Broadband Dipoles

One of the rarely discussed aspects of antenna construction is that the length/diame-ter ratio of the conductor used for the antenna element is a factor in determining thebandwidth of the antenna. In general, the rule of thumb states that large cross-sec-tional area makes the antenna more broadbanded. In some cases, this rule suggeststhe use of aluminum tubing instead of copper wire for the antenna radiator. On thehigher-frequency bands that is a viable solution. Aluminum tubing can be purchasedfor relatively small amounts of money, and is both lightweight and easily worked withordinary tools. But, as the frequency decreases, the weight becomes greater becausethe 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 sig-nificant 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 ofwide-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 mid-dle of one of them. The folded dipole is most often built from 300-Ωtelevision an-tenna 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 di-pole 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 the300-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 dipolea reasonable match to 52- or 75-Ω coaxial-cable transmission line.Another method for broadbanding the dipole is to use two identical dipoles fedfrom 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 theeffect 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 requirea 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 thecase of the cage dipole, however, spreader disk insulators are constructed to keepthe 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 properlytreated with varnish, polyurethene, or some other material that prevents it from be-coming waterlogged. The spreader disks are held in place with wire jumpers (see in-set 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 el-ements slightly different lengths. This “stagger tuning” method forces one dipole tofavor 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 theentire band. On the cage dipole, with four half-wavelength elements, it should bepossible to overlap even narrower sections of the band in order to create an evenflatter characteristic.

Sloping dipole (“sloper” or “slipole”)

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 di-pole 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 asobtained 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 suchthat they point in different directions around the compass (Fig. 6-9). A single four-position coaxial cable switch will allow switching a directional beam around the com-pass to favor various places in the world.

source : Practical Antenna Handbook by Joseph J Carr

Transmission line responses

In order to understand the operation of transmission lines, we need to consider two cases:step-function response and the steady-state ac response. The step-function case involves a single event when a voltage at the input of the line snaps from zero(or a steady value) to a new (or nonzero) value, and remains there until all action dies out. This response tells us something of the behavior of pulses in the line, and infact is used to describe the response to a single-pulse stimulus. The steady-state ac response tells us something of the behavior of the line under stimulation by a sinusoidal RF signal. 

Step-function response of a transmission line 
Figure 3-3 shows a parallel transmission line with characteristic impedance Zo connected to a load impedance ZL. The generator at the input of the line consists of a voltage source Vin series with a source impedance Zs and a switch S1. Assume for the present that all impedances are pure resistances (i.e., R+j0). Also, assume that Zs=Zo.When the switch is closed at time To(Fig. 3-4A), the voltage at the input of theline (Vin) jumps to V/2. In Fig. 3-2, you may have noticed that the LC circuit resem-bles a delay line circuit. As might be expected, therefore, the voltage wave front propagates along the line at a velocity v of,

 where v is the velocity, in meters per second L is the inductance, in henrys C is the capacitance, in farads

Inverted-vee dipole 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 possi-ble from the earth’s surface, but the ends droop to very close to the surface. Angle acan be almost anything convenient, provided that a> 90 degrees; typically, most in-verted-vee antennas use an angle of about 120 degrees. Although essentially a com-pensation antenna for use when the dipole is not practical, many operators believethat it is essentially a better performer on 40 and 80 m in cases where the dipole can-not 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. Al-though the concept of “absolute” length does not hold for regular dipoles, it is evenless 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 dipolefor the same frequency. The initial cut of the antenna element lengths (each quarter wavelength) 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 theantenna and narrows its bandwidth. Thus, some adjustment in these departments isin order. You might want to use an impedance matching scheme at the feedpoint, oran antenna tuner at the transmitter.

High-frequency dipole antennas

Dipole Antenna: What is it? (And the Types of Antennas) - 

What is a Dipole Antennas?

A dipole antenna (also known as a doublet or dipole aerial) is defined as a type of RF (Radio Frequency) antenna, consisting of two conductive elements such as rods or wires. The dipole is any one of the varieties of antenna that produce a radiation pattern approximating that of an elementary electric dipole. Dipole antennas are the simplest and most widely used type of antenna.

A ‘dipole’ means ‘two poles’ hence the dipole antenna consists of two identical conductive elements such as rods or metal wires. The length of the metal wires is approximately half of the maximum wavelength (i.e.,= Lambda/2) in free space at the frequency of operation.

A myth arose in radio communications circles some time ago. People came to believe (especially in the ham and CB communities) that large antenna arrays areabsolutely necessary for effective communications, especially over long distances.Overlooked, almost to the point of disdain, were effective (but simple) antennas that can be erected by in experienced people and made to work well. 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 itproduces a horizontally polarized signal.In its most common configuration (Fig. 6-1), the dipole is supported at each endby rope and end insulators. The rope supports are tied to trees, buildings, masts, orsome combination of such structures.The length of the antenna is a half-wavelength. Keep in mind that the physicallength of the antenna, and the theoretical electrical length, are often different byabout 5 percent. A free-space half-wavelength is found from

 In a perfect antenna, that is self-supported many wavelengths away from any ob-ject, 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 percentbecause 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 ahalf-wavelength antenna is

 where

 L is the length of a half-wavelength radiator, in feet 
FMHz is the operating frequency, in megahertz

 ExampleCalculate the approximate physical length for a half-wavelength di-pole operating on a frequency of 7.25 MHz.
Solution :

 or, restated another way:

 It is unfortunate that a lot of people accept Eq. 6.2 as a universal truth, a kind ofimmutable law of The Universe. Perhaps abetted by books and articles on antennasthat fail to reveal the full story, too many people install dipoles without regard for reality. The issue is resonance. An antenna is a complex RLCnetwork. At some fre-quency, 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 reac-tances 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 issaid to be resonant.The goal in erecting a dipole is to make the antenna resonant at a frequency that isinside the band of interest, and preferably in the portion of the band most often used bythe particular station. Some of the implications of this goal are covered later on, but forthe present, assume that the builder will have to custom-tailor the length of the an-tenna. Depending on several local factors (among them, nearby objects, the shape ofthe antenna conductor, and the length/diameter ratio of the conductor) it might provenecessary to add, or trim, the length a small amount to reach resonance.
Reference:Practical Antenna Handbook -Joseph Carr