Range Considerations for RF Networks Richard Wallace TI Technology Days 2010
Range Considerations for RF Networks - Texas …e2e.ti.com/.../Range-Considerations-for-RF-Networks.pdf1111.1 523.8 The distance in meters that it takes for a wave to complete a 360
The antenna can be one of the most daunting components of wireless designs. Most information available relates to large antenna’s related to Amateur Radio (HAM), Cellular Applications and expensive Whip
Antennas that can never be used in a low cost application.
This session covers the basics that most engineers would need to
know to predict the expected Range Distance between wireless devices and design parameters to achieve an optimum Range Distance.
At the end of this session, the attendees should be aware of antenna requirements, the Antenna Documentation Support available and be
able to select an Antenna Reference Design for their application.
•
Antenna Theory
•
Antenna Measurements
•
Range Estimation–
Friis Basic Formula–
Improved Range Estimation–
Best Accuracy Estimation
•
Antenna Reference Designs–
2.4 GHz–
2.4 GHz & 868 MHz Dual Band–
868 / 915 / 955 MHz–
433 / 315 / 169 MHz–
CC-Antenna-DK
•
Antenna Support Documentation–
Antenna Selection Quick Guide (DN035)–
Comprehensive Antenna Selection Guide (AN058)
Agenda
•
Transmit mode:
Transform RF signals into electromagnetic waves, propagating into free space
•
Receive mode:
Transform electromagnetic waves into RF signals
TX
RX
Antenna Theory –
Basic Function of an Antenna
OBS: The antenna
is a key
component for the successful design of a wireless communication system.
Range
Presenter
Presentation Notes
Principle of the antenna Crucial component. Huge effect of the overall performance for an RF system. The purpose of an antenna: Transmit mode: Transform RF signals into electromagnetic waves, propagating into free space Receive mode: Transform electromagnetic waves into RF signals Regulatory issues will also be affected by the antenna since radiated testing is often required. Antennas are reciprocal and have therefore the same performance in RX as TX. See Application Note 058 (AN058) for more information about antenna theory, antenna measurements and antenna reference designs: http://www.ti.com/lit/swra161.
Antenna Theory –
All Monopole Antennas are Derivatives of Dipole
AC current through an inductor lags the voltage by 90 degrees
*1: /2 Dipole produces the most power at the ends of the antenna with little power at the feedline.
/4 /4
All monopole antennas are derivatives of a simple dipole where one ¼
wavelength radiator is in air and one ¼
wavelength radiator is imaged into the GND and serves as the second radiator.
CurrentVoltagePower
*1 *1
Presenter
Presentation Notes
Dipole Antennas A dipole antenna most commonly refers to a half-wavelength (λ/2) dipole. The antenna is constructed of conductive elements whose combined length is about half of a wavelength at its intended frequency of operation. This is a simple antenna that radiates its energy out toward the horizon (perpendicular to the antenna). The resulting 3D pattern looks like a donut with the antenna sitting in the hole and radiating energy outward. The strongest energy is radiated outward in the x-y plane, perpendicular to the antenna. Given these antenna patterns, you can see that a dipole antenna should be mounted so that it is vertically oriented with respect to the floor or ground. This results in the maximum amount of energy radiating out into the intended coverage area. The null in the middle of the pattern will point up and down. Power measurements for a theoretical isotropic antenna is in dBi. Monopole Dipole Antenna Power is related to a isotropic antenna by the relationship 0 dBd = 2.14dBi .
Antenna Theory –
Wavelength Calculations in Free Space
Wavelength is dependent on frequency which is referenced to the speed of light (299 792 458 m / s):
meters = 2.99792458E8
m/sec f (GHz) where GHz = 1E9
Wavelength for several frequency ranges, all units are in cm:
Frequencyλ
/ 4
air
λ
/ 4
FR4
λ
air
λ
FR4
2.4 GHz 3.1 1.5 12.5 5.9
915 MHz 8.2 3.9 32.8 15.5
868 MHz 8.6 4.1 34.6 16.3
433 MHz 17.3 8.2 69.3 32.7
315 MHz 23.8 11.2 95.2 44.9
27 MHz 277.8 130.9 1111.1 523.8
Presenter
Presentation Notes
The distance in meters that it takes for a wave to complete a 360 degree cycle is called the wavelength. This distance is based on the dielectric constant of the medium that the wave is travelling in. For air, the dielectric constant is 1.0. For other materials, such as PCB material, the dielectric constant is very different. To find the wavelength of a particular frequency, we have to divide the wavelength in air by the square root of the dielectric constant of the medium. FR4 material has a dielectric constant of 4.5. This gets further complicated when you consider that in reality, a PCB antenna is really surrounded by a variety of dielectric constants composed of materials like FR4, air, ABS plastic, metal (from batteries), PCB components, etc..
Antenna Theory –
Antenna Considerations
•
Numerous issues to consider when selecting the antenna:–
Antenna placement
–
Board size available for antenna layout
–
Operating frequency
–
Ground plane for ¼
wavelength antennas
–
Antenna mismatch (VSWR)
–
Objects that alter or disrupt Line of Sight (LOS)
–
Antenna gain characteristics
–
Antenna bandwidth
–
Antenna Radiation Efficiency
Presenter
Presentation Notes
Antennas seemed to be the last item looked at when designing a system. But it should be the first thing to pick based upon link budgets and patterns. There are several factors when looking for an antenna.
Antenna Theory –
Antenna Radiation Patterns –
Traditional Coordinate System
An Isotropic Antenna is a theoretical antenna spec in dBi and radiates equally in all directions of a sphere.
Antenna Measurement Coordinate System
•
x-y
plane (θ
= 90 deg) is the azimuth plane (horizontal plane)
•
y-z
plane (φ
= 90 deg) is the elevation plane (vertical plane)
Presenter
Presentation Notes
Antenna specs from the majority of suppliers will reference their designs to an ideal isotropic antenna. This is a model where the antenna is in a perfect sphere and isolated from all external influences. Most of the measurements of power is done in units of dBi where i refers to the condition of isotropic antenna. Antenna pattern. The radiation pattern or antenna pattern is the graphical representation of the radiation properties of the antenna as a function of space. That is, the antenna's pattern describes how the antenna radiates energy out into space (or how it receives energy). It is important to state that an antenna radiates energy in all directions, at least to some extent, so the antenna pattern is actually three-dimensional. It is common, however, to describe this 3D pattern with two planar patterns, called the principal plane patterns. These principal plane patterns can be obtained by making two slices through the 3D pattern through the maximum value of the pattern or by direct measurement. It is these principal plane patterns that are commonly referred to as the antenna patterns. Principal plane patterns or even antenna patterns, you will frequently encounter the terms azimuth plane pattern and elevation plane pattern. The term azimuth is commonly found in reference to the horizontal whereas the term elevation commonly refers to "the vertical". When used to describe antenna patterns, these terms assume that the antenna is mounted (or measured) in the orientation in which it will be used. The azimuth plane pattern is measured when the measurement is made traversing the entire x-y plane around the antenna under test. The elevation plane is then a plane orthogonal to the x-y plane, say the y-z plane (φ = 90 deg). The elevation plane pattern is made traversing the entire y-z plane around the antenna under test. The antenna patterns (azimuth and elevation plane patterns) are frequently shown as plots in polar coordinates.
Antenna Theory –
Antenna Radiation Patterns –
3D OTA CTIA Measurements
Example 1: 2.4 GHz Yagi
Directional Antenna (DN034)
Example 2: 868 MHz Meandering Monopole Antenna (Dual Band Option, 2.4 GHz, DN024, Rev E)
Presenter
Presentation Notes
The azimuth plane pattern is formed by slicing through the 3D pattern in the horizontal plane, the x-y plane in this case. Notice that the azimuth plane pattern is directional, that is, the antenna does not radiate its energy equally in all directions in the azimuth plane. The elevation plane pattern is formed by slicing the 3D pattern through an orthogonal plane (either the x-z plane or the y-z plane).
Antenna Theory –
Antenna Parameters
Important parameters
•
WaveLength, .
Antenna size for dipole relative to the wavelength of transmission.
•
Polarization
the direction of the electric field to the electromagntic wave.
•
Impedance, Z.
A measure of the total opposition to current flow in an alternating current circuit, made up of two components, ohmic resistance and reactance, and usually represented in complex notation.
•
Bandwidth
is the range of frequencies where the return loss is below VSWR of 2
•
Efficiency ()
is the ratio of power in watts actually radiated to the power into the antenna terminals %100*
in
rad
PP
fc
Z = R + iX
BW = 100 ( (FH –
FL) / FC)
Presenter
Presentation Notes
Efficiency reductions occur from losses within the antenna structure, feed network or matching network.
Antenna Theory –
Antenna Parameters
Important parameters
•
Gain, G.
is the maximum radiation beam of the highest beam. This parameter takes into account VSWR mismatch and energy losses.
–
Important to remember that antennas do not amplify RF. Since antennas cannot create energy, the total power radiated is the same as an isotropic antenna. Any additional energy radiated in the directions it favors is offset by equally less energy radiated in all other directions.
–
IEEE Gain Definition: GIEEE = Radiated Power / Delivered Power =
D
•
Directivity, D.
Antenna directivity is usually measured in dBi, or decibels above isotropic sphere antenna.
–
The directional antenna has a maximum directivity greater than 0dB.
•
Resonance Frequency
is the electrical resonance is related to the electrical length of the antenna.
Presenter
Presentation Notes
Directivity High gain is not necessarily a good thing. It depends on the application and what factor that are causing the high gain. High gain is desirable for an RF system with fixed positioning of both receiving and transmitting antennas. Thus the antennas can be positioned with the optimum direction pointed towards each other. For mobile systems it is usually better to have an antenna which spreads the radiated power in a more omnidirectional manner. This will ensure that the performance of the system is not heavily affected by the positioning of the devices. Gain Gain is usually referred to an isotropic antenna and usually with the designation dBi. An isotropic antenna is a theoretical antenna that radiates equally in all directions. Gain in an antenna must not be related to gain from an amplifier. An antenna has not the capability to amplify the RF signal, but it can focus the energy in a specific direction and thus increase the gain. It can be compared to a flashlight were you always have the same amount of light, but it is possible to focus it to form a narrow beam. Thus you get a more intense light in a smaller area. Since antenna gain is often only given for the maximum direction it is important to also look at the radiation pattern or an average gain number when evaluating an antenna. Usually we are only interested in the maximum gain, which is the gain in the direction in which the antenna is radiating most of the power. An antenna gain of 3dB compared to an isotropic antenna would be written as 3dBi.
Antenna Theory –
Antenna Q and Bandwidth
Properly designed antenna’s should cover the range of frequencies over whichthe antenna can operate correctly with sufficient bandwidth.
TI defines the antenna’s bandwidth in Hertz when VSWR less than 2:1, or return loss of greater than -9.5dB
•
Bandwidth (BW) can be defined in percentage of the operating frequency:
BW = 100 ( (FH –
FL) / FC)
•
There is a direct relationship between Q and bandwidth
c
LH
fffQ
Presenter
Presentation Notes
The primary two specs an antenna must meet is its Q and bandwidth.
•
Two fundamental types of antennas
–
Single ended antennas•
Usually matched to 50 ohm•
Needs a balun if the chip has a differential output•
Easy to characterize with a network analyzer
–
Differential antennas•
Can be matched directly to the impedance of the radio•
Can be used to reduce the number of external components•
Complicated to make good design•
Difficult to measure the impedance and to characterize.
Antenna Theory –
Antenna Categories
Presenter
Presentation Notes
It is common for RF chips to have a differential RF interface. Thus a transformation network must be implemented to be able to use a single ended antenna with these devices. This network is called a balun. A balun can be implemented with discrete components, microstrip-lines or an IC balun. By using a differential antenna, matched directly to the impedance of the chip it is possible to reduce the BOM. Making a PCB antenna that is exactly matched to the impedance of the chip is complicated and simulation tools must be used. The impedance of a single ended antenna can be measured with a network analyzer. Measuring the impedance of a differential antenna requires a 4-port network analyzer which is an expensive equipment.
•
Resonant antennas are often used–
Monopole:
λ/4–
Bent Monopole:
λ/4–
Inverted F:
λ/4–
Dipole:
λ/2–
Folded dipole:
λ/2
Antenna Theory –
Resonant Antennas
Presenter
Presentation Notes
Monopole and Inverted F Antenna are common single ended antennas. The most common differential antennas are dipole, folded dipole and loop antennas. Theoretically, the imaginary part of the impedance equals zero for resonant antennas. In practical implementations you will often see that the impedance of these antennas also have an imaginary part and that the real part differs from the theoretical value. The impedance is affected by the size and the shape of the ground plane. Thus the exact same antenna can perform differently when implemented on different PCBs. The resonance frequency is the frequency where the antenna performs best and the length of the antenna determines the resonance frequency. Therefore will the operating frequency affect the required size of the antenna. Lower frequency will require larger antenna.
Antenna Theory –
Frequency “v”
Size
•
Lower frequency
increases
the range–
Theoretically, reducing the frequency by a factor of two doubles
the range (line of sight)
•
Lower frequency requires a larger antenna–
λ/4 at 433 MHz is 17.3 cm (6.77 in)–
λ/4 at 915 MHz is 8.2 cm (3.22 in)–
λ/4 at 2.4 GHz is 3.1 cm (1.20 in)
•
A meandered
structure is a compact dipole with inductive loading
-
λ/4 at 2.4 GHz
feedline
Presenter
Presentation Notes
You can take a dipole and use techniques like meandering to decrease the size. The bends act like inductors that increase the inductance of the antenna therefore allowing the length to be decreased. Here the feedline (smaller line provides an impedance change).
Antenna Theory –
Max. Power Transfer (VSWR)
Moritz Von Jacobi’s maximum power theory states that maximum power transfer happens when the source resistance equals the load resistance.
As impedances are mis-matched, part of the transmitted signal is reflected back into the source which is the Voltage Standing Wave Ratio (VSWR); the ratio of the reflected waveform to the transmitted waveform.
With antenna design: VSWR is a measure of how well the input impedance of the antenna matches the characteristic impedance of the output from the RF network.
Impedance mismatch will reduce performance !
Presenter
Presentation Notes
This slide shows the power delivered and reflected sets the total energy transfer. Refer to http://www.csgnetwork.com/vswrlosscalc.html for a VSWR Loss calculator
•
Commonly used antennas–
PCB antennas•
No extra cost•
Size demanding at sub 433 MHz•
Good performance at > 868 MHz
–
Whip antennas•
Expensive solutions for high volume•
Good performance•
Hard to fit in many applications
–
Chip antennas•
Medium cost•
Good performance at 2.4 GHz•
OK performance at 868-955 MHz•
Poor performance at 433-136 MHz
–
Wire / Helical antennas•
Low cost •
Ideal at sub 433 MHz
Antenna Theory –
Antenna Types -
Commonly Used antennas
Presenter
Presentation Notes
Designing compact and high performance PCB antennas requires a simulation tool and the skills to use such a tool. These tools are often very expensive and it is not straight forward to configure them to perform accurate simulations. It is also important to include the ground plane and correct PCB parameters to achieve correct results. The easiest approach is to copy an antennas reference design. Several PCB antenna designs can be found at www.ti.com/lpw. See Application Note 058 for more antenna theory and an overview of available references design: http://www.ti.com/lit/pdf/swra161 Whip antennas can either be implemented as a piece of wire or by using a prefabricated antenna with a connector on. Using a wire is a cheap and flexible solution. In most cases the wire can be bent to fit inside the plastic casing. The placement of the wire will affect the performance thus some kind of fixture should be used to hold the wire in place. The price of chip antennas varies from around 0.20 to 1.50 $, dependant on frequency, volume technology and manufacturer. Chip antennas tend to have narrow bandwidth and it is often required with matching component to achieve optimum performance at the desired frequency. Slot antennas can be created by making holes in the PCB. It is possible to achieve high gain with such antennas, but because of large size requirements are they rarely used.
Frequency Sweep Function (Ideal for bandwidth measurements)
Presenter
Presentation Notes
Many measurements can be done in the lab. This is example of the most common measurements carried out in a lab. See Application Note 058 (AN058) for more information about antenna theory, antenna measurements and antenna reference designs: http://www.ti.com/lit/swra161.
Antenna Measurements -
Impedance
The smith chart shows how the impedance varies with frequency. Useful tool to find the values of the antenna matching component values. VSWR circles can be used to see how well the antenna is matched.
Magnitude of mismatch in dB with respect to frequency. No exact impedance can be obtained from this format.
Question: The dashed red line is shown at 14dB. What is the VSWR ?
S11
S11
Presenter
Presentation Notes
The smith chart shows how the impedance vary with frequency. It is important to have a correct calibration of the phase when measuring the impedance. The smith chart can be used to calculate the value of matching component. A S11 measurement shows the magnitude of the impedance which is the distance from the center of the smith chart. This result show how much of the available power that is reflected at the feed point.
Antenna Measurements -
Impedance -
Smith Chart
Inductive
Capacitive
Short circuit Open circuitMatch
parallel C
parallel L series L
series C
Presenter
Presentation Notes
The smith chart is well used with RF design. There are many sites available on the internet where you can download more information; for example: http://www.rfcafe.com
Antenna Measurements -
Impedance Matching
Mismatch between the antenna and the feed line results in losses and will reduce the radiated power from the antenna.
Inductors and capacitors in shunt and series can be used to achieve the desired impedance matching.
Presenter
Presentation Notes
Inductors and capacitors in shunt and series can be used to achieve the desired impedance matching. Including the option for a shunt, series, shunt network at the feed point of the antenna gives a flexible option to tune the impedance. If no tuning is required then a 0 ohm resistor can be placed in series with the feed line.
Antenna Measurements -
Characterization
How to characterize antennas:
1.
Measure the reflected power at the feed point of the antenna
2.
Measure the radiated power across the bandwidth of interest
3.
Measure the radiation pattern in an Anechoic Chamber
1. 2. 3.
Presenter
Presentation Notes
A network analyzer and an anechoic chamber is needed to perform accurate characterization of an antenna. This is expensive equipment both to buy and rent. All measurements should be performed in an anechoic chamber but for practical reasons, tests 1 and 2 are usually performed in the standard lab. It is possible to perform antenna testing with less expensive equipment which still can give valuable information. By using a spectrum analyzer and an antenna from our development kit, it is possible to make relative measurements which shows how the radiated power varies with frequency or orientation of the antenna. If a spectrum analyzer is not available it is possible to perform measurements by using SmartRF Studio and our development kit. By using SmartRF Studio to put a transceiver, e.g.CC2500 or CC1101, in RX the Received Signal Strength will be displayed.
Antenna Measurements -
Anechoic Chamber
•
Patterns give directivity and gain of the antenna.
•
Radiated power is also measured.
•
Done in an anechoic chamber to eliminate multi-paths for accurate plots.
•
Done through contracted 3rd
party companies.
z
x
y
+ =
Presenter
Presentation Notes
To get an accurate measurement of antenna pattern and performance you must test in anechoic chamber.
The output power is dependent on polarization and direction. Note that the maximum can be at different frequencies for different directions. The receiving antenna can also affect the result, it should therefore have equal performance across the measured frequency band. With a spectrum analyzer: The advantage of this test is that it is not affected by cables or other test equipment and it is also easy to perform the test with the antenna placed in its operating environment, e.g. inside plastic casing or close to a persons body. Test software is needed to run this kind of test. A code that steps the carrier in 1 MHz steps and stays on each channel for approximately 5ms should be sufficient. The step size and dwell time can be adjusted to get the desired resolution when measuring. It is possible to measure the absolute level of emission if the test is performed in an anechoic chamber. Performing the test in a regular lab will not give absolute levels, but will give a good indication of the relative change in output power across the frequency band. With a network analyzer: The Q and bandwidth can be measure directly at the antennas input port, it is important to include the extra components that are used for matching.
Antenna Measurements -
Reflection
11log20)log(2011 VSWR
VSWRSdB
0
0
ZZZZ
l
l
11
VSWR
VSWR S11dB Reflected Delivered
power % power %
1 -∞ 0 100
1.1 -27 0.2 99.8
1.2 -21 0.8 99.2
1.5 -14 4 96
2 -9.5 11.1 89.9
3 -6 25 75
4 -4.4 36 64
5 -3.5 44.4 55.6
5.8 -3 50 50
10 -1.7 66.9 33.1
Presenter
Presentation Notes
A common definition of bandwidth is the band which has a reflection of less than -10 dB or VSWR less than 2. This ensures that 90 % of the available power is delivered to the antenna and is illustrated by the two vertical dashed lines in the plot. The reflection at the feed point of the antenna is determined by the difference between the impedance of the antennas (Zl) and the impedance of the feed line (Z0). For feed lines having a complex impedance will an antenna with complex conjugated impedance ensure optimum power transfer. If the feed line has impedance with only a real part the antennas should have the same impedance to ensure optimum transfer of power. Having a good VSWR is still just one parameter in the antenna design process. For example: having a 50ohm resistor as an antenna will give an excellent VSWR but will be a useless antenna due to the high ohmic losses. See Application Note 058 (AN058) for more information about antenna theory, antenna measurements and antenna reference designs: http://www.ti.com/lit/swra161.
Antenna Measurements -
Mounting of Cable for S11 Measurements
Tip:
It is invaluable
to have
semi-rigid
cables
in the lab
for debugging
RF.
Solder
first shielding
onto
an earth
plane and then
solder
the 50ohm connection. Minimize
risk for riping
off
tracks
when
connecting
to the semi-
rigid
cable.
Ready
made
semi-rigid
cables
are quite
expensive
but
can
be re-used
again.
Supplier: AMSKA (www.amska.se)
A50451229, ASC047-PSMAf-0,3-200, Cable Assembly, with SMA-f and 3mm stripped, 20cm
Presenter
Presentation Notes
A semi rigid coax cable is useful when performing measurements on prototypes. The outer conductor of the cable should be soldered to ground while the inner conductor is soldered to the feed line of the antenna. It is important that the antenna is disconnected from the rest of the circuitry when this measurement is performed. The unshielded part of the inner conductor should be as short as possible to avoid introducing extra inductance when measuring and the outer should be soldered to ground a close as possible to the end of the cable.
Ideal to have dedicated boards that are specifically used just for calibration purposes. Measuring one antenna design would require four boards:
•
Open : end connector in air; shield connected to GND1
•
Short : end connector to closest GND; shield connected to GND1
•
Load: 50ohm calibration, it is useful to use two 100ohm parallel resistors assembled at the end connection point; shield connected to GND1
•
Device Under Test Board.
•
By performing these steps then the semi-rigid cable is also taken care of during the calibration. By just using the network analyzer calibration kit; then the semi-rigid cables will be a part of the measurements.
Presenter
Presentation Notes
No one should think they can measure the RF sections with a oscilloscope probe and gain any useful information. The circuits at these high frequencies are often working with component values in low pF and nH. The common way to measure these values are using a vector analyzer and measuring S-parameters. Careful calibration is the key to reasonably good measurements.
Antenna Measurements -
Placement of Cable During S11 Measurement
Keep the cable in a constant direction and it is good practice to use cable ties
to maintain cable, including network analyzer cable in a fixed position.
Presenter
Presentation Notes
The placement of the cable can affect the measurement result, especially if there are strong currents traveling back and forth on the ground plane. Sharp dips and an unsymmetrical resonance are indications of incorrect results. North gives the best result on the plot and corresponds to the direction shown in the picture at the top right. The reason is that the cable uses the shortest path across the ground plane.
Antenna Measurements -
Use of Ferrites During S11 Measurement
Presenter
Presentation Notes
Ferrites can be used to reduce the influence from currents running at the outer of the cable. PCBs which have a ground plane with dimensions that are a fraction of a wavelength tend to have larger currents running on the ground plane. This could potentially cause more unstable results when trying to measure the reflection at the feed pint of antennas implemented on such PCBs The placement of the ferrite along the cable will also affect the result. Thus it is important to understand that there is a certain inaccuracy when performing this kind of measurement.
Antenna Measurements -
Influence of Plastic Encapsulation
•
Plastic encapsulation and body effects
Presenter
Presentation Notes
Plastic encapsulation tend to shift the resonance frequency down to a lower frequency. This shift in frequency can be of more than 50 MHz. It is therefore important to place the antenna where it is going to operate when tuning and characterizing the antenna. For PCB antennas it is often enough to reduce the length of the antenna to compensate for this shift. For chip antennas it might be necessary to adjust the value of tuning components to compensate for the influence of plastic encapsulation. Body effects and large metal pieces close to the antenna are other factors that can affect the performance.
Antenna Measurements -
Radiation Pattern -
Ground Plane Influence
Presenter
Presentation Notes
The radiation pattern and gain is affected by the size and shape of the ground plane.
What Range can I expect ?
Range Estimation -
Real Life Example
What Range can I expect ?
Example with CC2500 & C2591 & 7dBi Gain Antenna
Configuration:
•
Output Power: +21 dBm
•
Input Sensitivity: -104 dBm
(2.4kBaud)
•
LNA Gain (CC2591): 6 dB
•
Carrier Frequency: 2.4GHz
•
Antenna Gain: 7dBi
Range Estimation -
Link Budget
Link Budget = Output Power + Antenna gain –
Sensitivity
• Sensitivity is a negative number
•
The gain of the antennas is equal to the gain of the transmitting antenna + gain of the receiving antenna
•
Link budget is equal to how much loss you can have between the transmitter and receiver
With the CC2500 & C2591 & 7dBi Gain Antenna
Example:
•
Output Power: +21 dBm
•
Input Sensitivity: -104 dBm
(2.4kBaud)
•
LNA Gain (CC2591): 6 dB
•
Carrier Frequency: 2.4GHz
•
Antenna Gain: 7 dBi
The total link budget would be 145 dB
Presenter
Presentation Notes
High gain of an antenna is not necessarily a good thing. It depends on the application and what factor that are causing the high gain. High gain is desirable for an RF system with fixed positioning of both receiving and transmitting antennas. Thus the antennas can be positioned with the optimum direction pointed towards each other. For mobile systems it is usually better to have an antenna which spreads the radiated power in a more omnidirectional manner. This will ensure that the performance of the system is not heavily affected by the positioning of the devices. Gain is usually referred to an isotropic antenna and usually with the designation dBi. An isotropic antenna is a theoretical antenna that radiates equally in all directions. Gain in an antenna must not be related to gain from an amplifier. An antenna has not the capability to amplify the RF signal, but it can focus the energy in a specific direction and thus increase the gain. It can be compared to a flashlight were you always have the same amount of light, but it is possible to focus it to form a narrow beam. Thus you get a more intense light in a smaller area. Since antenna gain is often only given for the maximum direction it is important to also look at the radiation patter or an average gain number when evaluating an antenna.
Range Estimation -
Friis
Transmission Equation
= Wavelength in Meters
Pr
= Received Power in dBm
Pt
= Transmit Power in dBm
Gt
= Transmit Antenna Gain in dBi
Gr
= Receive Antenna Gain in dBi
R = Distance between Antenna in Meters
•
Predicts transmission distance based on applied and received power with no obstructions of Line of Sight.
r
rtt
PGGP
GHzfR
)(4m/s 80.29979245
)(m/s 80.29979245
GHzf
Using our example case, according to the Friis
Equation the distance would be approx 155 km !Correct ?
Presenter
Presentation Notes
Friis equation is the primary math model to predicting point to point communication links. This is the very elementary equation and has been expanded to include height of antenna above ground and difference in TX and RX antennas. The formula is very accurate once all the constants have been entered. Please refer to DN018 for further information concerning “Range Measurements in an Open Field Environment”. Friis equation (Free space loss) When discussing range and radio communication Friis Equation relates most system parameters. The equation relates received signal power to supply power to antenna for both transmitter and receiver. The frequency dependent free space loss are also include. However, in real life scenarios the achieved range will differ from the free space loss equations. Especially indoors with “no line of sight” the range will differ a lot from the range expected from the friis formula. This equation together with the sensitivity of the receiver determine the free space communication range for a particular radio link. The Friis equation give us the following relationships. Dubble the frequency => Half the range. 6 db increased sensitivity or output power => Dubbles the range. High gain antenna ca be utilized to increase the range.
Range Estimation -
Predicting Range with an Improved Estimation
Take into account the height distance to earth (H1 & H2).
The closer to earth, the shorter the range (H1 & H2 -> 0).
For further information, please refer to DN018
H
1
d
Receive antenna GR
Transmit antenna GT
Direct transmission
Reflected transmission
εr
H2H
1
d
Receive antenna GR
Transmit antenna GT
Direct transmission
Reflected transmission
εr
H
1
d
Receive antenna GR
Transmit antenna GT
Direct transmission
Reflected transmission
εr
Θi Θr
Reflection law
Θi
=Θr
Using our example case, with H1 & H2 at 1.3m; the distance would
be approx 9 km.
Presenter
Presentation Notes
Transmission budget The graph show the ideal received power as a function of distance predicted by Friis equation compared to the ground model which takes into account H1 & H2 parameters. The transmission budget is the difference between received power PR and sensitivity. To maintain communication the received power should be larger than the sensitivity. LPRF defines sensitivity as the signal level where the package error rate (PER) exceeds 1%.
Range Estimation -
Predicting Range with the Highest Accuracy
If even higher accuracy is required, then measure the actual Transmitted Radiated Power in a chamber and re-enter the measured values into the ground model formula.
Using our example case, with H1 at 4700m (Pike’s Peak, Colorado USA) & H2 at 1.3m; the distance would be approx 90 km. Measured at least 65 km +
The main point is to take into account the height above ground for the transmitter antenna (H1) and receiver antenna (H2) whilst calculating the expected range since this will strongly effect the range.
2.4 GHz Reference Designs –
Single Ended Antennas
AN043 DN007DN034
AN048
Presenter
Presentation Notes
TI offers several 2.4 GHz antenna designs. Since many of these are matched to 50 ohm they can be used with all 2.4 GHz reference design. The documentation can easily be found on www.ti.com by typing in the documentation name, e.g. DN007, in the keyword search field at the top of the web page. See Application Note 058 (AN058) for more information about antenna theory, antenna measurements and antenna reference designs: http://www.ti.com/lit/swra161.
To reduce the number of external components required by a balun, it is possible to design a differential antenna that is matched directly to the impedance of the RF port of the radio. In some cases a few external components are required to obtain proper impedance matching or filtering. CC2500, CC2510, CC2511 and CC2550 have all the same impedance. This makes it possible to use the antenna shown with all these products. This antenna design and the measured performance are presented in Design Note 004. The only external components needed are two capacitors to ensure compliance with ETSI regulations. Thus for FCC compliance no external components are required if the proper output power and AUT cycling are used. CC2400, CC2420 and CC243x have all slightly different impedances. It is therefore necessary to use external components to tune the impedance so the same antenna structure can be used for all these products. The antenna presented in Application Note 040 can be used with CC2400, CC2420 and CC243x if the inductor sitting between the RF pins is tuned accordingly. In addition to the tuning inductor it is recommended to use an inductor in series with the TXRX switch pin. This inductor works as a RF choke at 2.4 GHz.
This PCB antenna consists of a meandering monopole antenna and is a medium-size, low-cost solution which can operate at both 868 MHz & 2.4 GHz. More information about this design can be found in Design Note 024. This antenna is mostly used for a frequency of 868 MHz or 915 MHz and the efficiency is very good.
For 868/915/955 MHz operation, TI offers eight reference designs that can be used with all RF products capable of operating at these frequencies. Five designs are pure PCB antennas and the other three are chip antennas. All designs are matched to 50 ohm. Thus a balun is needed for all products with differential output.
For 433 MHz operation, TI offers three reference designs that can be used with all RF products capable of operating at these frequencies. Two designs are helical wire antennas and the third design is a chip antenna. All designs are matched to 50 ohm. For 315 MHz operation, TI offers a reference designs that can be used with all RF products capable of operating at this frequency. The design is matched to 50 ohm. For 169 MHz operation of in the frequency band 136 to 240 MHz, TI offers a reference designs that can be used with all RF products capable of operating at this frequency.
The CC-Antenna-DK contains 13 low cost antennas and 3 calibration boards. The antennas cover the frequency range as low as 136 MHz to 2.48 GHz. The main purpose of the CC-Antenna-DK is to ease the decision for which type of low cost antenna can be implemented as well as give an estimation of the performance that can be achieved. All antennas are tuned for connecting to an EM board on the EB platform. A matching network is used on each antenna design so the antenna boards can matched for other GND sizes than the EB board. Antennas are categorized under the operating frequency (169 MHz, 315 MHz, 433 MHz, 868 / 915 MHz or 2.44 GHz) and then the type of antenna (PCB Antennas, Chip Antennas, and Wire Antennas). The main focus is on PCB, Wire and Chip antennas, since these are mainly used in high volume products. • 169 MHz (136 – 240 MHz) Antenna • 315 MHz (273 – 348 MHz) Antenna • 433 MHz (387 – 510 MHz) Antenna • 868 MHz (779 – 960 MHz) Antenna • 915 MHz (779 – 960 MHz) Antenna • 2440 MHz Antenna
This application note describes important parameters to consider when deciding what kind of antenna to use in a short range device application. Important antenna parameters, different antenna types, design aspects and techniques for characterizing antennas are presented. Radiation pattern, gain, impedance matching, bandwidth, size and cost are some of the parameters discussed in this document. Antenna theory and practical measurement are also covered. In addition different antenna types are presented, with their pros and cons. All of the antenna reference designs available on www.ti.com/lpw are presented including the CC-Antenna-DK. The last section in this document contains references to additional antenna resources such as literature, applicable EM simulation tools and a list of antenna manufacturer and consultants. See Application Note 058 (AN058) for more information about antenna theory, antenna measurements and antenna reference designs: http://www.ti.com/lit/swra161.
