and fed 90 degrees out of phase. The arrow represents the axis of the elements, with the element on the right being the one of lagging phase.
resonated too low in frequency, about 6.9 MHz. This was to be expected as the fundamental equation for the quarter-wave vertical, 234/f, is only reasonably correct for very small-diameter tubing or antenna wire. When larger diameter tubing (1-1/4 inch and larger) is used, the physical length will be shorter than this, as described in Chapter Two. Using the antenna noise bridge, an inch at a time was cut off the top until the resonant frequency was 7100 kHz. This resulted in 6 inches being cut off, thus making the vertical exactly 32 feet long.
The ground system is very important in the operation of a vertical. The two usual methods of obtaining a ground system with verticals are shown in Fig. 8-8.
In order to obtain the unidirectional pattern shown in Fig. 8-7, the two verticals must be separated by a quarter wavelength, and one vertical must be fed 90 degrees behind the other. Two suggested feed methods are shown in Fig. 8-9. An electrical section of line cannot be used by itself to connect the two verticals together to obtain the 90-degree lag because of the velocity factor of RG-8/U. The length of an electrical wavelength of transmission line is based on the calculation:
246 X 0,66 / 7,1 MHz = 22' 10"
(Further information concerning velocity factor and transmission lines can be found in Chapter Three in the section on electrical length.)
Fig. 8-8 - An 8- to 10-ft. ground rod may provide a satisfactory ground system in marshy or beach areas, but in most locations a system of radial wires will be necessary.
Obviously, 22 feet, 10 inches of coax cannot be used, as the verticals are spaced 34.6 feet apart. This is overcome and a 90-degree lag is still obtained by using a 3/4-wavelength section of transmission line between the two verticals, Fig. 8-9A. The SWR is less than 1.25 to 1 across the entire band, using 52-ohm coax and no matching network.
Phased arrays with horizontal elements can be used to advantage at 7 MHz, if they can be placed at least 40 feet above ground. Any of the usual combinations will be effective. If a bidirectional characteristic is desired, the W8JK type of array, shown at A in Fig. 8-10, is a good one. If a unidirectional characteristic is required, two elements can be mounted about 20 feet apart and provision included for tuning one of the elementsas either a director or reflector,
as shown in Fig. 8-10B. The parasitic element is tuned at the end of its feed line with a series- or parallel-tuned circuit (whichever would normally be required to couple power into the line), and the proper tuning condition can be found by using the system for receiving and listening to distant stations along the line of maximum radiation of the antenna. Tuning the feeder to the parasitic element will peak up the signal.
Fig. 8-9 - Two methods of feeding the phased verticals.
An effective but simple 40-meter antenna that has a theoretical gain of approximately 2 dB over a dipole is a full-wave, closed loop. A full-wavelength closed loop need not be square. It can be trapezoidal, rectangular, circular, or some distorted configuration in between those shapes. For best results, however, the builder should attempt to make the loop as square as possible. The more rectangular the shape the greater the cancellation of energy in the system, and the less effective it will be. The effect is similar to that of a dipole, its effectiveness becoming impaired as the ends of the dipole are brought closer and closer together.
Fig. 8-10 - Directional antennas for 7 MHz. To realize any advantage from these antennas, they should be at least 40 feet high. The system at A is bidirectional, and that at В is unidirectional in a direction depending upon the tuning conditions of the parasitic element. The length of the elements in either antenna should be exactly the same, but any length from 60 to 150 feet can be used. If the length of the antenna at A is between 60 and 80 feet, the antenna will be bidirectional along the same line on both 7 and 14 MHz. The system at 8 can be made to work on 7 and 14 MHz in the same way, by keeping the length between 60 and 80 feet.
The practical limit can be seen in the "inverted-V" antenna, where a 90-degree apex angle between the legs is the minimum value ordinarily used. Angles that are less than 90 degrees cause serious cancellation of the rf energy.
The loop can be fed in the center of one of the vertical sides if vertical polarization is desired. For horizontal polarization it is necessary to feed either of the horizontal sides at the center.
Optimum directivity occurs at right angles to the plane of the loop, or in more simple terms, broadside from the loop. Therefore, one should try to hang the system from available supports which will enable the antenna to radiate the maximum amount in some favored direction.
Just how the wire is erected will depend on what is available in one's yard. Trees are always handy for supporting antennas, and in many instances the house is high enough to be included in the lineup of solid objects from which to hang a radiator. If only one supporting structure is available it should be a simple matter to put up an A frame or pipe mast to use as a second support. (Also, tower owners see Fig. 8-11 inset.)
The overall length of the wire used in a loop is determined in feet from the formula 1005/f (MHz). Hence, for operation at 7125 kHz the overall wire length will be 141 feet. The matching transformer, an electrical quarter wavelength of 75-ohm coax cable, can be computed by dividing 246 by the operating frequency in MHz, then multiplying that number by the velocity factor of the cable being used. Thus, for operation at 7125 kHz, 246/7.125 MHz = 34.53 feet. If coax with solid polyethylene insulation is used a velocity factor of 0.66 must be employed. Foam- polyethylene coax has a velocity factor of 0.80. Assuming RG-59/U is used, the length of the matching transformer becomes 34.53 (feet) x 0.66 = 22.79 feet or 22 feet, 9-1/2 inches.
Fig. 8-11 - Details of the full-wave loop. The dimensions given are for operation at the low end of 40 meters (7050 kHz). The height above ground was 7 feet in this instance, though improved performance should result if the builder can install the loop higher above ground without sacrificing length on the vertical sides. The inset illustrates how a single supporting structure can be used to hold the loop in a diamond-shaped configuration. Feeding the diamond at the lower tip provides radiation in the horizontal plane. Feeding the system at either side will result in vertical polarization of the radiated signal.
40-METER "SLOPER" SYSTEM
One of the more popular antennas for 3.5 and 7 MHz is the sloping dipole. David Pietraszewski, K1THQ, has made an extensive study of sloping dipoles at different heights with reflectors at the 3-GHz frequency range. From his experiments, he developed the novel 40-meter antenna system described here. With several sloping dipoles supported by a single mast and a switching network, an antenna with directional characteristics and forward gain can be simply constructed. This 40-meter system uses several "slopers" equally spaced around a common center support. Each dipole is cut to a half wavelength and fed at the center with 52-ohm coax. The length of each feed line is 36 feet. This length is just over 3/8 which provides a useful quality. All of the feed lines go to a common point on the support (tower) where the switching takes place. At 7 MHz, the 36-foot length of coax looks inductive to the antenna when the end at the switching box is open circuited. This has the effect of adding inductance at the center of the sloping dipole element, which electrically lengthens the element. The 36-foot length of feed line serves to increase the length of the element about 5%. This makes any unused element appear to be a reflector.
The array is simple and effective. By selecting one of the slopers through a relay box located at the tower, the system becomes a parasitic array which can be electrically rotated. All but one element of the array become reflectors, while one element is driven.
The basic physical layout is shown in Fig. 8-12. The height of the support point should be about 60 feet, but can be less and still give reasonable results. The upper portion of the sloper is five feet
Double This same loop antenna may be used on the twenty- and fifteen-meter bands, although its pattern will be somewhat different than on its fundamental frequency. Also, a slight mismatch will occur, but this can be overcome by a simple matching network. When the loop is mounted in a vertical plane, it tends to favor low-angle signals. If a high-angle system is desired, say for 80 meters, the full-wave loop can be mounted in a horizontal plane, thirty or more feet above ground. This arrangement will direct most of the energy virtually straight up, providing optimum sky-wave coverage on a short-haul basis.to edit
from the tower, suspended by rope, and makes an angle of 60 degrees with the ground. In Fig. 8-13, the switch box is shown containing all the necessary relays required to select the proper feed line for the desired direction. One feed line is selected at a time and opens the feed lines of those remaining. In this way the array is electrically rotated. These relays are controlled from inside the shack with an appropriate power supply and rotary switch. For safety reasons and simplicity, 12-volt dc relays are used. The control line consists of a five conductor cable, one wire used as a common connection; the others go to the four relays. By using diodes in series with the relays and a dual-polarity power supply, the number of control wires can be reduced, as shown in Fig. 8-15B.
Measurements indicate that this sloper array provides up to 20 dB front-to-back ratio and forward gain of about 4 dB. If one direction is the only concern, the switching system can be eliminated and the reflectors should be cut 5
Fig. 8-12 - Five sloping dipoles suspended from one support. Directivity and forward gain can be obtained from this simple array. Top view shows how the elements should be spaced around the support.
Fig. 8-13 - Inside view of relay box. Four relays provide control over five antennas. See text. The relays pictured here are Potter and Brumfield type MR11D.
Fig. 8-15 - Schematic diagram for sloper control system. All relays are 12-volt dc, dpdt with 8-A contact ratings. In A, the basic layout, excluding control cable and antennas. Note that the braid of the coax is also open-circuited when not in use. Each relay is bypassed with .001 -juF capacitors. The power supply is a low-current type. In B, diodes are used to reduce the number of control wires when using dc relays. See text.
system properly, a null can be placed in an unwanted direction, thus making it an effective receiving antenna. In the tests conducted with this antenna, the number of reflectors used were as few as one and as many as five. The optimum combination appeared to occur with four reflectors and one driven element. No tests were conducted with more than five reflectors. This same array can be scaled to 80 meters for similar results.
Source material and more extended discussion of topics covered in this chapter can be found in the references given below.
Elliott, "Phased Verticals for 40," QST, April, 1972.
Hubbell, "Feeding Grounded Towers as Radiators," QST, June, 1960.