APPLICATIONNOTE23 [ETC]

MICRF001 Antenna Design Tutorial ; MICRF001天线设计教程\n


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描述：	MICRF001 Antenna Design Tutorial MICRF001天线设计教程\n
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Application Note 23

Micrel

Application Note 23

MICRF001 Antenna Design Tutorial

by Tom Yestrebsky

using some basic terms. The problem is further simplified

because antenna systems in these applications are usually

connected directly to the transmitting and receiving units.

Introduction

Every wireless system is composed of the following five

components:

It has been determined that the best overall antenna for such

applications is simply a “piece of wire”. Certainly no antenna

is less expensive, especially when the “wire” is built into the

electronic circuit board. It only remains then to choose the

form factor of this “wire.” By this we mean whether the wire is

straight, coil, or a single loop. In many instances even the

form factor is dictated by product packaging constraints. For

example, when the package must be very small and com-

pletely enclosed, a coil or loop will be the preferred choice,

assuming the range constraint can also be met.

• Data encoder

• Baseband-to-RF transducer

• Antenna system

• RF-to-baseband transducer

• Data decoder

This is illustrated in the block diagram of Figure 1.

The MICRF001 UHF receiver IC, developed by Micrel, pro-

vides a low-cost solution for the RF-to-baseband transducer

in Figure 1, for applications in the 300MHz to 440MHz

frequencyband.Integratedanddiscretesolutionsalsoreadily

existforthedataencoder/decoderfunctionsandforthebase-

band-to-RF transducer (commonly called the transmitter).

The MICRF001 UHF receiver is designed to be connected

directly to the antennas described above and achieve range

performance adequate for most applications. Other high-

performance antennas exist, but cost constraints prohibit

theirconsiderationinallbutthehighest-performanceapplica-

tions. This application note will only discuss relative perfor-

mance characteristics of the three most popular antennas—

straight wire (monopole), (helical) coil, and loop—in the

context of what is generally important to the user (range

performance, size, and ease of design). For a more thorough

treatmentofthetheory, consultoneormoreofthereferences

in the bibliography.

Undeniably, of all the elements in Figure 1, the antenna

system is the most difficult to design and optimize. There are

several reasons for this. First, many designers lack sufficient

working experience with antennas to gain an intuitive feel,

especiallyinlow-power,low-costapplications.Antennamea-

surement and characterization requires sophisticated and

expensive test equipment, which may not be readily avail-

able. Also, antenna analysis often relies on simplifying as-

sumptions, which may not hold in all cases, and often leads

to measurement inconsistency.

The intent of this application note is to provide the user with

sufficient guidance to develop an antenna system for the

MICRF001—simply, quickly, and with a reasonable degree

of performance—especially for inexperienced users. If after

applying the concepts discussed here, rage performance still

is not adequate, further antenna optimization may be at-

tempted; however, one should not expect significant range

improvements to come from these further efforts. Antenna

systemoptimizationiscloselylinkedtothe“lawofdiminishing

returns.” This simply means that one can derive most of the

optimum antenna performance with a modest amount of

effort, and some simple guidelines. Beyond this point, incre-

mental improvements become increasingly costly, and yield

only marginal range benefit.

Reading this application note will not make one an antenna

expert. Antenna design and optimization is too complex and

driven by variables which are often beyond the designer’s

control. To add insult to injury, the entire problem is further

complicated if the antenna is located remotely from the

receiver through a transmission line. In these cases imped-

ance matching networks may need to be designed.

Fortunately, the problem of selecting an appropriate antenna

is not as overwhelming as it seems. Most low power remote-

control wireless applications are sensitive not only to range,

but to cost and packaging constraints as well. And the most

appropriate antennas for these applications are fairly simple

structures. They can be easily characterized and compared

Antenna System

Data

Encoder

Baseband-to-RF

Transducer

RF-to-Baseband

Transducer

Data

Decoder

Data

Out

Figure 1. Wireless Communication System—Simplified Block Diagram

QwikRadio is a trademark of Micrel, Inc. The QwikRadio ICs were developed under a partnership agreement with AIT of Orlando, Florida

Micrel, Inc. • 1849 Fortune Drive • San Jose, CA 95131 • USA • tel + 1 (408) 944-0800 • fax + 1 (408) 944-0970 • http://www.micrel.com

July 1999

Application Note 23

Micrel

Perhaps a better approach, where significant further range

improvement is needed, is to consider other more efficient

antenna types, assuming all other constraints (for example,

packaging) can be met. Discussion of such other solutions is

beyond the scope of this application note.

lobed in Figure 2c. Notice also that the radiation pattern in

Figure 2b is more highly directive than that of Figure 2a.

Directivity is anther characteristic of antennas, which the

reader may investigate further through the references.

90°

20°

60°

Each section of this application note is self-contained, with

significant passages italicized. This should help the reader to

quickly identify and digest the most important passages in

each section without getting bogged down in unwanted

detail.

150°

30°

X-Axis

180°

0°

Antenna Characteristics

Before discussing individual antenna types, it may help the

reader to understand basic characteristics common to all

antennas. However, this section is not required reading for

anyone who simply wants to quickly select and apply an

antenna to the MICRF001. Those individuals should read

“Comparison of Antenna Types” describing the desired an-

tenna.

210°

330°

240°

300°

270°

Figure 2a. Half-Wave (¹⁄2λ) Dipole Radiation Pattern

Reciprocity Theorem of Antennas

90°

20°

60°

The “reciprocal nature of antennas” means that the electro-

magneticcharacteristicsofatransmitantennaareequivalent

to those of a receive antenna, assuming the antennas are

identical in form-factor and orientation. A more general theo-

rem known as the “reciprocity theorem of antennas” is as

150°

30°

follows : If a voltage is applied to the terminals of antenna A,

180°

0°

and the current is measured at the terminals of another

antenna B, then an equal current (in both amplitude and

phase) will be obtained at the terminals of antenna A if the

same voltage is applied to the terminals of antenna B. This

simply means that any antenna can function equally as

well as a transmit antenna or receive antenna.

210°

330°

240°

300°

270°

Radiation Pattern and Orientation Effects

Figure 2b. Full-Wave (1λ) Dipole Radiation Pattern

Every antenna exhibits its own unique energy profile in the 3-

dimensional space around the antenna. This 3-dimensional

energyprofileiscalledtheantenna’sradiationpattern. These

patterns are derived theoretically, assuming a uniform, sinu-

soidal current distribution in the antenna, and that the an-

tenna is located in free-space away from other objects and

ground, unless otherwise stated. The real radiation pattern

will then vary from the theoretical pattern as these assump-

tions break down.

90°

20°

60°

Lobes

30°

150°

180°

0°

As an example, the radiation patterns for three different wave

lengths of linear dipole antenna are illustrated in Figures 2a–

2c. The angle of view in Figure 2a–2c. is from the side of a

vertically oriented straight wire.

Null

210°

330°

Peak

240°

300°

Thepatternsindicaterelativeresponseintensityasafunction

of(polar)angleintheX-Yaxis(the“planeofthepaper” X-axis

oriented horizontally). Since these are only 2-dimensional

figures, the intensity in the Z-direction (the direction “coming

out of the paper” when the X-axis is oriented horizontally) is

not shown. It should be understood that the field pattern

wraps around the antenna in the X-Z plane to form a torus

pattern.

270°

Figure 2c. 1¹⁄2λ Dipole Radiation Pattern

Thisexamplealsodemonstratesanantennaradiationpattern’s

dependence on length. Dipole antenna pattern is fundamen-

tally determined by antenna length, although this is not true

for all antenna types. The multilobe response in Figure 2c

comes about from the fact that the antenna is longer than 1

wavelength of the operating frequency, which elicits addi-

tional constructive and destructive interference of the energy

emanating from the antenna in 3-dimensional space. One

These patterns are made up of lobes. Peaks are simply lobe

maximums,andnullsaresimplylobeminimums.InFigures 2a

and 2b, only a single lobe exists, while the pattern is multi-

Application Note 23

July 1999

Application Note 23

Micrel

further observation is that, for the dipole antenna, no energy

emanates from the ends of the antenna.

antenna through the concept of gain. A half-wave dipole

antenna is commonly used. Another common reference

antenna is called an isotropic radiator. This is an idealized,

lossless antenna that radiates equally well in all directions.

These two antennas are described analytically in Reference

X-Axis

Data for Radio Engineers , Chapter 27. Antenna gain is then

defined as:

Dipole

Antenna

max. radiation intensity

Y-Axis

Evaluation Antenna

gain =

max. radiation intensity

Reference Antenna

provided the input power is the same for the reference

antenna and the antenna under evaluation.

Z-Axis

90°

Figure 2d. Typical Dipole Antenna

20°

60°

¹⁄2λ dipole

30°

It should be obvious that two antennas (one transmitting, and

theotherreceiving),whoseorientationsaresuchthatthelobe

maximumsfaceoneanother, areoptimallyaligned. Thusone

would not normally choose to orient a transmit antenna

verticallyandreceiveantennahorizontallyinthesameplane,

since the receive antenna would only pick up a small amount

of the energy delivered into the 3-dimensional space around

the transmit antenna. This is illustrated in Figure 3a. How-

ever, one could simply turn the receive antenna so that both

antennasareorientedinthesame(vertical)direction,andthe

antenna would be optimally aligned. This is illustrated in

Figure 3b.

150°

1λ dipole

180°

0°

Peak

210°

330°

240°

300°

270°

Figure 4. Antenna Gain and Directivity

Antenna Polarization

Antenna polarization is a characterization of the directional

behavior of the electric vector of the electromagnetic (EM)

wave emanating from the antenna. Figures 5a–5d illustrates

three types of polarization: linear, elliptic, and circular. These

names refer to the figure (line, circle, or ellipse) traced out by

the tip of the electric vector as it travels through space. Linear

polarization further breaks down into horizontal and vertical

polarization, depending on whether the antenna is oriented

horizontally or vertically. Polarization characteristics vary

with antenna type. For example, linear antennas like mono-

poles, exhibit linear polarization, while helical antennas are

fundamentally circularly polarized. Ideally, transmit and

receive antennas should exhibit compatible polarization

for optimum performance. However, as with orientation,

this may not always be possible due to other system or

packaging constraints. Once again, the designer should

try to mitigate this problem as much as possible, but

expect range variations to occur.

Figure 3a. Misaligned Antenna Radiation Patterns

Figure 3b. Fully-Aligned Antenna Radiation Patterns

Antenna radiation pattern misalignment is a problem

that exists in just about every system application. These

orientationeffectsmanifestthemselvesassystemrange

variations and are usually best understood through

experimentation. Many times, the user does not have the

luxurytooptimizeantennaorientation, duetopackaging

constraints, for example. The system designer should

try to improve the orientation characteristics as much as

possible, but expect application-dependent range varia-

tions to occur.

X-Axis

Antenna Gain

For the sake of completeness we shall define antenna gain.

Theconceptisnot,strictlyspeaking,soimportant,butdefines

antenna radiation performance relative to a reference an-

tenna.

Y-Axis

The reference antenna may be any antenna type arbitrarily

chosen by the user. Performance of the antenna under

consideration can then be compared with the reference

Z-Axis

Figure 5a. Linear (Vertical) Polarization

Application Note 23

July 1999

Application Note 23

Micrel

X-Axis

This implies that antenna size should be maximized to the

extentpossible.Antennasizeisgenerallynotsoimportantfor

the transmitters in these low-power applications, since regu-

latory agencies usually limit the allowable effective radiated

power or field strength. It is assumed the signal current could

be increased, no matter what the radiation resistance (that is,

increase current to offset antenna inefficiency). However,

due to the reciprocity theorem of antennas, higher radiation

resistanceisdesirableatthereceiveantennasinceefficiency

is important there, so the system designer should maximize

this parameter to the extent possible at the receiver.

Y-Axis

Z-Axis

Figure 5b. Linear (Horizontal) Polarization

Antenna (Terminal) Impedance

X-Axis

The impedance looking into the terminals of an antenna is

usually only important for signal power matching into a

transmission line (see “Impedance Matching”). Terminal im-

pedance is generally composed of a (real) resistance term

plus a reactive term. For an antenna whose radiative losses

are much greater than its resistive losses, the resistive term

is called the antenna’s radiation resistance, previously de-

scribed.

Y-Axix

If the antenna is small and placed close to the input pin of the

MICRF001, asismostoftenthecase, theentirestructurecan

be treated as a lumped, rather than distributed, circuit. In this

case, impedance matching the antenna to the input of the IC

will yield little improvement in range.

Z-Axis

Figure 5c. Eliptical Polarization

X-Axis

IftheantennaislocatedawayfromtheIC,theantennashould

be coupled to the IC via a transmission line. In this case, the

antennaimpedancemustbeknown,sothatitcanbematched

into the characteristic impedance of the transmission line.

This requires a matching circuit at the antenna-transmission

line interface. A similar circuit is necessary to match the

transmission line to the input of the MICRF001. These

additional matching networks are only required when the

antenna is located away from the input pin of the IC. This

subject is further discussed in “Impedance Matching”.

Y-Axis

Z-Axis

Figure 5d. Circular Polarization

Antenna Radiation Resistance

Antenna Resonance and Tuning

An antenna is defined as resonant if its terminal impedance

isequaltoitsradiationresistance. Thisisequivalenttosaying

that the terminal impedance contains no reactive impedance

component. Since the antenna impedance equals the radia-

tion resistance at resonance, it can be said that the antenna

is operating at maximum radiating (or receiving) efficiency.

An antenna’s radiation resistance is a measure of its ability to

radiate an applied signal into space, or to receive a signal

from space. To calculate the radiation resistance, the an-

tenna is assumed to be lossless. Then, for a given applied

signal, thetotalradiatedpower(P)iscalculatedormeasured,

along with the current (I) in the antenna. Using the equation

An antenna may be “tuned to resonance” at a given

frequency by incrementally adjusting the length or form

factor of the antenna structure. The antenna will be

detuned by placing it in the vicinity of other metallic

objects (which introduces parasitic capacitance to the

antenna). The antenna’s radiation pattern will also be

modified by proximity to such objects. When tuning and

measuring an antenna system, it is important that the

antenna be in its normally deployed state to account for

these parasitics. Otherwise, avoid placing the antenna

close to other metallic components.

P = I ×R

where:

P = total radiated power (W)

I = rms antenna current (A)

R = antenna radiation resistance (Ω)

we associate the radiated power with a radiation “resistance”

R . The radiation resistance is not a real (dissipative) resis-

tance,butameasureofthepowerradiatedintofree-spacefor

a given input current. The important observation about Ra-

diation resistance is that, for a given current into the antenna,

as radiation resistance increases, so does the antenna’s

efficiency. It will be established later that, in general, larger

antennas are more effective “signal collectors,” and also

exhibit higher radiation resistance than smaller antennas.

Antenna Bandwidth

As one might expect, an antenna’s characteristics are valid

over only a finite bandwidth. For narrow-band transmitters,

commonlyusedwiththeMICRF001, bandwidthofcommonly

used antennas is not an issue. Instances where bandwidth

Application Note 23

July 1999

Application Note 23

Micrel

might be important are where the MICRF001 is used to

receive one of several channelized frequencies and the

frequencies are spaced widely. It is difficult to quantize

bandwidth, sincetheamountthattheantennacharacteristics

can vary from resonance, is application dependent.

antenna type. This section contains rule-of-thumb informa-

tionwhichappliesgenerally.However,relativeperformances

can be modulated by such variables as antenna length,

orientation, and location to ground plane or parasitics. For

this comparison, the monopole is assumed to be a quarter-

wavelength long.

Ground-Plane Effect on Antenna Performance

Parameter

Loop

Helical

Monopole

The presence or absence of a ground plane and the need

for a ground plane with an antenna is commonly misun-

derstood. Unless otherwise stated, antenna characteris-

tics are generally derived by assuming the antenna to be

infree-space,withoutanygroundplane.(Therareexcep-

tion to this is the monopole, as introduction of a perfect

ground plan allows the monopole to be easily resolved

to, and analyzed as, a dipole.) In the absence of a ground

plane, the most important characteristics, antenna pat-

tern and terminal impedance, can be determined. When

agroundplaneisbroughtintothevicinityoftheantenna,

these characteristics can be altered, in a manner that

may or may not improve system (range) performance

Design Simplicity

Range

Size

Parasitic Immunity

Overall Performance

Key: 1 = best relative performance

3 = worst relative performance

Table 1. Antenna Performance Summary

Monopole antennas are physically larger structures in-

tended for applications which demand the best range.

Monopole antennas are also by far the easiest antennas

to design and apply. Monopoles can be a single straight

wireprotrudingfromPCB(theprintedcircuitboard)ormaybe

a (metal) trace built into the PCB (which can lower costs by

removing another assembly step). Often, straight wire mono-

pole antennas protrude from the housing assembly, simply

due to their size (for example, a 315MHz quarter-wave

monopole is 8.9 inches long). Inductively loaded monopoles

are available which provide similar performance in a smaller

length, but at higher cost than a simple piece of wire. Range

of monopole antennas is generally up to 100 meters when

used with micropower OOK (on-off keyed) transmitters.

Antennas in the presence of a ground plane are generally

analyzed by the method of images. This approach removes

the ground from the analysis, and places an image antenna

in space at the appropriate dimensions to mimic the signal

reflection associated with the ground plane. The image is not

a real antenna at all, but simply a mathematical construct to

account for the ground plane signal reflection.

One often sees it stated that the antenna must be located

above a “good” ground plane. “Good” usually refers here to

a ground plane that is sufficiently large and conductive to

allow prediction of the antenna’s characteristics with only a

small error to a (theoretical) infinite, perfectly conducting

plane. This is not strictly necessary. Even without a good

ground plane, the antenna will still radiate, but with a pattern

and impedance different than if the antenna were above a

good ground plane. The best way to think of the ground plane

is as an energy reflector from the antenna itself, which,

depending on the distance from ground plane to antenna,

sets up constructive and destructive interference of signal in

space which alters the antenna pattern. The terminal imped-

ance is altered due to the parasitic capacitance from antenna

to ground plane. A good description of all this may be found

Small helical antennas are a good compromise, espe-

cially where small size is important. The resulting as-

semblygenerallycanbecompletelyenclosed, andmade

quite compact. Helical antennas are more difficult to set

up and optimize than monopoles since the antenna’s

characteristics are strongly influenced by coil diameter

and compactness of turns along the axial dimension.

Further, small helical antennas are used in what is commonly

called the radial mode of emission, which is not treated in the

literature as thoroughly as axial mode operation of large

in Antennas , Sections 11.7 and 11.8.

helical antennas . Range is generally up to 60 meters when

used with micropower OOK transmitters.

For applications where one has the luxury to use or not

use a ground plane, the choice is not particularly clear.

If,byusingagroundplane,themodifiedantennapattern,

directionality, and terminal impedance yields the best

system performance, then it should be used. Otherwise

it should not. For applications where a ground plane

must exist, or where no good ground plane can be

allowed, the antenna should be optimized for that par-

ticular condition. Finally, there is no reason an adequate

antenna cannot be constructed, even if there is no good

ground plane to work against.

Loop antennas provide the poorest range of the three

antennasunderconsideration, generallyupto30meters

when used with micropower OOK transmitters. Size is

not particularly attractive, but is smaller than a quarter-

wave monopole. Loop antennas can be rugged and low

cost when the antenna is completely integrated into the

PCB. An alternative consideration is to use a less-than-

quarter-wave monopole built into the PCB rather than a loop

antenna. Such an antenna might provide the advantages of

a loop (ruggedness, cost) while providing better range.

It is convenient to think of the helical antenna as the general

structure, and that the monopole and loop antennas are

simply degenerate forms of the helical. For example, com-

pletely stretching out the helical antenna yields a monopole,

and compressing a helical antenna inwards yields a loop

Antenna Types

It is beneficial for users to appreciate how the three antenna

types (monopole, helical, loop) compare in general terms

before getting too involved in the theory surrounding each

July 1999

Application Note 23

Micrel

antenna. So it is not unexpected that the helical performance

is generally between the two extremes of monopole and loop

antenna.

To design a monopole antenna, simply calculate the

appropriate length, cut a wire, and attach directly to the

ANT (antenna) pin of the MICRF001. That’s all there is to

it.

Monopole Antennas

For example, the appropriate length for a quarter-wave

monopole at 433.92MHz would be 2808 ÷ 433.92 = 6.47

inches. Sophisticated antenna measurements are generally

not necessary unless a highly optimized design is desired.

This makes the monopole very popular and easy to apply.

Monopole antennas are commonly used in applications with

the MICRF001 where range is important. These antennas

are also very easy to design and tune simply by slight

changes in length. It is assumed the antenna is a quarter-

wavelengthlong,whichistypicalofmonopoleantennasinthe

UHF band.

Helical Antennas

90°

Ahelical(coil)antennaisshowninFigure8. Helicalantennas

may be constructed from copper, steel, or brass; from an

electronic component standpoint, it is simply an inductor.

Compared to the monopole, which is essentially a two-

dimensional structure, the helical antenna is a 3-dimensional

structure. As stated earlier, a monopole can be thought of as

a “stretched-out” helical antenna. Helicals are difficult to

analyze because of their 3-dimensional nature, and are

usually empirically optimized.

20°

60°

150°

30°

180°

0°

210°

330°

240°

300°

270°

Drive Point

(connect to ANT input)

Figure 6. 0.25λ Monopole Over Ground Plane

Figure 6 illustrates the radiation pattern of a quarter-wave-

length monopole above a ground plane. The radiation is

linearly polarized, either horizontally or vertically, depending

on antenna orientation. Radiation resistance of a quarter-

wave monopole is approximately 37Ω, and does not vary

Figure 8. Helical Antenna

Helical antennas are characterized as either small helicals,

which operate in normal mode, or large helicals, which

operate in axial mode. By axial or normal, we convey the

direction of the radiation pattern: axial being along the axis of

the helix, and normal being at right angles to the helix axis. A

helical antenna is small if its diameter and length are both

much smaller than one wavelength. Helical antennas used

with the MICRF001 are almost exclusively small helicals,

with a normal radiation pattern.

much with presence or absence of ground plane . Figure 7

indicates that the radiation resistance of monopole antennas

is length dependent. Resonance of a quarter-wavelength

monopole occurs when its length is slightly less than a

quarter-wavelength.

150

120

Figure 9 illustrates the radiation pattern of the small helix. We

observe the radiation pattern is similar in nature to the

monopole, and is also fairly insensitive to dimensional

changes, provided such changes are much smaller than a

wavelength.

90°

20°

60°

150°

30°

0.2

0.4

0.6

0.8

1.0

ANTENNA LENGTH ( )

Figure 7. Radiation Resistance of

Monopole Over Ground Plane

180°

0°

Length of a resonant quarter-wavelength monopole antenna

made of wire may be calculated from the following equation

which takes into account the slight shortening for resonance:

210°

330°

2808

length =

240°

300°

270°

frequency

Figure 9. Helix Radiation Pattern

where:

length = inches

frequency = MHz

Application Note 23

July 1999

Application Note 23

Micrel

Terminal impedance of the helical antenna is far less well

characterized, simply because the impedance depends on

numerous parameters: coil diameter, coil loop pitch, coil

length (or number of turns), and frequency. Variations in any

of these parameters can “detune” the antenna away from

resonance. For this reason the helical antenna is considered

to be more narrow-band than the monopole. As a result,

designing and optimizing helical antennas is usually

done empirically. But even with this shortcoming, the

helical is very popular, since it provides reasonable

range and very small size.

the loop antenna is fundamentally circular. Finally, when the

loopareaA<0.01λ ,squareandcircularloopscanbetreated

identically as long as the areas of the two loops are the same.

Thismeansthatforsmallloopsitisnotatallimportantthatthe

loop be circular, but it can be any closed loop structure.

Loop antennas find applications mostly at the transmit-

ter, especially where ruggedness, size, and ease of

construction are required.

A good application for the loop antenna is the push-

buttontransmitterwhichattachestoakeychain, forRKE

(remote keyless entry) applications. Such designs must

be rugged, cheap, very small, and fully integrated. Fur-

ther, the typical packaging is elliptical or circular in

nature,allowingaloopantennatobeconstructedaround

the periphery of the assembly with little additional im-

pact to PCB space.

Radiation from small helical antennas is fundamentally ellip-

tically polarized. A good discussion on the design of helical

antennas and coils is given in Reference Data for Radio

Engineers ,Chapter27,pages27-11through27-13andThe

Design of Impedance Matching Networks… , Section 2.3.6.

Helical antennas are commonly found on LC (inductor-

capacitor)transmitters, wheretheL(helicalcoil)isbothapart

of the resonant network and the antenna—a very inexpen-

sive solution.

To construct a loop antenna, make the loop as large as

possible, then simply “tune” the antenna to resonance

with a parallel capacitor. Typical values are 1pF to 5pF in

the UHF band, and the capacitor may be fixed or variable

depending on the application.

Unfortunately, no simple expression exists for the de-

sign of a helical antenna, like exists in the previous

section for the monopole. It is possible to calculate the

length of a (resonant) helical once its diameter, coil

spacing, and material type are known. In most cases,

however,itisjustaseasytoarriveatadesignempirically

by taking an overly long coil, and tuning it by clipping

away pieces until the antenna is resonant at the desired

frequency. Strictly speaking, this will require a piece of

specialized test equipment, such as a network analyzer.

Otherwise, trim the structure for maximum range.

1000

100

0.1

0.01

0.001

0.0001

0.00001

0.2

0.4

0.6

0.8

1.0

PCB Loop Antennas

ANTENNA CIRCUMFERENCE ( )

Loop antennas are perhaps the least used antenna at the

receiver. These antennas have very low radiation resis-

tances and must be relatively large to be efficient signal

collectors, an important attribute at the receiver. Figures 10a

and 10b illustrate the radiation pattern and radiation resis-

tance of the loop antenna, respectively. Radiation resistance

Figure 10b. Loop Radiation Resistance

vs. Loop Circumference

Impedance Matching

Where electrically small antennas (that is, physical dimen-

sions significantly less than 1 wavelength) are connected

directly to the MICRF001 ANT pin, the structure can be

treated as a lumped circuit. This is because the phase across

the antenna is negligible. In such instances, impedance

matchingtheantennatotheICwillnotimprovesystemrange.

In applications where the antenna and IC are collocated,

impedance matching is not required.

is given as a function of C , the loop circumference in

wavelengths. Even for C = 0.5 wavelengths, the radiation

resistance is under 10Ω.

90°

20°

60°

150°

30°

In applications where the antenna is located away from the

IC, they must be interconnected using a transmission line. A

transmission line is simply a way of conveying a signal

between two points without distortion or loss, as the line

180°

0°

provides constant incremental impedance . For the trans-

mission line to function properly, the antenna impedance

must be “matched” into the transmission line impedance at

one end and the transmission line “matched” to the IC

impedance at the other end. A commonly used type of

transmission line is coaxial cable which is available in a

number of standard impedance values.

210°

330°

240°

300°

270°

Figure 10a. Loop Radiation Pattern

The concept of transmission line matching is too extensive to

be covered in detail in this section. Impedance matching is

Theradiationpatternforasmallloopissimilartothoseofboth

the small helical and quarter-wave monopole. Polarization of

July 1999

Application Note 23

Micrel

generally regarded as an RF engineering problem, and there

are entire textbooks devoted to the subject.

Antenna Testing and Measurement

An antenna’s theoretical and measured characteristics can

vary widely, due to factors such as ground plane, antenna

orientation, form-factor changes, and proximity to other ob-

jects in the product assembly. Further modifications arise

from objects at the installation sites, and elicit multipath

fading, for which little can usually be done. In many cases,

designers of MICRF001-like applications just empirically

optimize their antenna systems. If this is not adequate, more

thorough methods do exist to measure an antenna’s charac-

teristics. Such methods are too extensive to be completely

covered here, and can be found in numerous references, for

example, Antennas, Chapter 15. Unfortunately, such mea-

surements require an RF expertise and more sophisticated

test equipment. An alternative, if cost permits, is to “contract

out” such antenna characterization work. This will greatly

improve the chances that the work will get done right the first

time.

Users who require the antenna to be remote from the IC,

and don’t already posses impedance matching exper-

tise, should seek outside guidance. Several references

4,5

for constructing matching networks

are provided in

the bibliography. If it is at all possible, Micrel recom-

mends that the antenna be attached directly to the IC to

avoid impedance matching issues.

Multipath Fading

Multipath fading is a form of signal fading caused by signals

arriving at he receive antenna with differing phases. This

results because signals from the transmitter may follow

different paths in traveling to the receiver. Portions of the

original signal may travel in a direct path, while others may

arrive at the receiver by reflecting off ground or other objects

inthelocale.Thesedifferencesinphaseresultinconstructive

and destructive interference at the receiving antenna, which

affects the amplitude of the signal developed at the antenna.

While a solution exists for this problem (called diversity

switchingwithmultipleantennas),itisusuallycost-prohibitive

for MICRF001 applications.

Bibliography

1. Kraus, J. D., Antennas, McGraw-Hill Co., 1950.

ISBN 07-035410-3

2. Reference Data for Radio Engineers, 6th ed., compiled

by ITT (International Telephone and Telegraph),

Howard W. Sams and Co. Publishers, 1968.

Library of Congress No. 75-28960

Antennatestingisusuallyperformedinanopenfieldasaway

ofkeepingmultipathfadingfromcorruptingthemeasurement

process. Multipath fading effects are not related to the an-

tenna, but to the local environment. While there is little one

can do to mitigate the problem, it is important that the user

understand that multipath fading will cause system range

variations from site to site.

3. Jasik, H., Antenna Engineering Handbook, McGraw-Hill

Co., 1961

4. Caron, W. N., Antenna Impedance Matching, ARRL

Press, 1989.

ISBN 0-87259-220-0.

5. Abrie, P. L. D., The Design of Impedance-Matching

Networks for Radio-Frequency and Microwave Amplifi-

ers, Artech House, Inc., 1985.

ISBN 0-89006-172-6.

MICREL INC. 1849 FORTUNE DRIVE SAN JOSE, CA 95131 USA

TEL + 1 (408) 944-0800 FAX + 1 (408) 944-0970 WEB http://www.micrel.com

This information is believed to be accurate and reliable, however no responsibility is assumed by Micrel for its use nor for any infringement of patents or

other rights of third parties resulting from its use. No license is granted by implication or otherwise under any patent or patent right of Micrel Inc.

Application Note 23

July 1999

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