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

Transmission line characteristics

Velocity factor
 In the section preceding this section, we discovered that the velocity of the wave (orsignal) in the transmission line is less than the free-space velocity (i.e., less than thespeed of light). Further, we discovered in Eq. 3.3 that velocity is related to the di-electric constant of the insulating material that separates the conductors in thetransmission line. Velocity factor vis usually specified as a decimal fraction of c,the speed of light (3 ×108m/s). For example, if the velocity factor of a transmissionline is rated at “0.66,” then the velocity of the wave is 0.66c, or (0.66) (3 ×108m/s)= 1.98 ×108m/s.Velocity factor becomes important when designing things like transmission linetransformers, or any other device in which the length of the line is important. In mostcases, the transmission line length is specified in terms of electrical length, whichcan be either an angular measurement (e.g., 180° orπradians), or a relative measurekeyed to wavelength (e.g., one-half wavelength, which is the same as 180°). Thephysical lengthof the line is longer than the equivalent electrical length. For exam-ple, let’s consider a 1-GHz half-wavelength transmission line.A rule of thumb tells us that the length of a wave (in meters) in free space is 0.30/F,where frequencyFis expressed in gigahertz; therefore, a half-wavelength line is 0.15/F

 At 1 GHz, the line must be 0.15 m/1 GHz = 0.15 m. If the velocity factor is 0.80, then thephysical lengthof the transmission line that will achieve the desired electrical lengthis [(0.15 m) (v)]/F= [(0.15 m) (0.80)]/1 GHz = 0.12 m. The derivation of the rule ofthumb is “left as an exercise for the student.” (Hint: It comes from the relationship be-tween wavelength, frequency, and velocity of propagation for any form of wave.)There are certain practical considerations regarding velocity factor that resultfrom the fact that the physical and electrical lengths are not equal. For example, ina certain type of phased-array antenna design, radiating elements are spaced a half-wavelength apart, and must be fed 180° (half-wave) out of phase with each other.The simplest interconnect is to use a half-wave transmission line between the 0°element and the 180° element. According to the standard wisdom, the transmissionline will create the 180° phase delay required for the correct operation of the an-tenna. Unfortunately, because of the velocity factor, the physical length for a one-half electrical wavelength cable is shorter than the free-space half-wave distancebetween elements. In other words, the cable will be too short to reach between theradiating elements by the amount of the velocity factor!Clearly, velocity factor is a topic that must be understood before transmissionlines can be used in practical situations. Table 3-1 shows the velocity factors for sev-eral types of popular transmission line. Because these are nominal values, the actualvelocity factor for any given line should be measured.

 Transmission line noise 
Transmission lines are capable of generating noise and spurious voltages that areseen by the system as valid signals. Several such sources exist. One source is thecoupling between noise currents flowing in the outer conductor and the inner con-ductor. Such currents are induced by nearby electromagnetic interference and othersources (e.g., connection to a noisy groundplane). Although coaxial design reducesnoise pickup, compared with parallel line, the potential for EMI exists. Selection ofhigh-grade line, with a high degree of shielding, reduces the problem.Another source of noise is thermal noises in the resistances and conductances.This type of noise is proportional to resistance and temperature.
 There is also noise created by mechanical movement of the cable. One speciesresults from the movement of the dielectric against the two conductors. This form ofnoise is caused by electrostatic discharges in much the same manner as the sparkcreated by rubbing a piece of plastic against woolen cloth.A second species of mechanically generated noise is piezoelectricityin the di-electric. Although more common in cheap cables, one should be aware of it. Me-chanical deformation of the dielectric causes electrical potentials to be generated.Both species of mechanically generated noise can be reduced or eliminated byproper mounting of the cable. Although rarely a problem at lower frequencies, suchnoise can be significant at microwave frequencies when signals are low.

 Coaxial cable capacitance 
A coaxial transmission line possesses a certain capacitance per unit of length. This capacitance is defined by :

 A long run of coaxial cable can build up a large capacitance. For example, acommon type of coax is rated at 65 pF/m. A 150-m roll thus has a capacitance of 65 pF/m ×(150 m), or 9750 pF. When charged with a high voltage, as is done inbreakdown voltage tests at the factory, the cable acts like a charged high-voltagecapacitor. Although rarely (if ever) lethal to humans, the stored voltage in newcable can deliver a nasty electrical shock and can irreparably damage electronic components.
  
Coaxial cable cutoff frequency Fo 
The normal mode in which a coaxial cable propagates a signal is as a transverseelectromagnetic (TEM) wave, but others are possible—and usually undesirable.There is a maximum frequency above which TEM propagation becomes a prob-lem, and higher modes dominate. Coaxial cable should notbe used above a fre-quency of

where 

F is the TEM-mode cutoff frequency
D is the diameter of the outer conductor, in inchesdis the diameter of the inner conductor, in inches 
e is the dielectric constant 

When maximum operating frequencies for cable are listed, it is the TEM modethat is cited. Beware of attenuation, however, when making selections for microwavefrequencies. A particular cable may have a sufficiently high TEM-mode frequency,but still exhibit a high attenuation per unit length at X or Ku bands.
(from Practical Antenna :by Joseph Carr) If you wantget hardcopy of this Practical Antenna Theory ,You canbuy this book :Practical Antenna Handbook by Joseph Carr:

Transmission line characteristic impedance (Zo)

The transmission line is an RLC network (see Fig. 3-2), so it has a characteristic impedance Zo, also sometimes called a surge impedance. Network analysis will show that Zo is a function of the per unit of length parameters resistance R,con-ductance G,inductance L, and capacitance C, and is found from equation :

where Zo is the characteristic impedance, in ohms 
 R is the resistance per unit length, in ohms 
G is the conductance per unit length, in mhos  
L is the inductance per unit length, in henrys  
C is the capacitance per unit length, in farads 
ω is the angular frequency in radians per second (2πF)

 In microwave systems the resistances are typically very low compared with the reactances, so Eq. 3.1 can be reduced to the simplified form:

 Example 3-1A nearly lossless transmission line (Ris very small) has a unit length inductance of 3.75 nH and a unit length capacitance of 1.5 pF. Find the char-acteristic impedance Zo,
Solution:

The characteristic impedance for a specific type of line is a function of the con-ductor size, the conductor spacing, the conductor geometry (see again Fig. 3-1), andthe dielectric constant of the insulating material used between the conductors. Thedielectric constant eis equal to the reciprocal of the velocity (squared) of the wavewhen a specific medium is used

where e is the dielectric constant (for a perfect vacuum e= 1.000) v is the velocity of the wave in the medium

(a)Parallel line

where Zo is the characteristic impedance, in ohms e is the dielectric constant S is the center-to-center spacing of the conductors d is the diameter of the conductors

 (b) Coaxial line

where D is the diameter of the outer conductor d is the diameter of the inner conductor 

(c)Shielded parallel line

where A=s/d 

          B=s/D

 (d)Stripline

 where  

et ,is the relative dielectric constant of the printed wiring board (PWB) 

T is the thickness of the printed wiring board 

W is the width of the stripline conductor 

The relative dielectric constant e tused above differs from the normal dielectric constant of the material used in the PWB. The relative and normal dielectric con-stants move closer together for larger values of the ratio W/T.Example 3-2A stripline transmission line is built on a 4-mm-thick printed wiring board that has a relative dielectric constant of 5.5. Calculate the characteris-tic impedance if the width of the strip is 2 mm.

Solution :

 

 

In practical situations, we usually don’t need to calculate the characteristic im-pedance of a stripline, but rather design the line to fit a specific system impedance(e.g., 50 Ω). We can make some choices of printed circuit material (hence dielectricconstant) and thickness, but even these are usually limited in practice by the avail-ability of standardized boards. Thus, stripline widthis the variable parameter. Equa-tion 3.2 can be arranged to the form:

 The impedance of 50 Ωis accepted as standard for RF systems, except in the cable TV industry. The reason for this diversity is that power handling ability and lowloss operation don’t occur at the same characteristic impedance. For example, the maximum power handling ability for coaxial cables occurs at 30 Ω, while the lowest loss occurs at 77 Ω; 50 Ωis therefore a reasonable tradeoff between the two points.In the cable TV industry, however, the RF power levels are minuscule, but lines arelong. The tradeoff for TV is to use 75 Ωas the standard system impedance in orderto take advantage of the reduced attenuation factor.

  If you want get hardcopy of this Practical Antenna Theory ,You canbuy this book :Practical Antenna Handbook by Joseph Carr:

Transmission Lines

CONDUITS FOR TRANSPORTING RF SIGNALS between elements of a system. For example, transmission lines are used between anexciter output and transmitter input, and between the transmitter input and its out-put, and between the transmitter output and the antenna. Although often erro-neously characterized as a “length of shielded wire,” transmission lines are actuallycomplex networks containing the equivalent of all the three basic electrical compo-nents: resistance, capacitance, and inductance. Because of this fact,
transmissionlines must be analyzed in terms of an RLC network. 

Parallel and coaxial lines
 
This article will consider several types of transmission lines. Both step-functionand sine-wave ac responses will be studied. Because the subject is both conceptualand analytical, both analogy and mathematical approaches to the theory of trans-mission lines will be used.Figure 3-1 shows several basic types of transmission line. Perhaps the oldest andsimplest form is the parallel lineshown in Figs. 3-1A through 3-1D. Figure 3-1A shows an end view of the parallel conductor transmission line. The two conductors,of diameter d, are separated by a dielectric (which might be air) by a spacing S.These designations will be used in calculations later. Figure 3-1B shows a type ofparallel line called twin lead. This is the old-fashioned television antenna transmis-sion line. It consists of a pair of parallel conductors separated by a plastic dielectric.TV-type twin lead has a characteristic impedance of 300 Ω, while certain radio trans-mitting-antenna twin lead has an impedance of 450 Ω. Another form of twin lead isopen line, shown in Fig. 3-1C. In this case, the wire conductors are separated by anair dielectric, with support provided by stiff (usually ceramic) insulators. A tie wire(only one shown) is used to fasten each insulator end to the main conductor. Someusers of open line prefer the form of insulator or supporter shown in Fig. 3-1D. 

This form of insulator is made of either plastic or ceramic, and is in the form of a U. Thepurpose of this shape is to reduce losses, especially in rainy weather, by increasingthe leakage currents path relative to spacing S.Parallel lines have been used at VLF, MW, and HF frequencies for decades. Evenantennas into the low VHF are often found using parallel lines. The higher imped-ance of these lines (relative to coaxial cable) yields lower loss in high-power appli-cations. For years, the VHF, UHF, and microwave application of parallel lines waslimited to educational laboratories, where they are well suited to performing exper-iments (to about 2 GHz) with simple, low-cost instruments. Today, however, printedcircuit and hybrid semiconductor packaging has given parallel lines a new lease onlife, if not an overwhelming market presence.Figure 3-1E shows a form of parallel line called shielded twin lead. This type of lineuses the same form of construction as TV-type twin lead, but it also has a braided shield-ing surrounding it. This feature makes it less susceptible to noise and other problems.The second form of transmission line, which finds considerable application atmicrowave frequencies, is coaxial cable(Figs. 3-1F through 3-1L). This form ofline consists of two cylindrical conductors sharing the same axis (hence “coaxial”),and separated by a dielectric (Fig. 3-1F). For low frequencies (in flexible cables)the dielectric may be polyethylene or polyethylene foam, but at higher frequenciesTeflonand other materials are used. Also used, in some applications, are dry air anddry nitrogen.

 Several forms of coaxial line are available. Flexible coaxial cable is perhaps themost common form. The outer conductor in such cable is made of either braid or foil(Fig. 3-1G). Television broadcast receiver antennas provide an example of such cablefrom common experience. Another form of flexible or semiflexible coaxial line is heli-cal line(Fig. 3-1H) in which the outer conductor is spiral wound.Hardline(Fig.3-1I) is coaxial cable that uses a thin-wall pipe as the outer conductor. Some hardlinecoax used at microwave frequencies has a rigid outer conductor and a solid dielectric.Gas-filled lineis a special case of hardline that is hollow (Fig. 3-1J), the centerconductor is supported by a series of thin ceramic or Teflon insulators. The dielec-tric is either anhydrous (i.e., dry) nitrogen or some other inert gas.Some flexible microwave coaxial cable uses a solid “air-articulated” dielectric(Fig. 3-1K), in which the inner insulator is not continuous around the center con-ductor, but rather is ridged. Reduced dielectric losses increase the usefulness of the
cable at higher frequencies. Double-shielded coaxial cable (Fig. 3-1L) provides anextra measure of protection against radiation from the line, and EMI from outsidesources, from getting into the system.Stripline, also called microstripline(Fig. 3-1M), is a form of transmission lineused at high UHF and microwave frequencies. The stripline consists of a criticallysized conductor over a ground-plane conductor, and separated from it by a dielec-tric. Some striplines are sandwiched between two groundplanes and are separatedfrom each by a dielectric.

(from  Practical Antenna Handbook by Joseph J Carr)

Inflatable Antenna/Bandpass Antenna Element

ANTENNA ARRAYS -- INFLATABLE STRUCTURES by Nasa/Langley Reaearch  Center
Source : https://archive.org/details/2002-L-02197