Application Note 23
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.
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 completely enclosed, a coil or loop will be the preferred choice,
assuming the range constraint can also be met.
Every wireless system is composed of the following five
• 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, provides a low-cost solution for the RF-to-baseband transducer
in Figure 1, for applications in the 300MHz to 440MHz
frequency band. Integrated and discrete solutions also readily
exist for the data encoder/decoder functions and for the baseband-to-RF transducer (commonly called the transmitter).
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,
especially in low-power, low-cost applications. Antenna measurement and characterization requires sophisticated and
expensive test equipment, which may not be readily available. Also, antenna analysis often relies on simplifying assumptions, which may not hold in all cases, and often leads
to measurement inconsistency.
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 impedance 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 remotecontrol 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
The MICRF001 UHF receiver is designed to be connected
directly to the antennas described above and achieve range
performance adequate for most applications. Other highperformance antennas exist, but cost constraints prohibit
their consideration in all but the highest-performance applications. This application note will only discuss relative performance 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
treatment of the theory, consult one or more of the references
in the bibliography.
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 attempted; however, one should not expect significant range
improvements to come from these further efforts. Antenna
system optimization is closely linked to the “law of diminishing
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, incremental improvements become increasingly costly, and yield
only marginal range benefit.
Antenna System
Figure 1. Wireless Communication System—Simplified Block Diagram
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July 1999
Application Note 23
Application Note 23
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.
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
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.
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 antenna.
Reciprocity Theorem of Antennas
The “reciprocal nature of antennas” means that the electromagnetic characteristics of a transmit antenna are equivalent
to those of a receive antenna, assuming the antennas are
identical in form-factor and orientation. A more general theorem known as the “reciprocity theorem of antennas” is as
follows1: If a voltage is applied to the terminals of antenna A,
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.
Radiation Pattern and Orientation Effects
Every antenna exhibits its own unique energy profile in the 3dimensional space around the antenna. This 3-dimensional
energy profile is called the antenna’s radiation pattern. These
patterns are derived theoretically, assuming a uniform, sinusoidal current distribution in the antenna, and that the antenna 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 assumptions break down.
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.
The patterns indicate relative response intensity as a function
of (polar) angle in the X-Y axis (the “plane of the paper” 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
These patterns are made up of lobes. Peaks are simply lobe
maximums, and nulls are simply lobe minimums. In Figures 2a
and 2b, only a single lobe exists, while the pattern is multi-
Antenna Characteristics
Application Note 23
Figure 2a. Half-Wave (1⁄2λ) Dipole Radiation Pattern
λ) Dipole Radiation Pattern
Figure 2b. Full-Wave (1λ
Figure 2c.
Dipole Radiation Pattern
This example also demonstrates an antenna radiation pattern’s
dependence on length. Dipole antenna pattern is fundamentally 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 additional constructive and destructive interference of the energy
emanating from the antenna in 3-dimensional space. One
July 1999
Application Note 23
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
Data for Radio Engineers 2, Chapter 27. Antenna gain is then
defined as:
gain =
max. radiation intensityEvaluation Antenna
max. radiation intensityReference Antenna
provided the input power is the same for the reference
antenna and the antenna under evaluation.
Figure 2d. Typical Dipole Antenna
It should be obvious that two antennas (one transmitting, and
the other receiving), whose orientations are such that the lobe
maximums face one another, are optimally aligned. Thus one
would not normally choose to orient a transmit antenna
vertically and receive antenna horizontally in the same plane,
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. However, one could simply turn the receive antenna so that both
antennas are oriented in the same (vertical) direction, and the
antenna would be optimally aligned. This is illustrated in
Figure 3b.
1λ dipole
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 monopoles, 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
orientation effects manifest themselves as system range
variations and are usually best understood through
experimentation. Many times, the user does not have the
luxury to optimize antenna orientation, due to packaging
constraints, for example. The system designer should
try to improve the orientation characteristics as much as
possible, but expect application-dependent range variations to occur.
Antenna Gain
For the sake of completeness we shall define antenna gain.
The concept is not, strictly speaking, so important, but defines
antenna radiation performance relative to a reference antenna.
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
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Figure 5a. Linear (Vertical) Polarization
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This implies that antenna size should be maximized to the
extent possible. Antenna size is generally not so important for
the transmitters in these low-power applications, since regulatory 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
resistance is desirable at the receive antenna since efficiency
is important there, so the system designer should maximize
this parameter to the extent possible at the receiver.
Antenna (Terminal) Impedance
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 impedance 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 described.
If the antenna is small and placed close to the input pin of the
MICRF001, as is most often the case, the entire structure can
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.
If the antenna is located away from the IC, the antenna should
be coupled to the IC via a transmission line. In this case, the
antenna impedance must be known, so that it can be matched
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”.
Antenna Resonance and Tuning
An antenna is defined as resonant if its terminal impedance
is equal to its radiation resistance. This is equivalent to saying
that the terminal impedance contains no reactive impedance
component. Since the antenna impedance equals the radiation resistance at resonance, it can be said that the antenna
is operating at maximum radiating (or receiving) efficiency.
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.
Antenna Bandwidth
As one might expect, an antenna’s characteristics are valid
over only a finite bandwidth. For narrow-band transmitters,
commonly used with the MICRF001, bandwidth of commonly
used antennas is not an issue. Instances where bandwidth
Figure 5b. Linear (Horizontal) Polarization
Figure 5c. Eliptical Polarization
Figure 5d. Circular Polarization
Antenna Radiation Resistance
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 antenna is assumed to be lossless. Then, for a given applied
signal, the total radiated power (P) is calculated or measured,
along with the current (I) in the antenna. Using the equation
P = I2 × R
P = total radiated power (W)
I = rms antenna current (A)
RR = antenna radiation resistance (Ω)
we associate the radiated power with a radiation “resistance”
RR. The radiation resistance is not a real (dissipative) resistance, but a measure of the power radiated into free-space for
a given input current. The important observation about Radiation 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.
Application Note 23
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Application Note 23
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, since the amount that the antenna characteristics
can vary from resonance, is application dependent.
Ground-Plane Effect on Antenna Performance
The presence or absence of a ground plane and the need
for a ground plane with an antenna is commonly misunderstood. Unless otherwise stated, antenna characteristics are generally derived by assuming the antenna to be
in free-space, without any ground plane. (The rare exception 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 pattern and terminal impedance, can be determined. When
a ground plane is brought into the vicinity of the antenna,
these characteristics can be altered, in a manner that
may or may not improve system (range) performance
antenna type. This section contains rule-of-thumb information which applies generally. However, relative performances
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 quarterwavelength long.
Design Simplicity
Parasitic Immunity
Overall Performance
Key: 1 = best relative performance
3 = worst relative performance
Table 1. Antenna Performance Summary
Monopole antennas are physically larger structures intended 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
wire protruding from PCB (the printed circuit board) or may be
a (metal) trace built into the PCB (which can lower costs by
removing another assembly step). Often, straight wire monopole 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.
Small helical antennas are a good compromise, especially where small size is important. The resulting assembly generally can be completely enclosed, and made
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
helical antennas1. Range is generally up to 60 meters when
used with micropower OOK transmitters.
Loop antennas provide the poorest range of the three
antennas under consideration, generally up to 30 meters
when used with micropower OOK transmitters. Size is
not particularly attractive, but is smaller than a quarterwave 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-thanquarter-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, completely stretching out the helical antenna yields a monopole,
and compressing a helical antenna inwards yields a loop
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 impedance is altered due to the parasitic capacitance from antenna
to ground plane. A good description of all this may be found
in Antennas1, Sections 11.7 and 11.8.
For applications where one has the luxury to use or not
use a ground plane, the choice is not particularly clear.
If, by using a ground plane, the modified antenna pattern,
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 particular condition. Finally, there is no reason an adequate
antenna cannot be constructed, even if there is no good
ground plane to work against.
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
Application Note 23
antenna. So it is not unexpected that the helical performance
is generally between the two extremes of monopole and loop
Monopole Antennas
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 quarterwavelength long, which is typical of monopole antennas in the
UHF band.
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
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.
Helical Antennas
A helical (coil) antenna is shown in Figure 8. Helical antennas
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 twodimensional 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.
Drive Point
(connect to ANT input)
Figure 6. 0.25λ
λ Monopole Over Ground Plane
Figure 6 illustrates the radiation pattern of a quarter-wavelength monopole above a ground plane. The radiation is
linearly polarized, either horizontally or vertically, depending
on antenna orientation. Radiation resistance of a quarterwave monopole is approximately 37Ω, and does not vary
much with presence or absence of ground plane3. 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
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.
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
Figure 7. Radiation Resistance of
Monopole Over Ground Plane
Figure 9. Helix Radiation Pattern
length = inches
frequency = MHz
Application Note 23
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:
length =
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Application Note 23
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
loop area A < 0.01λ2, square and circular loops can be treated
identically as long as the areas of the two loops are the same.
This means that for small loops it is not at all important that the
loop be circular, but it can be any closed loop structure.
Loop antennas find applications mostly at the transmitter, especially where ruggedness, size, and ease of
construction are required.
A good application for the loop antenna is the pushbutton transmitter which attaches to a key chain, for RKE
(remote keyless entry) applications. Such designs must
be rugged, cheap, very small, and fully integrated. Further, the typical packaging is elliptical or circular in
nature, allowing a loop antenna to be constructed around
the periphery of the assembly with little additional impact to PCB space.
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.
Radiation from small helical antennas is fundamentally elliptically polarized. A good discussion on the design of helical
antennas and coils is given in Reference Data for Radio
Engineers 2, Chapter 27, pages 27-11 through 27-13 and The
Design of Impedance Matching Networks…5, Section 2.3.6.
Helical antennas are commonly found on LC (inductorcapacitor) transmitters, where the L (helical coil) is both a part
of the resonant network and the antenna—a very inexpensive solution.
Unfortunately, no simple expression exists for the design 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, it is just as easy to arrive at a design empirically
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.
PCB Loop Antennas
Loop antennas are perhaps the least used antenna at the
receiver. These antennas have very low radiation resistances 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 resistance of the loop antenna, respectively. Radiation resistance
is given as a function of Cλ, the loop circumference in
wavelengths. Even for Cλ = 0.5 wavelengths, the radiation
resistance is under 10Ω.
Figure 10a. Loop Radiation Pattern
The radiation pattern for a small loop is similar to those of both
the small helical and quarter-wave monopole. Polarization of
July 1999
Where electrically small antennas (that is, physical dimensions 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
matching the antenna to the IC will not improve system range.
In applications where the antenna and IC are collocated,
impedance matching is not required.
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
provides constant incremental impedance4. For the transmission 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.
The concept of transmission line matching is too extensive to
be covered in detail in this section. Impedance matching is
Impedance Matching
Figure 10b. Loop Radiation Resistance
vs. Loop Circumference
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generally regarded as an RF engineering problem, and there
are entire textbooks devoted to the subject.
Users who require the antenna to be remote from the IC,
and don’t already posses impedance matching expertise, should seek outside guidance. Several references
for constructing matching networks 4,5 are provided in
the bibliography. If it is at all possible, Micrel recommends that the antenna be attached directly to the IC to
avoid impedance matching issues.
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 objects 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 characteristics. Such methods are too extensive to be completely
covered here, and can be found in numerous references, for
example, Antennas, Chapter 15. Unfortunately, such measurements 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
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
in the locale. These differences in phase result in constructive
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
switching with multiple antennas), it is usually cost-prohibitive
for MICRF001 applications.
Antenna testing is usually performed in an open field as a way
of keeping multipath fading from corrupting the measurement
process. Multipath fading effects are not related to the antenna, 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.
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
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 Amplifiers, Artech House, Inc., 1985.
ISBN 0-89006-172-6.
+ 1 (408) 944-0800
+ 1 (408) 944-0970
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.
© 1999 Micrel Incorporated
Application Note 23
July 1999