GPS antenna connection
Design guide
application note
© GNS-GmbH 2011
V1.2, May. 13th 2013
1
The GPS antenna connection
Design guide
V1.2
This design guide demonstrates several typical application circuits and should help the application
engineer to achieve the best GPS performance.
www.forum.gns-gmbh.com .
GNS Global Navigation Systems GmbH
Adenauerstrasse 18
D – 52146 Würselen
Germany
www.gns-gmbh.com
info@gns-gmbh.com
GPS antenna connection
Design guide
application note
© GNS-GmbH 2011
V1.2, May. 13th 2013
2
Index
1 Introduction ......................................................................................................................... 3
2 Antenna design .................................................................................................................... 4
2.1 Type of antenna ........................................................................................................... 4
2.1.1 Chip antenna ............................................................................................................ 4
2.1.2 Patch antenna ........................................................................................................... 5
2.1.3 Active patch antenna ................................................................................................. 6
2.1.4 Other antenna types .................................................................................................. 7
2.2 Antenna matching ......................................................................................................... 7
2.2.1 General .................................................................................................................... 7
2.2.2 Transmission line type: Microstrip ................................................................................ 8
2.2.3 Transmission line type: Coplanar Waveguide ................................................................ 9
2.2.4 Evaluating matching network values .......................................................................... 10
2.3 Antenna placement and directivity ................................................................................ 10
2.4 Indoor reception ......................................................................................................... 11
2.5 External LNA .............................................................................................................. 11
2.6 Noise impact at antenna input ...................................................................................... 12
3 Available evaluation kits .................................................................................................... 13
4 Document revision history ................................................................................................. 13
GPS antenna connection
Design guide
application note
© GNS-GmbH 2011
V1.2, May. 13th 2013
3
1 Introduction
The “GPS antenna connection design guide” shows the application engineer a practical and simple way
to implement RF transmission lines and RF impedance matching for his GPS application.
RF basics are required to follow up this documentation.
It illustrates an overview and comparison about the functional parameters at the market available
various GPS antenna types.
Discussed are several noise impacts, which can be generated at a mixed-signal application PCB, and the
processes, how to verify and reduce or eliminate them.
GPS antenna connection
Design guide
application note
© GNS-GmbH 2011
V1.2, May. 13th 2013
4
2 Antenna design
2.1 Type of antenna
Antenna design is critical because it determines the performance of the entire product.
A poor antenna design will ruin most of the efforts that have been made in receiver development.
The main reasons for a bad performing antenna are: Mismatched antenna frequency and bandwidth,
mismatched antenna impedance and noise impacts.
The TC6000 series supports both active and passive antennas. It includes an on-chip LNA (Low-Noise-
Amplifier) and an on-module SAW filter. Therefore, a passive antenna can be directly connected to the
RF_IN input without any further LNA or filters.
However, a good compromise must be found between small size and antenna performance.
Assuming, that bandwidth and matching is optimized, a bigger (larger area) antenna can generally
deliver a better signal than a small one.
2.1.1 Chip antenna
Chip antennas are used for applications which offer very
small space, for example mobile phones. Generally, chip
antennas are lower in performance due to their small
size and linear polarization. Although chip antennas are
very small, they are highly dependant on a ground plane
and on an "isolation distance" (typically 6mm) where no
other components should be placed. Most manufacturers
provide designs guides for their GPS chip antennas.
In high density applications like phones and other
handhelds, the reception of a chip antenna must be
carefully tested in prototype stage, because chip
antennas are quite sensitive regarding the components
or mechanical parts in their direct proximity.
Chip antennas are not recommended for applications
that must reliably work under critical conditions.
For example, the small chip antenna, that is used on the
TC6000GN-P1_EM1 board, performs sufficiently under
open sky conditions, but performance will degrade, when
the receiver is used in deep urban canyons or indoor.
TC6000 Series
GPS
ENGINE
LNA
SAW
GPS antenna connection
Design guide
application note
© GNS-GmbH 2011
V1.2, May. 13th 2013
5
2.1.2 Patch antenna
A common patch antenna of 25mm by 25mm mounted on a metal ground plane is the best choice
regarding costs and performance. It should be used, whenever the space conditions allow. The size of
the ground plane will essentially influence the performance of the antenna. Best performance (a gain of
+4..5 dBi) can be achieved with larger ground plane areas, e.g. 70 by 70mm, while a 30mm by 30mm
ground plane will result in a gain of ~1 dBi. The patch antenna should always be placed in the
geometrical center of the ground plane. No components should be placed and no tracks should be
routed in the area of the ground plane or the patch antenna.
2.1.2.1 Patch antenna size
GPS patch antennas are available in different sizes from 6x6mm2 to 25x25mm2 and different thickness.
While the 25x25x4mm2 size is quite robust and high perfoming, the smaller sizes are lower in
performance and also more critical regarding their operating parameters.
For a small patch antenna, the resulting center frequency and gain must be carefully observed. For this
reason, patch antenna manufacturers offer their products with different center frequencies which allows
to choose the antenna that works on the correct frequency together with the customer application.
The center frequency alignment should be made using a vector network analyzer (VNA) measuring the
return loss (S11).
The following two graphs, as an example, show the relationship between the GND size area and the
center frequency as well as the GND size vs. the antenna gain of a typical GPS patch antenna.
frequency vs. ground plane size gain vs. ground plane size
GPS antenna connection
Design guide
application note
© GNS-GmbH 2011
V1.2, May. 13th 2013
6
2.1.3 Active patch antenna
In use cases where the antenna will be installed separately (more than 10cm away) from the receiver,
the antenna should be equipped with an internal LNA, that is placed near to the antenna. The LNA will
effectively compensate for cable losses and will minimize the impact of noise.
Such active GPS antennas which contain a LNA and an additional SAW filter are available from many
sources on the market.
They require a power supply to power the internal LNA. This power supply is fed over the same wire as
the RF (a "phantom" supply) and must be decoupled from the RF_IN line. Please use the following
simple circuit to feed-in the antenna supply voltage. The typical power consumption of active antennas
is 3 to 30mA, the voltage 1.5 to 5 Volts.
Inductor : L=270nH Capacitor : C=100pF, use low parasitic components. DC voltage : see active antenna spec
The inductor will block the RF against the DC source, the capacitor avoids DC loading of the GPS_RF pin
TC6000GN
Antenna
DC supply
L
C
GPS_RF
LNA
SAW
cable
GPS antenna connection
Design guide
application note
© GNS-GmbH 2011
V1.2, May. 13th 2013
7
2.1.4 Other antenna types
There are some more antenna types available which may be taken into consideration, but are not
discussed here because they are rarely used and in special applications, only.
The following table lists some properties of the antenna types.
Type
Performance
Polarisation
Directivity
Costs
Space requirements
implementation
patch
Good
Circular
Hemispheric
Medium
Medium
Easy
Chip
Medium
Linear
Hemispheric,
sensitive for
disturbances
Low
Very low, but larger
area required for
good performance
Easy , medium
critical
Helix
Good
Circular
Omni
High
Large, best outside of
the device
Medium
PCB
Low
Linear
Critical
Very low
PCB space, virtually
no hight
Very critical
Fractal
Medium to low
Linear
NN
Low
medium
Medium
F-antenna
Medium
Linear
Omni
Low
Medium
Critical
Loop antenna
Good
NN
Double lobe
Medium
For external use
w/cable, large
Easy
Active patch
Very good
Circular
Hemispheric
High
For external use
w/cable, large
Very easy
Dipole
Medium to
good
Linear
Double lobe
Low
For external use
w/cable, large
Critical
2.2 Antenna matching
2.2.1 General
All common GPS antennas are electrically prepared to drive a 50 Ohms load. Therefore the connection
(cable or PCB track) and the input of the receiver should be 50 Ohm as well. A deviation will result in
power loss and in a possible shift of center frequency.
The antenna must be impedance matched to the input of the TC6000 series, to the PCB, the wiring and
the enclosure. The antenna must be impedance matched to the RF input of the TC6000 series for every
application. Proper matching will optimize the transmission of antenna input power to the receiver input
and therefore improve the overall performance.
Furthermore, please verify your design for correct frequency matching. GPS antennas are relatively
narrow band, so there's a risk to tune the antenna out of the band center.
GNS offers support for the antenna matching and provides an evaluation board for a passive GPS chip
antenna with an SPDT switch IC on board, which switches between the passive GPS chip or an
active/passive GPS patch antenna, GNS part #4037735104549 “GPS Antenna switch evaluation
module”.
GPS antenna connection
Design guide
application note
© GNS-GmbH 2011
V1.2, May. 13th 2013
8
2.2.2 Transmission line type: Microstrip
To connect a GPS antenna it is required to use a 50Ohm impedance transmission line between the RF
input of the TC6000GN and the antenna. These impedance lines are GND related transmission lines,
which have to be calculated. A free RF software tool from Agilent Technologies “AppCAD” can be used.
To setup up calculating transmission line, the “εr -relative Permitivity” of the PCB material, for example:
εr =4.6 for FR-4 material, and the distance between signal/top layer and GND layer (layer2 or bottom)
“H” must be known. These values are provided by PCB manufacturer.
Example: H=0,2mm; εr = 4,6; T=35um copper cladding
Using a microstrip line it is recommended to have a distance between the microstrip line and enveloping
GND plane of minimum 3xW (Rule of thumb)!
If the GPS patch antenna will be on the same layer as the GPS module or thickness of the Microstrip
line will be too thick, because of the PCB build-up or relative Permitivity, a “Coplanar waveguide line
(CPW)” should be used, refer to GNS part #4037735104549 “GPS Antenna switch evaluation module”.
3. Insert: H
2. Select: Length Units
4. Insert: T
5. Insert start value (e.g. 1mm): W (thickness Microstrip line)
6. Calculate Microstrip line Impedance
7. Read out: Impedance. Repeat iterative step 5 to 7 till 50Ohm is reached
1. Choose PCB material
GND
GND
W
Minimum: 3xW
GND Via stitching
GPS antenna connection
Design guide
application note
© GNS-GmbH 2011
V1.2, May. 13th 2013
9
2.2.3 Transmission line type: Coplanar Waveguide
Example: H=1,6mm; εr = 4,6; T=35um copper cladding
Designing the transmission line between the RF input of the GPS module and the GPS antenna is
finished. To match the GPS antenna to the application board, a matching network is required and
should be placed as near as possible to the feed point of the GPS antenna. Three lumped elements
should be placed as a PI-network closely to the GPS antenna feed point. These three elements are
placeholders for the antenna matching network, which has to be simulated or calculated and verified by
measurement.
Repeat steps 1 to 5 shown at example above
6. Insert start value (e.g. 1mm): W (thickness coplanar waveguide line)
7. Insert start value (e.g. 0.5mm): W (thickness coplanar waveguide line)
8. Read out: Impedance. Repeat iterative step 6 to 7 till 50Ohm is reached
GPS Patch antenna
Pi-Matching Network
GND
Measurement Reference Plane
Coplanar Waveguide
Antenna Feed Point
GPS antenna connection
Design guide
application note
© GNS-GmbH 2011
V1.2, May. 13th 2013
10
2.2.4 Evaluating matching network values
The following steps should be done for evaluating the matching network components
1. Measure S-Parameter-set using a network analyzer (VNA). Measurement reference plane should
be at the first shunt element (marked in above figure)
2. Use the network analyzer measurement results file (*.s2p) as input for a RF simulation software
(e.g. Agilent ADS, Ansoft Designer or similar), which supports an impedance matching tool.
3. Selecting the same frequency grid in the simulation as in the s2p file will provide the best
simulation result.
4. After identification of the two matching elements and their values, they should be soldered on
the application board carefully at the appropriate positions
5. Next, verify the return loss (S11) again using a VNA. The return loss should be -10dB or better
at 1575.42 MHz.
6. In most cases, a fine tuning of the matching components will be necessary to tune the point of
lowest return loss to the exact frequency. This tuning should be made by trying the next value
from the E12 series and repeating the return loss measurement.
The graph shows a measurement of return loss with a good result of more than -16dB.
-24
-21
-18
-15
-12
-9
-6
-3
0 0
1 of 4 (Max)
Pwr
-20 dBm
Ch1
Start
1.5 GHz
Stop
1.65 GHz
Trc1
Mem2[Trc1]
S11
S11
dB Mag
dB Mag
3 dB /
3 dB /
Ref 0 dB
Ref 0 dB
Cal
Invisible
•
Mkr 1
1.575420
GHz
-16.502
dB
S11
Mkr 1
Date: 20.DEC.2011 12:48:41
If no RF software tool is available, the impedance measurement procedure can be done alternatively by
measuring impedance at the reference line at 1575.42MHz. Then insert normalized impedance to a
Smith Chart and calculate the two elements there. The details of this procedure cannot be explained in
detail here, therefore please refer to specialized literature.
The GPS antenna matching is finished.
2.3 Antenna placement and directivity
The GPS system is based on a system of satellites, which are moving across the sky. For a good
performance of the receiver, the viewing angle of the antenna should be as wide as possible to be able
to track the biggest number of satellites. Ideally it should cover the whole hemisphere.
For this reason, the directivity of the antenna, the placement of the antenna and the placement of the
whole receiver should allow a free view to the whole hemisphere. When moving through urban canyons
or mountainous landscapes, a wide viewing angle will also optimize the view to the available satellites.
Furthermore, any material between the antenna and the satellites has more or less influence on the
signal. Metal, metalized glass and thick stone will block the signals almost totally, while plastics or glass
cause some attenuation.
GPS antenna connection
Design guide
application note
© GNS-GmbH 2011
V1.2, May. 13th 2013
11
2.4 Indoor reception
Modern high-sensitivity GPS receivers like the TC6000 series allow indoor operation in many cases.
However, it's important to know about the shortcomings of indoor GPS:
1. The indoor signal has high directivity. In many situations, the signals will come in through a
window and therefore the available viewing angles will be quite narrow. Since the satellites move
over the sky within ~6hrs, the signals will be lost when the satellites leave the viewing angle.
Maybe, new satellites will come into view, but in many scenarios the fix may get lost.
2. After turning on the GPS receiver, gaining a position solution depends on receiving the data
telegrams from the satellites. This process needs much more signal strength than the tracking
mode does. Therefore, time to first fix (TTFF) may be very long or the fix may be impossible.
3. When using GPS indoor in an urban environment, there might be a lot of signals that are
reflected from other buildings. These signals will have longer signal paths and therefore, the
calculated position will be moved away from the real position. Reflections can easily generate
position errors of 50m or more.
For indoor applications, we recommend to use an outdoor external antenna whenever possible.
Indoor antennas are always likely to cause uncertain (or no) position fixes.
2.5 External LNA
TC6000 series includes a complete RF frontend with integrated LNA and can be directly connected to a
passive antenna.
However, in some applications an external LNA can improve the overall performance. An external LNA
can be beneficial if the antenna tracks or cables have some loss or if the antenna has low gain.
Furthermore, a very low noise LNA might improve the GPS reception by lowering the noise figure of the
entire input stage.
When adding an additional LNA special care should be taken to avoid any saturation of the input.
GPS antenna connection
Design guide
application note
© GNS-GmbH 2011
V1.2, May. 13th 2013
12
2.6 Noise impact at antenna input
GPS receivers work at very low signal levels down to -162dBm which allows to track a position in very
demanding environments like urban canyons and indoor locations.
Therefore it is very important to avoid any noise signal impact on the antenna or the RF_INPUT tracks.
Thanks to advanced filtering and signal processing, this problem is primarily the result of noise that is in
band. For all out-band impacts there's a good suppression because of using a internal SAW filter on all
TC6000 series modules.
On the other hand, if there is in-band noise, the receiver may be extremely sensible. Even a very low
noise level (that might be really difficult to locate by Spectrum Analyzer measurements without external
high gain LNA) can have a strong impact on GPS performance.
If an in-band noise source is identified in your application, better try first to remove the source of
problem or move the frequency value away instead of trying to filter or to shield the noise.
If you observe a bad GPS sensitivity during development, please check the following:
Are there any frequencies at 1575.42MHz ±10 MHz in the system?
Try to locate the source. Move the source frequency away whenever possible.
Are there any (digital) oscillators (e.g. from the microcontroller) that have harmonics on GPS
frequency? Even high order harmonics of clocks can disturb GPS.
use a different clock frequency. Decouple RF GND and analog/digital GND.
Are these oscillators of R/C type? Such oscillators may move their working frequency with
temperature. A high order harmonic of a R/C clock may "move" into the GPS band by thermal
effects. Furthermore, R/Cs have big tolerances in initial frequency.
A safe way is to use an Xtal instead of R/C oscillator for your microcontroller.
Are there any broadband noise sources in the system? e.g. Switch-mode-voltage regulators ?
Try to optimize noise sources. Shielding or improved PCB tracks should help
Last but not least:
In case of unknown noise sources : check your laboratory for noise sources. Computers and even
measurement equipment may be sources of noise.
GPS antenna connection
Design guide
application note
© GNS-GmbH 2011
V1.2, May. 13th 2013
13
3 Available evaluation kits
type
description
picture
available
from
GPS Antenna switch evaluation
module
A GPS antenna
evaluation board to
switch between a
passive GPS chip
antenna and an
active/passive patch
antenna.
GNS
GNS part#:
4037735104549
4 Document revision history
date
author
comment
12/20/2011
MR
Initial document V 1.0
12/23/2011
PS
Reviewed V1.1
05/13/2013
MR
Document structure reworked V1.2
GNS GMBH 2011
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REPRODUCTION IN WHOLE OR IN PART IS PROHIBITED WITHOUT THE PRIOR WRITTEN CONSENT OF THE COPYRIGHT OWNER