0. Factors affecting RANGE
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Factors affecting RANGE
Prior to starting an RF design a realistic range requirement MUST be established
These are the factors affecting range of customers system:
The chosen frequency band of operation (315MHz/433MHz/868MHz/915MHz/2.4GHz)
In which countries shall the end product be sold (RF requirements: FCC, ETSI, Weak Power Radio etc.)
Maximum allowed transmitted power (ERP) for territory
RF output power and sensitivity of chosen nRF device
Available, suitable antennas with given gain figures
Necessary margins (system margin) set to allow for additional losses in air and surroundings (Buildings and objects)
Can the desired range requirement be achieved or not?
The importance of these factors can be shown in the Line-Of-Sight (LOS) link budget

1. Range- Theoretical LOS link budget
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Range- Theoretical LOS link budget
The parameters included in LOS link budget calculations are:
Allowed transmitter output power (POUT)
Transmitter antenna matching network losses (LM_TX)
Transmitter antenna gain (Gant_TX)
Free space loss (LP), given by frequency of operation and distance
Receiver antenna gain (Gant_RX)
Receiver antenna matching network losses (LM_RX)
Receiver sensitivity (S)
Line-Of-Sight range (R)
Path loss (LP)
Gant_RX
Gant_TX TX
POUT
Antenna matching network
LM_RX RX
Antenna matching network
LM_TX
Sensitivity (S)

2. Range- External losses
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Range- External losses
For a complete (realistic) range calculation, additional external factors must be considered:
Signal losses caused by objects in the path between the transmitter and receiver antennas (e.g. walls, floors, furniture, windows)
These losses increases with increasing frequency
Range variations caused by multi-path fading
Objects in close proximity to the antenna that affects antenna performance, e.g.:
Printed circuit board (PCB)
Enclosure, enclosure material
Human body
Quality of sampling, detection and processing of received data from the RF device
These external losses must be added to the LOS link budget for realistic calculation!
Multipath Fading
Multipath fading is a form of signal fading caused by signals arriving at the 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 effects the amplitude of the signal developed at the antenna.
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. So while there is little one can do to mitigate the problem, it is important that the user understand that Multipath fading will introduce system range variations from site to site.
Rule of thumb:
OUTDOOR RANGE = 1/2 of theoretical LOS link range
INDOOR RANGE (trough walls) = 1/10 of theoretical LOS link range

3. An example on how to achieve long and reliable range
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An example on how to achieve long and reliable range N5 N6 N7 N3 N4 N1 B N2 N5 N6 N7 N3 N4 N1 B N2
This solution implies:
This solution implies:
Coverage to/from base station from/to all nodes
Big, high gain antennas
High output power
Coverage only between neighbouring nodes
Small, low gain antennas
Low output power
Big and bulky radio devices
High current consumption
Jamming of other systems
Compliance with frequency regulations?
Small, light weight devices
Low current consumption
Low interference
Easy to comply with frequency regulations

4. Link budget calculation example
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Link budget calculation example
System specification:
Frequency of operation (f0) : MHz (wavelength =0.69m)
Wanted outdoor range (R) : 75m  Design for LOS = 2 • R = 2 • 75m = 150m
Transmitter output power (Pout) : 10dBm
Transmitter antenna gain (Gant_TX): -27dB (18x10mm loop antenna)
Receiver antenna gain (Gant_RX) : To be decided
Receiver sensitivity (S) : -105dBm
Frequency of operation (f0): From the frequency of operation (f0=433.92MHz) we can choose between the nRF401, nRF402 and nRF403 devices.
Path loss (free space loss) L: The free space loss is entirely decided by frequency of operation (f0) and distance (R) between the transmitter and receiver antennas.
Transmitter output power (POUT):
The transmitter output power must be set based on the transmitter antenna gain and the allowed ERP from the antenna. TX
STR-1
STR-2/STR-3 RX
STR-1/STR-3
Sensitivity (S) P
out

5. Link budget calculation example
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Link budget calculation example
Calculation 1: Can the 150m line-of-sight range requirement be achieved with a 25x15mm loop antenna at the receiver?
A 25x15mm loop antenna has a theoretical gain Gant_TX = -22dB.
Max allowed path loss is given by:
Which gives a line-of-sight range:
The 150m line-of-sight range requirement is NOT fulfilled. A receiver antenna with HIGHER gain is needed.

6. Link budget calculation example
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Link budget calculation example
Calculation 2: Can the 150m line-of-sight range requirement be achieved with a 35x20mm loop antenna at the receiver?
A 35x20mm loop antenna has a theoretical gain Gant_TX = -18dB.
Max allowed path loss is given by:
Which gives a line-of-sight range:
So, a maximum line-of-sight range of 150m can be reached with the combination of 18x10mm and 35x20mm loop antennas. This should assure that the outdoor range R = LOS/2 = 75m requirement can be achieved.

7. Link budget calculation, ERP specification
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Link budget calculation, ERP specification
Under Japans weak radio law and the FCC regulations in USA the max level of emitted power (ERP) from the TX antenna is specified instead of the device output power.
System specification (weak radio system):
Possible outdoor range (R) : to be decided
Frequency of operation (f0) : MHz (wavelength =0.95m)
Max. emitted power (ERP) : 500 3m => ERP ~ -42 dBm
TX & RX antenna gain (Gant_TX, Gant_RX): -22 dB (315 MHz, 35x20mm loop antenna)
Transmitter output power (Pout) : ERP - Gant_TX = -42 dBm - (-22dB) = -20 dB
Receiver sensitivity (S) : -105dBm TX
STR-1/
STR-2/STR-3 P
out
Frequency of operation (f0): From the frequency of operation (f0=433.92MHz) we can choose between the nRF401, nRF402 and nRF403 devices.
Path loss (free space loss) L: The free space loss is entirely decided by frequency of operation (f0) and distance (R) between the transmitter and receiver antennas.
Transmitter output power (POUT):
The transmitter output power must be set based on the transmitter antenna gain and the allowed ERP from the antenna. RX
STR-1/STR-3
Sensitivity (S)

8. Link budget calculation example, weak radio
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Link budget calculation example, weak radio
Calculation3: What outdoor range can be achieved on a link with 315MHz 35x20 loop antennas in both ends?
A 315 MHz 35x20mm loop antenna has a theoretical gain Gant = -22dB.
Max allowed path loss is now given by:
NOTE!
Improving Gant_TX will increase range.
In a transceiver solution (nRF403) both antennas need to be “equal” to meet regulations and maintain sensitivity both ways
Which gives a line-of-sight range:
The outdoor range are consequently (LOS / 2) = 4.25 m.

9. SUNBOW STR-1 range example
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SUNBOW STR-1 range example
18x10mm loop antenna: G = -27dB 25x15mm loop antenna: G = -22dB 35x20mm loop antenna: G = -18dB
Rule of thumb: OUTDOOR RANGE = 1/2 of theoretical LOS link range INDOOR RANGE = 1/10 of theoretical LOS link range

10. Free space (LOS) path loss vs. Frequency
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Free space (LOS) path loss vs. Frequency
Doubling the frequency  6dB added free space loss.
6dB added free space loss  cutting the range in half
If long range / “good” coverage is the primary key design parameter
 go down in frequency
Wavelength [m] 0, , , ,125
Frequency [MHz]
R[m]
1 -22, , , ,046
3 -31, , , ,5884
10 -42, , , ,046
20 -48, , , ,0666
30 -51, , , ,5884
40 -54, , , ,0872
50 -56, , , ,0254
60 -57, , , ,609
70 -59, , , ,948
80 -60, , , ,1078
90 -61, , , ,1308
, , , ,046
, , ,04 -80,8739
, , , ,6296
, , , ,3249
, , , ,9686
, , , ,5678
, , , ,1284
, , , ,655
, , , ,1514
, , , ,6211
, , , ,0666

11. To RF System
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Antenna connection methods, antenna impedance matching, Antennas for nRF designs
Customers major design goals:
Small size
Light weight
Low cost
Lowest possible current consumption
Maximum range
The choice of ANTENNA TYPE, CONNECTION METHOD and IMPEDANCE MATCHING of the antenna to the SUNBOW STR device is of MAJOR importance for the above issues.

12. Antenna connection methods
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Antenna connection methods
The SUNBOW STR devices have a differential (balanced) antenna interface for easy connection of differential type antennas (e.x loop antenna).
PWR_UP
VSS
XC1
ANT1
VDD
DIN
ANT2
FILT1
XC2
TXEN
RF_PWR
DOUT 14 20 19 17 16 15 18 1 2 3 4 5 6 7 8 9 10 11 12 13
VCO1
VCO2 CS
A single ended (unbalanced) antenna (e.g. helical antenna) can be connected to the SUNBOW STR device by the use of a differential to single ended matching network.
SUNBOW STR devices can be used for both differential and single ended antennas!

13. Antenna differential to single ended matching network
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Antenna differential to single ended matching network
Purpose of the differential to single ended matching network:
Differential to single ended conversion
Differential
Single ended
Differential
Design purpose and assumptions
50 antenna load impedance
Symmetrical PCB layout for minimum radiation of 2. harmonic content in transmit mode
Use of standard 1.6mm double-sided FR4 printed circuit board
Maximum output power in transmit mode
Maximum receiver sensitivity in receive mode
Use of only standard E12 series component values
No manually tuneable components
The differential to single ended matching networks are described in application notes and datasheets.
Layouts are available on the Nordic VLSI download pages.
Impedance matching (transformation) between the SUNBOW STR device recommended antenna port load impedance to the antenna impedance

14. Measurement of output power and sensitivity
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Measurement of output power and sensitivity
Remember the following when measuring transmitter output power and receiver sensitivity performance:
The differential to single ended matching network introduces an insertion loss of about 2-3dB
Losses in connectors and cables typically adds up to about 1-2dB
DIN
DOUT
RF in/out 50 Ohm
STR-EVBOARD
SPECTRUM ANALYZER
LF-GENERATOR
Bit rate : kbps
The maximum output power and maximum sensitivity figures given in the nRFTM devices datasheet, are values measured at the nRFTM device antenna pins.
Example: Typical carrier power/modulation bandwidth-testbench
Thus, when measuring performance related parameters, losses in differential to single ended matching network, connectors and cables must be taken into account. These losses are typically in the order of 3-5dB.

15. Impedance matching to a differential loop antenna
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Impedance matching to a differential loop antenna
Our loop antennas uses a T-match configuration for matching the high input impedance of the loop itself, to the recommended load impedance for the SUNBOW STR device
T-match
Under resonance the resistive input impedance of the loop is high, and has to be transformed down to a lower value to match the recommended nRFTM device load impedance. An effective shunt-matching technique is the T-match connection as shown in the figure. This method of matching is based on the fact that the impedance between any two points equidistant from the centre along a resonant antenna is resistive, and has a value that depends on the spacing between the two points (feed length). It is therefore possible to choose a pair of points between which the impedance will have the right value to match the recommended nRFTM device load impedance. By reducing the distance between the connection points the impedance is reduced. In practice, the transmitter/receiver cannot be connected directly at these points because the distance between them is much greater than the pin spacing of an integrated circuit. The T-match arrangement in the figure overcomes this difficulty by using a second conductor paralleling the antenna to form a matching section to which the transmitter/receiver may be connected.
A trial and error procedure is used to vary the feed length to make the total input impedance of the loop antenna equal to the transmitter output impedance/receiver input impedance. The estimated capacitor Cp (1/Cp = 1/C1+1/C2+1/C3) must be tuned for maximum radiated power from the antenna for every position of the connection points.
The loop antenna is sensitive to changes in layout!

16. Antennas suitable for SUNBOW STR devices
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Antennas suitable for SUNBOW STR devices
The most common / basic antenna types for SRD applications are:
Loop antenna
Quarter wave dipole antenna
Centre-fed half wave dipole antenna
Folded, half wave dipole antenna
Helical dipole antenna
Embedded (integrated) antennas TX
STR-1
STR-2/STR-3 P
out
STR-1/STR-3
Sensitivity (S) RX
The customer has to select the appropriate antenna - contact the antenna vendor.

17. Antennas for SUNBOW STR, Loop antenna
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Antennas for SUNBOW STR, Loop antenna
Differential (balanced) antenna
Can be connected “directly” to the differential antenna port of the nRFTM devices with a T-match
Radiation pattern:
Omi-directional in the plane of the loop
Typical gain in the maximum direction:
-20dB to -30dB
Bandwidth:
Narrow, Q  40-50
Impedance:
High, several k’s. Transformed down to the required value (e.g. 400) with a T-match
Other characteristics:
Suitable for frequency bands 315MHz, 433MHz, 868MHz, 915MHz
Inexpensive solution
Not dependent on a ground plane
Superior performance in handheld/body-worn applications
Radiation pattern: The field pattern of electrically small loop antennas of any shape (circular, elliptical, rectangular, square, etc.) is similar to that of an infinitesimal magnetic dipole. The loop antenna will have a radiation pattern being omnidirectional with its maximum along the plane of the loop, while having a minimum perpendicular to the plane of the loop. To get an omni-directional coverage, the loop antenna should be placed horizontally.
Bandwidth: Some data for the 9.5x9.5mm 868/915MHz loop antenna, Q=40
BW3dB=22MHz BW6dB=35MHz BW10dB=60MHz
Superior performance in handheld/body-worn applications, WHY: Traditionally, the loop antenna is the antenna type that is best suited for handheld and body-worn applications. To understand the effect the body has on the antenna, we have to look at the wave impedance close to an antenna. A small electric dipole (or monopole) antenna sets up primarily an electric field, leading to high wave impedance close to the antenna, rolling off towards the far-field wave impedance of 377 Ohm. A small magnetic loop sets up a primarily magnetic field and therefore leads to low wave impedance close to the antenna.
The intrinsic impedance of the body is found to be 38 – 57 Ohm in the range of radio frequencies 30 – 3000 MHz. The high wave impedance of a dipole (e.g. a quarter wave whip) parallel and close to the body is therefore effectively short-circuited by the body. On the other hand, the loop antenna exhibiting low wave impedance in the near field is less affected by the body, and is hence often the preferred choice for body-worn applications.

18. Antennas for SUNBOE STR, Quarter wave dipole
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Antennas for SUNBOE STR, Quarter wave dipole
Single ended (unbalanced) antenna
A differential to single ended matching network needed
Radiation pattern:
Omi-directional in the direction normal to the dipole axis
Theoretical gain in the maximum direction:
5.1dBi (with infinite ground plane)
Bandwidth:
Wide
Impedance:
About 36 when tuned to resonance
Other characteristics:
Suitable for the frequency bands 315MHz, 433MHz, 868MHz, 915MHz, 2.4GHz
Dependent on a ground plane
Gain decreases considerably when ground plane area spreads out less than a quarter wavelength around the base of the dipole
Gain:Theoretically the directivity is 3 dBd (over that of a dipole) because the radiated power is radiated only in the upper half plane due to the ground plane.
The quarter-wave monopole is a simple form of antenna derived from the centre-fed half-wave dipole antenna. The monopole is always used in conjunction with a ground plane, which acts as a sort of electrical mirror. The "image antenna" for the monopole made up in a "perfect" ground plane, forms the "missing" second half of the antenna, transforming a monopole into the functional equivalent of a center-fed half-wave dipole. The feed-point resistance of a quarter-wave long, thin vertical monopole over such a ground plane at resonance is half that of the center-fed dipole, and will approach the theoretical value of 36 ohm.
Ideally, a ground plane should spread out at least a quarter wavelength, or more, around the base of the quarter-wave monopole.
When something less than an ideal ground plane is used as the ground plane for a vertical monopole, the ground-return loss resistance increases rapidly when the ground plane area is reduced. This increase in ground-return loss resistance will decrease the antenna efficiency considerably.
The term VSWR describes the degree of mismatch between a transmission line and its source or load. In low power, low cost radio design like this, anything less than is generally acceptable. A ZL=36 ohms gives VSWR= Z0/ZL = 50/36 = The 36 ohm antenna impedance will reduce the output power by a factor (referred to optimal load) 10*log(36/50) = 1.43dB.
While an antenna can be matched more precisely, the characteristic impedance of a quarter-wave whip on an adequate ground plane is close enough to 50 ohms to provide a generally acceptable value of VSWR (Voltage Standing Wave Ratio) when connected to the differential to single ended matching network 50 ohms output.

19. Antennas for SUNBOW STR, Centre-fed half wave dipole
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Antennas for SUNBOW STR, Centre-fed half wave dipole
Single ended (unbalanced) antenna
A differential to single ended matching network needed
Radiation pattern:
Omi-directional in the direction normal to the dipole axis
Theoretical gain in the maximum direction:
2.1dBi
Bandwidth:
Wide
Impedance:
About 73 when tuned to resonance
Other characteristics:
Suitable for the frequency bands 315MHz, 433MHz, 868MHz, 915MHz, 2.4GHz
Not dependent on a ground plane
Space consuming
The single-ended half-wavelength dipole antenna has a resistive input impedance of about 73 ohm when cut to lambda length. This antenna can be connected directly to the output of the dufferential to single ended matching network.

20. Antennas for SUNBOW STR, Folded half wave dipole
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Antennas for SUNBOW STR, Folded half wave dipole
Differential (balanced) antenna
Can be connected directly to the differential antenna port of the STR devices
Radiation pattern:
Omi-directional in the direction normal to the dipole axis
Theoretical gain in the maximum direction:
2.1dBi
Bandwidth:
Wide
Impedance:
About 292 when tuned to resonance
Other characteristics:
Suitable for the frequency bands 315MHz, 433MHz, 868MHz, 915MHz, 2.4GHz
Not dependent on a ground plane
Space consuming
VDD connection:
The nRFTM device power amplifier VDD connection should be made at the centre of the dipole, opposite of the antenna terminals.
Impedance:
The impedance of the folded dipole is four times greater (4•73 =292) than that of a centre-fed half wave dipole when cut to 0.47-0.48 length.

21. Antennas for SUNBOW STR, Helical dipole
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Antennas for SUNBOW STR, Helical dipole
Single ended (unbalanced) antenna
A differential to single ended matching network needed
Radiation pattern:
Omi-directional in the direction normal to the helical axis
Typical gain in the maximum direction:
0dBi to -20dBi has been reported
Bandwidth:
Narrow, high Q
Impedance:
About 35  or less, depends on size of coil and orientation to ground
Other characteristics:
Suitable for frequency bands 315MHz, 433MHz, 868MHz, 915MHz
Dependent on a ground plane
Easily de-tuned (impedance and radiation pattern) by nearby objects
May not be good for handheld/body-worn use
Extremely high RF currents at feed-point
Helical antenna Impedance: 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.
Care must be exercised in placement,as a helical detunes badly when located in proximity to other conductive objects. Because a helical has a high Q factor, its bandwidth is very narrow and the spacing of the coils has a pronounced effect on antenna performance.
The normal mode helical antenna radiates in the direction normal to the helical axis. Thus, the radiation pattern is as for the monopole. It can be seen as a monopole antenna shorted by coiling up the whip itself. This makes the dimensions of the helix much smaller than a wavelength, and resonance can be achieved for an antenna length much shorter than that of a full-length monopole.
The efficiency of the helix can be higher than that of a non-helical structure of the same dimensions. But the gain decrease by 3-5 dB compared to a full size monopole.

22. Antennas for SUNBOW STR, Embedded antennas
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Antennas for SUNBOW STR, Embedded antennas
Antennas for integration inside enclosures
Available from an increasing number of manufacturers
Most often single ended (unbalanced) antennas
A differential to single ended matching network needed
Impedance:
Most often designed for standard impedance 50 
Other characteristics:
Available for all frequency bands 315MHz, 433MHz, 868MHz, 915MHz, 2.4GHz
Some antennas available are dependent on a ground plane, others have a self-contained ground plane
Expensive

23. This is valid for all types of antennas
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Antenna gain vs. Size
Antenna gain increases proportionally with the effective area size of the antenna. So does range!
18x10 mm: G = -27 dB
25x15 mm: G = -22 dB
35x20 mm: G = -18 dB
This is valid for all types of antennas
Remember: +6dB increase of total antenna gain = twice the range!