Broadband Dipoles

One of the rarely discussed aspects of antenna construction is that the length/diameter ratio of the conductor used for the antenna element is a factor in determining the bandwidth of the antenna. In general, the rule of thumb states that large cross-sectional area makes the antenna more broadbanded. In some cases, this rule suggests the use of aluminum tubing instead of copper wire for the antenna radiator. On the higher-frequency bands that is a viable solution. Aluminum tubing can be purchased for relatively small amounts of money, and is both lightweight and easily worked with ordinary tools. But, as the frequency decreases, the weight becomes greater because the tubing is both longer and (for structural strength) must be of greater diameter. On 80 m, aluminum tubing is impractical, and at 40 m it is nearly so. Yet, 80 m is a significant problem, especially for older transmitters, because the band is 500 kHz wide, and the transmitters often lack the tuning range for the entire band. Some other solution is needed. Here are three basic solutions to the problem of  wide-bandwidth dipole antennas: folded dipole, bowtie dipole, and cage dipole.

Figure 6-10A shows the folded dipole antenna. This antenna basically consists of two half-wavelength conductors shorted together at the ends and fed in the middle of one of them. The folded dipole is most often built from 300-Ω television antenna twin-lead transmission line. Because the feedpoint impedance is nearly 300 Ω, the same type of twin lead can also be used for the transmission line. The folded dipole will exhibit excellent wide-bandwidth properties, especially on the lower bands.

A disadvantage of this form of antenna is that the transmitter has to match the 300-Ω balanced transmission line. Unfortunately, most modern radio transmitters are designed to feed coaxial-cable transmission line. Although an antenna tuner can be placed at the transmitter end of the feedline, it is also possible to use a 4:1 balun transformer at the feedpoint (Fig. 6-10B). This arrangement makes the folded dipole a reasonable match to 52- or 75-Ω coaxial-cable transmission line.

Another method for broadbanding the dipole is to use two identical dipoles fed from the same transmission line, and arranged to form a “bowtie” as shown in Fig. 6-11. The use of two identical dipole elements on each side of the transmission line has the effect of increasing the conductor cross sectional area so that the antenna has a slightly improved length/diameter ratio.

The bowtie dipole was popular in the 1930s and 1940s, and became the basis for the earliest television receiver antennas (TV signals are 3 to 5 MHz wide, so they require a broadbanded antenna). It was also popular during the 1950s as the so-called Wonder Bar antenna for 10 m. It still finds use, but it has faded somewhat in popularity.

The cage dipole (Fig. 6-12) is similar in concept, if not construction, to the bowtie. Again, the idea is to connect several parallel dipoles together from the same transmission line in an effort to increase the apparent cross-sectional area. In the case of the cage dipole, however, spreader disk insulators are constructed to keep the wires separated. The insulators can be built from plexiglass, lucite, or ceramic.


They can also be constructed of materials such as wood, if the wood is properly treated with varnish, polyurethene, or some other material that prevents it from becoming waterlogged. The spreader disks are held in place with wire jumpers (see inset to Fig. 6-12) that are soldered to the main element wires.

A tactic used by some builders of both bowtie and cage dipoles is to make the elements slightly different lengths. This “stagger tuning” method forces one dipole to favor the upper end of the band, and the other to favor the lower end of the band. The overall result is a slightly flatter frequency response characteristic across the entire band. On the cage dipole, with four half-wavelength elements, it should be possible to overlap even narrower sections of the band in order to create an even flatter characteristic.

From Book : Practical Antenna Handbook - Joseph P. Carr

Sloping Dipole

The sloping dipole (Fig. 6-8) 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, so please see that section also. Some operators like to arrange four sloping dipoles from the same mast such that they point in different directions around the compass (Fig. 6-9). A single fourposition coaxial cable switch will allow switching a directional beam around the compass to favor various places in the world.

From Practical Antenna Handbook - Joseph P Carr

Inverted-Vee Dipole

The inverted-vee dipoleis a half-wavelength antenna fed in the center like a dipole. By the rigorous definition, the inverted-vee is merely a variation on the dipole theme. 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 shown in Fig. 6-7), the resonant frequency is effectively lowered. Thus, the antenna will

By sloping the antenna elements down from the horizontal to an angle (as shown in 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 rigorous 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 followed 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 quarter wavelength) is L = ft [6.16]
After this length is determined, the actual length is found from the same cutand-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.

The dipole feedpoint

The dipole is a half-wavelength antenna fed in the center. Figure 6-2 shows the voltage (V) and current (I) distributions along the length of the half-wavelength radiator element. The feedpoint is at a voltage minimum and a current maximum, so you can assume that the feedpoint is a current antinode.
At resonance, the impedance of the feedpoint is Ro = V/I. 

There are two resistances that make up Ro. The first is the ohmic losses that generate nothing but heat when the transmitter is turned on. These ohmic losses come from the fact that conductors have electrical resistance and electrical connections are not perfect (even when properly soldered). Fortunately, in a well-made dipole these losses are almost negligible. 

The second contributor is the radiation resistance Rr of the antenna. This resistance is a hypothetical concept that accounts for the fact that RF power is radiated by the antenna. The radiation resistance is the fictional resistance that would dissipate the amount of power that is radiated away from the antenna.



For example, suppose we have a large-diameter conductor used as an antenna, and it has negligible ohmic losses. If 1000 W of RF power is applied to the feedpoint, and a current of 3.7 A is measured, what is the radiation resistance?


It is always important to match the feedpoint impedance of an antenna to the transmission-line impedance. Maximum power transfer always occurs (in any system) when the source and load impedances are matched. In addition, if some applied power is not absorbed by the antenna (as happens in a mismatched system), then the unabsorbed portion is reflected back down the transmission line toward the transmitter. This fact gives rise to standing waves, and the so-called standing wave ratio (SWR or VSWR) discussed in Chap. 3. This is a problem to overcome. Matching antenna feedpoint impedance seems to be simplicity itself because the free-space feedpoint impedance of a simple dipole is about 73 Ω, seemingly a good match to 75-Ω coaxial cable. Unfortunately, the 73-Ω feedpoint impedance is almost a myth. Figure 6-3 shows a plot of approximate radiation resistance (Rr) versus height above ground (as measured in wavelengths). As before, we deal in approximations in Fig. 6-3; in this case, the ambiguity is introduced by ground losses. Despite the fact that Fig. 6-3 is based on approximations, you can see that radiation resistance varies from less than 10 Ω, to around 100 Ω, as a function of height. At heights of many wavelengths, this oscillation of the curve settles down to the freespace impedance (72 Ω). At the higher frequencies, it might be possible to install a dipole at a height of many wavelengths. In the 2-m amateur radio band (144 to 148 MHz), one wavelength is around 6.5 ft (i.e., 2 m ×3.28 ft/m), so “many wavelengths” is relatively easy to achieve at reasonably attainable heights. In the 80-m band (3.5 to 4.0 MHz), however, one wavelength is on the order of 262 ft, so “many wavelengths” is a practical impossibility. There are three tactics that can be followed. First, ignore the problem altogether. In many installations, the height above ground will be such that the radiation resistance will be close enough to present only a slight impedance mismatch to a standard coaxial cable. The VSWR is calculated (among other ways) as the ratio:

good engineering practice (there are sometimes practical reasons) it is nonetheless necessary to install a dipole at less than optimum height. So, if that becomes necessary, what are the implications of feeding a 60-Ω antenna with either 52- or 75-Ω standard coaxial cable? Some calculations are revealing: For 75-Ω coaxial cable:

In neither case is the VSWR created by the mismatch too terribly upsetting. The second approach is to mount the antenna at a convenient height, and use an impedance matching scheme to reduce the VSWR. In Chap. 23, you will find information on various suitable (relatively) broadbanded impedance matching methods including Q-sections, coaxial impedance transformers, and broadband RF transformers.“Homebrew” and commercially available transformers are available to cover most impedance transformation tasks. The third approach is to mount the antenna at a height (Fig. 6-3) at which the expected radiation resistance crosses a standard coaxial cable characteristic impedance. The best candidate seems to be a height of a half-wavelength because the radiation resistance is close to the free-space value of 72 Ω, and is thus a good match for 75-Ω coaxial cable (such as RG-11/U or RG-59/U).

From Practical Antenna Handbook : Joseph J. Carr

Simple Halfwave Dipole Antennas

The simple dipole, or doublet, is a case in point. This antenna is also sometimes called the Hertz, or hertzian, antenna because radio pioneer Heinrich Hertz reportedly used this form in his experiments. The half-wavelength dipole is a balanced antenna consisting of two radiators (Fig. 6-1) that are each a quarter-wavelength, making a total of a half-wavelength. The antenna is usually installed horizontally with respect to the earth’s surface, so it produces a horizontally polarized signal.  

Let say you have L long dipole antenna .

for halfwave dipole antenna L = 1/2L = 1/4 L + 1/4L

In its most common configuration (Fig. 6-1), the dipole is supported at each end by rope and end insulators. The rope supports are tied to trees, buildings, masts, or some combination of such structures. The length of the antenna is a half-wavelength. Keep in mind that the physical length of the antenna, and the theoretical electrical length, are often different by about 5 percent. A free-space half-wavelength is found from
 
                                                         L = 492/Fmhz   feet   [6.1]





In a perfect antenna, that is self-supported many wavelengths away from any object, Eq. 6.1 will yield the physical length. But in real antennas, the length calculated above is too long. The average physical length is shortened by up to about 5 percent because of the velocity factor of the wire and capacitive effects of the end insulators. A more nearly correct approximation (remember that word, it's important) of a half-wavelength antenna is
 
                                                                L = 468/Fmhz  ft [6.2]
 
where L is the length of a half-wavelength radiator, in feet F MHz is the operating frequency, in megahertz
 
Example Calculate the approximate physical length for a half-wavelength dipole operating on a frequency of 7.25 MHz. Solution:
                                                                L = 468/Fmhz  ft

                                                                   = 468/7.25 ft 

                                                                   = 64.55 ft
 
or, restated another way:
                                                                 L = 64 ft 6.6 in
 
It is unfortunate that a lot of people accept Eq. 6.2 as a universal truth, a kind of immutable law of The Universe. Perhaps abetted by books and articles on antennas that fail to reveal the full story, too many people install dipoles without regard for reality. The issue is resonance. An antenna is a complex RLC network. At some frequency, it will appear like an inductive reactance (X = +jXL), and at others it will appear like a capacitive reactance (X =–jXC). At a specific frequency, the reactances are equal in magnitude, but opposite in sense, so they cancel each other out: XL – XC = 0. At this frequency, the impedance is purely resistive, and the antenna is said to be resonant. The goal in erecting a dipole is to make the antenna resonant at a frequency that is inside the band of interest, and preferably in the portion of the band most often used by the particular station. Some of the implications of this goal are covered later on, but for the present, assume that the builder will have to custom-tailor the length of the antenna. Depending on several local factors (among them, nearby objects, the shape of the antenna conductor, and the length/diameter ratio of the conductor) it might prove necessary to add, or trim, the length a small amount to reach resonance.