Contents

 

 

6.0 Antennas

 

6.2 Antennas

6.2.1 Marconi Antennas

6.2.2 Hertzian Antennas

6.2.3 Dipole Antenna

6.2.4 Halfwave Dipole

6.2.4.1 Loop Antenna

6.2.4.3 Log Periodic Dipole Array [LPDA]

6.2.4.4 Yagi Uda Antenna

6.2.5 Array Factor

6.2.5.1 Broadside Array

6.2.5.2 End-fire Array

6.2.5.4 Phased Array

 

6.3 TV Broadcast Antennas

Max Height [ft]

6.3.1 Superturnstile or Batwing

6.3.2 Panel Antennas

6.3.3 Circularly Polarized Antennas

6.3.4 Helical Antennas

6.3.5 Slot Antenna

 

Assignment Questions

 

For Further Research

 

 

 


6.0 Antennas

 

Antenna Basics by the US Navy

Antenna Characteristics by Harris

Antenna Gain Measurements Part 1 by Brown

Antenna Gain Measurements Part 2 by Brown

Parabolic Antennas & Their Feeds by Comly

Smart Antenna Systems by ArrayCom

Shortwave Antenna by Gmbh

http://news.wirelessdesignonline.com/content/hubs/dir.asp?hub=news

http://www.borg.com/~warrend/guru.html

 

The purpose of an antenna is to collect and convert electromagnetic waves to electronic signals. Transmission lines then guide these to the receiver front end.

In order for a picture to be usable, a high signal to nose ratio must be achieved. A video signal with an S/N of 10 dB is not usable whereas a S/N of 40 dB results in an excellent picture.

Although most TV receiving antennas are simply a piece of bent wire, their interaction with electromagnetic fields is quite complex, and a whole array of terms is needed to characterize them:

Beamwidth: the angle defines by the radiation pattern where the signal strength drops 3 dB of its maximum value in a given plane.

Polarization: the plane of electric field polarization with respect to the earth.

Gain: a figure of merit used to quantify the signal capturing ability of the antenna. It is closely related to directivity and beamwidth.

Effective area: a measure of the antenna’s ability to collect energy. It is related to gain by the expression: A = Gl2/4p.

Input impedance: The impedance, which is necessary in the receiver for maximum power transfer to occur.

Radiation resistance: the ratio of the power driving the antenna to the square of the current driving its terminals.

Bandwidth: the usable frequency band associated with the antenna.

Before any antenna can be selected, the center frequency and operating bandwidth must be known. In general, the higher the operating frequency, the smaller the antenna.

Antenna gain is always measured against a known reference such as an isotropic source (dBi) or a half wave dipole (dBd).

Antenna Type

Typical Gain [dBd]

Dipole

0

Omni

0

Gain Omni

3 – 12

Mobile Whip

-0.6 to + 5.5

Corner Reflector

4 – 10

Log Periodic

3 – 8

Horn

5 – 12

Helix

5 – 12

Microstrip Patch

3 – 15

Yagi

3 – 20

Panel

5 – 20

Parabolic Dish

10 ‑ 30

 

Increasing antenna gain by 3 dB generally requires increasing the size by a factor of 2-3 or by reducing the beamwidth.

Vertical omni directional antennas and collinear arrays are used for line-of-sight communications with ground based mobile units. Sectoring can be accomplished by panel antennas. Fixed point-to-point links generally use a yagi or parabolic dish.

Polarization

Polarization by US Navy

 

Most wireless systems use vertical, horizontal, or elliptical polarization. Elliptical polarization can be either RHC or LHC. Circular or elliptical polarization is used for in-building applications since handheld devices often point off-axis. Polarization can provide 20-dB isolation between adjacent systems.

6.2   Antennas

Antennas exhibit reciprocity, which means they have the same gain whether used for transmitting or receiving.

The relevant electric fields associated with an antenna are extremely complex and have the general form[1]:

The near field or Fresnel region consists of all three fields. The electrostatic and inductive fields fall off in intensity quite quickly. The far field or Fraunhoffer region consists entirely of the radiated field.

Antenna Near Field Characteristics

Antenna Near Field by US Navy

Phase & Frequency Effects by US Navy

Radiation Patterns by US Navy

 

6.2.1  Marconi Antennas

Marconi antennas are usually 1/4 wavelength long and require a path to ground. The ground plane itself acts as a reflector of energy, and combines with the directly radiated wave to create the overall radiation pattern. If the ground is dry or otherwise a poor conductor, a copper grid is generally laid out on the ground. The impedance of a 1/4 l Marconi antenna is 36.6 W.

Increasing the antenna length has a significant impact on the radiation pattern:

6.2.2  Hertzian Antennas

Hertzian antennas do not require a path to either ground or a ground plane. The simplest antenna of this type is the elementary doublet. It is a hypothetical antenna where the instantaneous current magnitude is constant along its length.

The radiation pattern for this antenna is donut shaped, with the antenna rod running through the hole. The bulk of the energy is radiated at right angles to the rod and nothing off the ends.

This antenna is often used as a reference instead of an isotropic radiator, since close approximations of it can be constructed. Other antennas can be considered as being composed of a series of doublets.

The field strength at any distance and angle can be calculated from:

6.2.3  Dipole Antenna

A dipole is sometimes referred to as a Hertzian dipole. Since it has a relatively simple construction and it’s radiation characteristics are well defined, it is often used as the standard to which all other antennas are compared.

The dipole radiation pattern is shaped like a slightly flattened donut.

The simplest antenna is the dipole. The relationship between antenna current and electric field is given by:[2]

A ½ l dipole has an impedance of about 70 W. To increase this impedance and more closely match the characteristics of a twin lead cable, the dipole may be folded. A ½ l folded dipole has an input impedance of about 280 W, and is used as the driving element in many other types of antennas.

Most TV receivers are equipped with two indoor antennas, one to cover the VHF band and the other the UHF band.

The most common VHF antennas are the extendible monopole and vee dipole colloquially known as the rabbit ears. These are available with either a 75 W or 300 W impedance and have a typical gain of -4 dB with respect to a ½ l dipole. The vee dipole has a lower input impedance than a straight dipole of the same length, but under some conditions, it can exhibit a higher directivity due to the reduction of sidelobes.

The common UHF antennas are the circular loop and triangular dipole. They typically have a 300 W impedance. The dipole version sometimes has a reflecting screen to improve the gain and directivity.

6.2.4  Halfwave Dipole

The radiation pattern of the half wave dipole is very much like a donut.

The distribution of voltage, current, and impedance resemble:

By increasing the length of the dipole, the donut tends to flatten out and then explodes into complex multi lobed shapes.

Short Wave Antenna

 

6.2.4.1 Loop Antenna

The entire UHF band can be received on a single 20.3 cm diameter loop. The circumference varies from one wavelength at 470 MHz to 1.7 wavelengths at 806 MHz. The directivity is about 3.5 dB. The mid band gain is 3 dB higher than a ½ l dipole, but falls off to about 1 dB at either end.

 

Loop antennas that are much smaller than wavelength they are attempting to catch, exhibit a null in the direction of the loop axis. This makes it suitable for radio direction finding equipment. If the loop size is increased, it begins to generate a lobe across the axis and in line with the feed.

6.2.4.2 Triangular Dipole [Bowtie]

The bowtie antenna is formed of two triangular sheets connected to a transmission line and provides a 3 dB gain over a simple dipole. It can also be constructed of a wire mesh if the spacing is less than 1/10 wavelength. The input impedance is a function of length and flare angle. For television applications, the flare angle a is between 60o and 80o. If the antenna is mounted ¼ l in front of a reflecting surface, the gain increases to approximately 9 dB.  Stacking two of them vertically one wavelength apart, increases the overall gain to about 12 dB.

If the receiver is located at a great distance from the broadcast tower, it is often necessary to use an outdoor antenna. These often have a gain of 15 dB. Placing the antenna on a tall mast also increases the received signal strength by as much as an additional 14 dB. A further improvement occurs because these antennas have a greater immunity to interference, due to their complex structure.

Most outdoor antennas are a combination of two antennas [UHF and VHF] in a single structure. The VHF antenna is generally a log-periodic dipole array. The UHF antenna may be an LPDA, Yagi-Uda, corner reflector, parabolic reflector, or triangular dipole array with reflecting screen.

6.2.4.3 Log Periodic Dipole Array [LPDA]

This antenna is called a log periodic array because the impedance variations across the usable band are periodic functions of frequency. The high impedance version is mounted on an insulated boom and feed by a balanced cable. The average domestic antenna of this type has a gain of about 4.5 dB in the low VHF band, rising to 7 dB in the high VHF band.

Basic Design Formulas

Most CATV applications use a 75W unbalanced configuration, because it is more compatible with their cable feeds and equipment.

Two parallel conducting booms form a low impedance transmission line. Phase reversal between dipoles is obtained by alternate attachment to the booms.

An UHF LPDA can be constructed from V-shaped dipoles. The dipoles are used in their ½ l and 3/2 l modes and eliminate the need for higher frequency dipoles.

6.2.4.4 Yagi Uda Antenna

The dipole is typically 0.45 to 0.49 wavelengths long. The reflector is normally 0.55 wavelengths long and placed anywhere from 0.1 to 0.25 wavelengths behind the dipole. The reflector spacing has no affect on the forward gain, but does influence the front to back ratio and input impedance.

The directors are normally 0.4 to 0.45 wavelengths long and are spaced at 0.3 to 0.4 wavelengths in front of the dipole. An antenna will usually have 6 to 12 directors.

6.2.4.5 Parabolic Antenna

Parabolic Antennas and Their Feeds

http://www.pinnaclecomm.com/parabolic.html

http://www.afcsat.com/profile.html

http://www.mlesat.com/antennas.html

http://deepspace.jpl.nasa.gov/dsn/antennas/index.html

 

Arecibo, with a diameter of 305 meters, is the world’s largest parabolic reflector.

The NASA Deep Space Network uses a number of parabolic antennas including 34-meter beam waveguide antennas.

The 70-meter antenna is the largest, and most sensitive, in the DSN. The reflector must remain accurate within a fraction of the signal wavelength [one centimeter].

Antenna Coupling

RF Balun Transformers by MiaCom

 

Many low power antenna transmission lines are be either 75 W unbalanced or 300 W balanced cables. A baluns transformer is needed to match to a 75 W cable to a 300 W input. They have an insertion loss of about 1 or 2 dB.

Many home use radio receivers have two inputs; one accepts a 300 W twin lead cable on a screw terminal and the other accepts a 75 W coax cable on a type F connector.

Most higher power radio systems are designed to operate over a 50 W transmission line.

VSWR

http://www.antennex.com/preview/vswr.htm

http://www.l0pht.com/~oblivion/radionet/reference/quick_ref/vswr.html

http://home.epix.net/~donaldso/E_Tools/VSWR.html

 

The VSWR is a measure of how well the radio system is impedance matched to the antenna. If VSWR is too high, transmission efficiency is reduced.

The typical value for VSWR is 1.5. This means that the antenna impedance must be between 37.5 and 75 W.

In general, a VSWR of >2.0 is undesirable since it represents increased transmission loss. Reducing VSWR below 1.5 increases expense with little improvement in performance.

Smith Chart

http://www.educatorscorner.com/experiments/spectral/SpecAn9.html

http://sss-mag.com/smith.html

http://www.ee.surrey.ac.uk/Personal/D.Jefferies/smith.html

http://fbox.vt.edu:10021/B/brindlec/smith/smith.html

 

Simplified Smith Chart

The Smith Chart is a graphical tool that is used to examine the performance of radio transmission lines.

The tangential circles represent axis of constant resistance. The curved lines radiating up and down represent constant reactance. The intersection of any two lines therefore represents a complex impedance. In order to increase the utility of the chart, the impedances are normalized. Therefore, the center represents the ideal or matched impedance regardless of the system being examined.

In most systems, the actual impedance is a function of frequency. It can also be a function of position with respect to the source or load. Since the degree of impedance mismatch is related to VSWR, the Smith chart also has several scales plotted below and around the actual chart.

6.2.5  Antenna Arrays

Flat Panel Arrays by Harris

Smart Antenna Tutorial

http://server5550.itd.nrl.navy.mil/projects/HAARP/ant3.html

 

By combining several radiating elements together, the overall radiation pattern can be modified to suit a particular application.

The array factor is the increase in field strength in an array when compared to a single element radiating the same power:

The array factor AF, is a maximum when  and a minimum when .

HAARP Array

http://server5550.itd.nrl.navy.mil/projects/haarp/index.html

Photo of HAARP Array

 

The HAARP Ionospheric facility conducts upper atmospheric research in Alaska. It broadcasts 3.6 Mw into a rectangular array of 180 crossed dipoles operating in the HF band.

SETI

http://www.seti-inst.edu/

 

The ATA array, managed by SETI is expected to consist of 700 4 m diameter antennas. It will operate in the 1 – 10 GHz range.

 

6.2.5.1 Broadside Array

If all of the elements are fed in-phase, there will always be a broadside radiation pattern. However, depending on the relative spacing, an end fire pattern can also be created.

The radiating elements in the above illustration can be placed such that they reinforce one another along the array axis, or not. An end fire pattern is recreated when they reinforce.

By varying the space or phase shift between the elements, the size and direction of the side lobes can be adjusted between these two extremes. Increasing the number of radiating elements increases the overall array gain.

Determining the array factor is sometimes relatively straightforward. By definition, the signal strength for a broadside array is a maximum when  and a minimum when

Since the array factor is a maximum when  we can determine the current phase shift a, required to create a broadside radiation pattern for a given frequency or element spacing:

Therefore the array factor for a broadside array is:

Varying the spacing in a 6-element array produces the following patterns:

Each element may have its own feed or there may be a single feed:

       

This form of antenna is often deployed in vertical stacks, with a reflector spaced 1/4 wavelength behind the curtain. This broadband dipole curtain array is the standard antenna for 100 to 500 kW short-wave broadcasting stations.

CBC Radio International operates eight curtain arrays at Sackville NB. Three have an output power of 100 kW and five have an output of 250 kW. They are tunable over the range of 4.9 to 21.7 MHz. Signals are beamed to Africa, Europe, Latin America, The Caribbean, the USA, and Mexico.

6.2.5.2 End-fire Array

If all of the elements are positioned in such a way that the combined wave fronts reinforce along the array axis.

Calculate the array factor is relatively straightforward. The signal strength is a maximum when  and a minimum when

Since the array factor is a maximum when  we can determine the value of a:

Therefore the array factor for an end-fire array is:

Varying the spacing for a 6-element array produced the following patterns:

6.2.5.4 Phased Array

By varying the phase shift between elements, a beam or multiple beams can be pointed towards a given direction. This forms the basis of the large electronically steered radar system currently being deployed. Collectively theses systems are known as phased arrays.

The overall radiation pattern may resemble:

 

The PAVE PAWS[3] early warning radar for example has 1792 active crossed dipole antennas on a 102-foot face. Each face can scan 120o in azimuth and 80o in elevation. The array has a range of 300 miles and can produce multiple beams, which can be redirected in milliseconds.

http://www.spacecom.af.mil/hqafspc/library/facts/pavepaws.htm

Smart Antennas

 

6.3 TV Broadcast Antennas

In order to minimize interference between various broadcasting stations, the FCC has established maximum limits on antenna height and horizontally polarized ERP.

Channel

ERP [kW]

Max Height [ft]

2 - 6

100

1000

7 - 13

316

2000

14 - 83

5000

2000

 

This is a lot of power, which must be transported to the antenna. The feed arrangement generally consists of two coaxial copper tubes, with a characteristic impedance of 51.5 W or 75 W. The impedance is largely determined by the diameter of the two pipes. The upper UHF channels do not use a center conductor, and the signal is feed over a waveguide.

Although most antennas may be regarded as a bent piece of wire, they are very complex structures. Some of their more important characteristics include: horizontal and vertical directivity and gain, impedance, bandwidth, and power rating.

TV antennas can be either horizontally or elliptically polarized. Elliptically polarized signals have a decided advantage since the orientation of the receiving antenna is not as important, and ghosting is reduced. Furthermore, the total output power can twice as large since the regulations permit a broadcaster to transmit as much power in the other orientations as in the horizontal one.

To make certain that the main lobe of the radiation pattern actually strikes the earth, TV antennas are generally given a slight downward tilt of 1o or 2o. Normally this would also produce sharp nulls in the broadcast area. The radiation pattern is therefor adjusted to prevent this through a process called null fill.

6.3.1  Superturnstile or Batwing

This is the first antenna developed for commercial broadcast applications. Each radiator is a modified dipole. Four of them are evenly spaced around a central support to form a section. Dipole pairs are feed in phase quadrature and thus produce a horizontal polarization pattern.

Sections are stacked one wavelength apart to obtain a gain of anywhere from 3 to 12 for the low VHF channels, and 6 to 18 for the high VHF channels. This provides for a nearly circular pattern in the horizontal plane.

The batwing is a broadband antenna, since it is formed not simply by a rod, but by means of a partially filled plane surface. Consequently, it can be used to carry two channels simultaneously.

6.3.2  Panel Antennas

Flat Panel Array

 

These antennas come in a wide variety of shapes. The radiating elements are often a rhombus [H-panel] or delta [butterfly] shape. Each panel is comprised of two dipoles or a variation, with a wideband characteristic capable of carrying two simultaneous channels.

6.3.3  Circularly Polarized Antennas

One way to create a circularly polarized signal is to feed crossed dipoles in quadrature.

6.3.4  Helical Antennas

This antenna consists of two helixes wrapped in opposite directions around a central support. In this arrangement, the vertical components from each antenna tend to cancel out, but the horizontal ones reinforce.

6.3.5  Slot Antenna[4]

The traveling wave or slot antennas consist of an inner and outer conductor where pairs of slots are cut in the outer conductor. These are spaced at 1/4 wavelength intervals. Probes are placed from the slot center inward. These distort the internal fields and induce voltages across the slot forcing it to act as a radiator.

               

John Hancock Center, Chicago[5]

Mt. Sutro, San Francisco[6]

World Trade Center, New York[7]

Co-Sited FM Transmission Systems by Harris

 

Assignment Questions

 

Quick Quiz

1.  The impedance of a 1/4 l Marconi antenna is [36.6, 73, 292] Ω.

2.  The [Hertzian, Marconi] antenna requires a path to ground.

3.  The impedance of a vacuum to an electromagnetic field is [0, 377, ∞] Ω.

4.  Antenna reflectors are short parasitic elements. [True, False]

5.  Antenna radiation patterns can be enhanced by adding several [reflectors, directors] along the antenna axis.

6.  Hertzian antennas [can, cannot] be arranged to form an array.

7.  The optical horizon is farther than the radio horizon. [True, False]

8.  A broadside array can be turned into an end-fire array by adjusting the phase shift between elements. [True, False]

9.  Array factor is the increase in field strength in an array when compared to an isotropic source. [True, False]

10. A turnstile antenna has a nearly circular radiation pattern. [True, False]

11. A helical antenna produces [vertically, horizontally, circularly] polarized waves.

12. The [impedance, element spacing] is a function of the log of the frequency in a log periodic antenna.

13. A [Marconi, Hertzian] antenna often needs a counterpoise.

Analytical Questions

1.  Assuming that the ideal matching impedance is 50 W , plot the position of the following impedances on a Smith Chart, and determine the reflection coefficient and VSWR:

a)  75 W

b)  40 + 20 j  W

c)  20 - 60 j  W

Composition Questions

1.  Suggest two applications for the following types of antennas:

a)  curtain array

b)  phase array

c)  slot antennas

 

4.  Sketch the following for a half wave dipole antenna:

a)    radiation pattern

b)    voltage distribution along the antenna

c)    current distribution along the antenna

d)    impedance distribution along the antenna

e)    show how the directivity can be improved by the use of parasitic elements

For Further Research

 

http://www.lingoinc.com/antenna.htm

http://www.jampro.com/jtvidx.html

HTTP://www-ncce.ceg.uiuc.edu/~singh/javahtml/Antenna/ant.html

http://wkweb4.cableinet.co.uk/simon.collings/Propagation/Propagation.htm

http://www.webproforum.com/arraycomm/

http://www.ac6v.com/pageant.html

http://www.borg.com/~warrend/guru.html

http://www.ac6v.com/pageans.html

http://www.bsoftware.com/v2/a19c72p2.htm

http://news.wirelessdesignonline.com/content/hubs/dir.asp?hub=news

 

Packet Radio

http://www.packetradio.com/

 



       Right Hand Circular

       Left Hand Circular

[1]       Antenna Engineering Handbook, Johnson & Jasik

[2]       Antenna Theory & Design, Stutzmann & Thiele, eq. 5-6

[3]         Scientific American, February 1985

[4]       Television Engineering Handbook, K. Blair Benson, 1986, FIG 8-4

[5]       Television Engineering Handbook, K. Blair Benson, 1986, FIG 8-2

[6]       Television Engineering Handbook, K. Blair Benson, 1986, FIG 8-3

[7]       Television Engineering Handbook, K. Blair Benson, 1986, FIG 8-1