PA-25 Antenna Integration Application Note Version 2

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CONTENTS 1. BASICS 2. APPLICATIONS 3. GROUND PLANE EFFECTS 4. IMPEDANCE 5. BANDWIDTH 6. VSWR 7. GAIN 8. EFFICIENCY 9. POLARIZATION 10. ADVANTAGES 11. MOUNTING 12. ENVIRONMENTAL CONSIDERATIONS 13. TUNING 14. ISOLATION 15. TRANSMISSION LINE 16. MAJOR NO-NO’S

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1. BASICS

The PA-25 is a monopole PIFA SMD antenna.

Knowledge of the best environment to use such an antenna is crucial to optimizing it’s performance in the device.

The PA-25 Planar Inverted F Antenna (PIFA) consists of a PIFA antenna with rectangular planar element located above a ground plane, a short circuiting plate, and a feeding mechanism for the planar element.

The PIFA is a variant of the monopole where the top section has been folded down so as to be parallel with the ground plane. This is done to reduce the height of the antenna, while maintaining a resonant trace length. This parallel section introduces capacitance to the input impedance of the antenna, which is compensated by implementing a short-circuit stub. The stub’s end is connected to the ground plane via the short circuiting plate on the edge of the antenna, whose function is to extend the bandwidth of the PIFA.

The ground plane of the antenna plays a significant role in its operation. Excitation of currents in the PIFA causes excitation of currents in the ground plane. The resulting electromagnetic field is formed by the interaction of the PIFA and an ‘image’ of itself below the ground plane.

Its behaviour as a perfect energy reflector is consistent only when the ground plane is infinite or very much larger in its dimensions than the monopole itself. In practice the groundplane area is of comparable dimensions to the monopole and forms the ‘image antenna’ which is one half of the dipole.

The antenna/groundplane combination will behave as an asymmetric dipole, the differences in current distribution on the two-dipole arms being responsible for some distortion of the radiation pattern.

This design application note is intended to help the antenna integrator understand the relevant parameters affecting the antenna performance. Taoglas recommends that the integrator strictly follow the guidelines in this application

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note, upon your device prototype completion Taoglas offers further optimization by custom tuning and testing service of the antenna in your device.

2. APPLICATIONS

The PA-25 is suitable for mobile cellular applications where internal antennas are required and where it can be placed on the shorter side of the device main-board with enough clearance to radiate efficiently.

3. GROUND PLANE EFFECTS

In general for a PIFA, the required PCB ground plane length should be at least one quarter ( λ/4) of the operating wavelength.

o If the ground plane is much longer than λ/4, the radiation patterns will become increasingly ‘multilobed’. o On the other hand, if the ground plane is significantly smaller than λ/4, then tuning becomes increasingly difficult and the overall performance degrades.

The optimum location of the PIFA in order to achieve an omni-directional far-field pattern and 50 Ω impedance match was found to be close to the edge of the Printed Circuit Board.

Basically in terms of PA-25 performance a larger ground-plane improves return loss and efficiency. The length of the ground-plane is also the critical dimension. The longer the better.

A matching circuit can compensate by shifting the frequency up or down but it will only improve performance by a few percent.

Test data below shows the affect of different ground plane lengths on the antenna in terms of antenna efficiency on all frequencies in free-space.

Reference efficiency data with different ground plane length :

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9.5cm 8.5cm 7.5cm 6.5cm 5.5cm 4.5cm 800MHz 51% 33% 24% 18% 11% 10% 815MHz 57% 38% 27% 20% 12% 10% 830MHz 63% 44% 33% 24% 14% 13% 845MHz 70% 51% 39% 29% 17% 15% 860MHz 70% 53% 42% 32% 20% 17% 875MHz 71% 57% 45% 35% 22% 19% 890MHz 65% 53% 42% 33% 20% 17% 905MHz 65% 53% 43% 33% 19% 16% 920MHz 66% 55% 46% 36% 22% 19% 935MHz 68% 57% 48% 38% 23% 20% 950MHz 60% 50% 42% 34% 22% 19%

965MHz 67% 55% 46% 38% 24% 21% 980MHz 65% 54% 45% 37% 23% 20% 995MHz 63% 51% 43% 34% 22% 19% 1010MHz 64% 52% 43% 35% 22% 20% 1025MHz 65% 52% 42% 34% 22% 20% 1040MHz 57% 45% 37% 29% 18% 16% 1055MHz 58% 45% 36% 28% 18% 16% 1070MHz 59% 46% 37% 29% 18% 16% 1085MHz 54% 43% 34% 26% 16% 14% 1100MHz 52% 41% 32% 24% 14% 13% 1115MHz 49% 39% 30% 22% 13% 11% 1130MHz 43% 35% 27% 20% 11% 10% 1145MHz 38% 32% 24% 18% 10% 8% 1160MHz 32% 28% 22% 16% 9% 7% 1175MHz 28% 25% 20% 14% 8% 7% 1190MHz 26% 24% 19% 14% 7% 6% 1205MHz 21% 21% 17% 12% 7% 6% 1220MHz 14% 15% 12% 9% 5% 4% 1235MHz 13% 14% 12% 9% 5% 4% 1250MHz 11% 12% 11% 8% 4% 4% 1265MHz 10% 11% 10% 8% 4% 4% 1280MHz 8% 9% 9% 7% 4% 3% 1295MHz 8% 9% 9% 7% 4% 4% 1310MHz 6% 7% 7% 6% 4% 3% 1325MHz 5% 6% 6% 5% 3% 3% 1340MHz 5% 5% 5% 5% 4% 3% 1355MHz 5% 5% 5% 5% 4% 4% 1370MHz 5% 5% 5% 5% 4% 4% 1385MHz 5% 5% 5% 5% 5% 5% 1400MHz 5% 5% 5% 5% 5% 5% 1415MHz 5% 5% 5% 5% 5% 6% 1430MHz 6% 6% 6% 6% 6% 6% 1445MHz 6% 6% 7% 7% 6% 6% 1460MHz 7% 7% 8% 9% 7% 7% 1475MHz 8% 9% 10% 11% 9% 8% 1490MHz 9% 11% 12% 14% 11% 10% Copyright 2009 All Rights Reserved. Taoglas Limited Unit 3, Enniscorthy Technology Centre, Mile House Road, Enniscorthy, Co. Wexford, Ireland. [email protected] www.taoglas.com the antenna solutions provider

1505MHz 11% 13% 15% 17% 13% 11% 1520MHz 12% 14% 17% 20% 16% 13% 1535MHz 13% 15% 18% 21% 18% 15% 1550MHz 15% 17% 21% 25% 22% 17% 1565MHz 19% 21% 25% 30% 27% 21% 1580MHz 22% 24% 28% 34% 33% 25% 1595MHz 25% 28% 32% 39% 39% 30% 1610MHz 28% 31% 35% 42% 43% 34% 1625MHz 31% 34% 38% 46% 48% 38% 1640MHz 36% 38% 42% 50% 54% 44% 1655MHz 41% 42% 47% 55% 61% 50% 1670MHz 45% 47% 51% 59% 66% 56% 1685MHz 47% 49% 52% 60% 66% 56% 1700MHz 51% 52% 56% 63% 70% 59% 1715MHz 57% 58% 63% 70% 76% 65% 1730MHz 51% 52% 55% 61% 65% 57% 1745MHz 53% 54% 58% 63% 67% 59% 1760MHz 61% 62% 67% 72% 74% 65% 1775MHz 60% 62% 66% 71% 73% 64% 1790MHz 59% 60% 64% 70% 70% 61% 1805MHz 63% 65% 71% 77% 75% 65% 1820MHz 61% 63% 69% 75% 74% 63% 1835MHz 61% 63% 68% 75% 73% 62% 1850MHz 62% 65% 70% 77% 75% 64% 1865MHz 63% 66% 72% 80% 77% 64% 1880MHz 63% 66% 73% 81% 78% 65% 1895MHz 62% 65% 72% 80% 78% 65% 1910MHz 60% 62% 68% 77% 75% 62% 1925MHz 58% 60% 66% 75% 74% 60% 1940MHz 56% 58% 64% 72% 73% 59% 1955MHz 54% 55% 61% 69% 71% 58% 1970MHz 53% 53% 58% 67% 70% 57% 1985MHz 52% 51% 56% 64% 69% 56% 2000MHz 51% 50% 54% 62% 68% 55% 2015MHz 49% 47% 51% 58% 64% 53% 2030MHz 50% 48% 51% 59% 66% 54% Copyright 2009 All Rights Reserved. Taoglas Limited Unit 3, Enniscorthy Technology Centre, Mile House Road, Enniscorthy, Co. Wexford, Ireland. [email protected] www.taoglas.com

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2045MHz 48% 45% 47% 54% 62% 52% 2060MHz 49% 45% 47% 53% 62% 53% 2075MHz 53% 48% 49% 56% 67% 57% 2090MHz 54% 48% 49% 56% 66% 58% 2105MHz 56% 49% 49% 56% 67% 59% 2120MHz 58% 50% 50% 56% 68% 61% 2135MHz 58% 51% 50% 55% 66% 61% 2150MHz 62% 54% 52% 57% 69% 64% 2165MHz 66% 57% 55% 59% 72% 68% 2180MHz 65% 56% 53% 56% 68% 66% 2195MHz 67% 59% 55% 58% 68% 68% 2210MHz 67% 60% 56% 57% 66% 66% 2225MHz 66% 60% 56% 57% 63% 64% 2240MHz 63% 59% 55% 55% 60% 61% 2255MHz 62% 59% 56% 55% 58% 59% 2270MHz 63% 61% 58% 57% 57% 58% 2285MHz 58% 58% 56% 54% 52% 52% 2300MHz 58% 59% 58% 57% 51% 52%

4. IMPEDANCE

RF circuits in mobile devices should be designed for a 50 Ohm characteristic impedance at the source (RF module), transmission line (PCB trace or coax cable) and load (antenna). In practice sometimes the characteristic impedance of the circuit is not 50 Ohms at different transmitting and receiving bands. This necessitates the antenna impedance to be changed to match the actual characteristic impedance of the circuit. For a cellular antenna this is most effective when tuning the antenna at the over the air active testing stage in a 3D Copyright 2009 All Rights Reserved. Taoglas Limited Unit 3, Enniscorthy Technology Centre, Mile House Road, Enniscorthy, Co. Wexford, Ireland. [email protected] www.taoglas.com the antenna solutions provider radiation chamber when the device is turned on and using the TRP and TIS numbers as guide to find the best impedance match for the antenna.

5. BANDWIDTH

Bandwidth is defined as the frequency band below -10dB return loss.

For hexa-band antennas in general in small mobile devices it is more realistic to accept a minimum of -5dB return loss at the band edges for the targeted application bands (for example GSM 850/900/1700/1800/1900 UMTS 2100). A return loss of below -10dB is targeted for the centre of the bands.

The size of the ground-plane and clearance to metal defines the return loss of the PA-25. The longer the main-board the deeper the return loss. Also if you go below 20mm in clearance to metal the return loss degrades the closer the metal is to the antenna.

6. VSWR

In principle the target is to be below 2.0, ideally below 1.5. In practice in multi- band and challenging environments it may go to 3.0.

7. GAIN

The gain of the antenna is closely linked to the surface area or volume of the antenna. The larger the surface area or volume of the antenna the higher the gain. The ideal target for a cellular band antenna in a mobile device which needs omni-directional radiation characteristics is 0dBi. Higher gain skews the radiation pattern in some directions and reduces the gain in another area of the pattern.

Care must be taken that clearances of minimum 10mm are kept from other metal components in the device or metallized substances which will absorb or reflect the electro-magnetic radiation, substantially reducing the gain. The larger the clearance, the better the radiation characteristics of the antenna. We recommend 20mm or more for best gain and radiation efficiency. Copyright 2009 All Rights Reserved. Taoglas Limited Unit 3, Enniscorthy Technology Centre, Mile House Road, Enniscorthy, Co. Wexford, Ireland. [email protected] www.taoglas.com

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8. EFFICIENCY

Efficiency is the passive antenna measurement of choice for an omni-directional antenna for cellular mobile communication systems. These days it comes down to a simple maxim- the higher the efficiency the better the antenna.

In general 20% efficiency or higher at all bands is good enough for worldwide mobile use. However it is recommended to always aim for 40%+ in efficiency if this antenna must pass operator approvals after PTCRB in the US, especially where AT&T approval is required. In this case a ground-plane length of 85mm or above would be needed. Higher efficiency also translates into better device performance in weak signal areas and lower battery consumption.

Efficiency of the antenna directly relates to the TRP/TIS results of a device in OTA testing if the module has a 50 Ohm impedance. However it is only one factor and care must be taken to not single out antenna efficiency as the only reason why a device does not meet certain TRP/TIS targets. Impedance mismatches, conducted power from the module and noise can sometimes have a larger effect on TRP/TIS than the antenna efficiency itself.

9. POLARIZATION

Polarization describes the orientation of the wave oscillation. All our cellular and broadband antennas are linearly polarized to most efficiently match with the signals broadcast and the antennas mounted on cellular base-stations. Whether it is horizontally or vertically polarized depends just depends on how it is mounted from your frame of reference. Standing directly in front of the antenna the linear polarization is horizontal if the antenna is placed in a horizontal position and vertical if the antennas are placed in a vertical position. In practice the radiation emitted and received by internal antennas will be to some degree cross- polarized due to reflections from the environment and scattering in the atmosphere.

10. ADVANTAGES of the PA-25 are as follows;

o Compact volume, minimum footprint - It can be placed into the housing of the mobile/handheld device, unlike most whip/rod/helix antennas. o It can be mounted directly on edge of device main-board o Transmission losses are kept to a minimum Copyright 2009 All Rights Reserved. Taoglas Limited Unit 3, Enniscorthy Technology Centre, Mile House Road, Enniscorthy, Co. Wexford, Ireland. [email protected] www.taoglas.com the antenna solutions provider

o It exhibits a reduction in backward radiation toward the user’s head compared to other antenna technologies, thus minimizing the electromagnetic wave power absorption (SAR) which in turn enhances the antenna’s performance. o Achieves moderate to high gain in both vertical and horizontal polarization planes. This feature is very useful in certain wireless communications where the antenna orientation is not fixed and the reflections or multipath signals may be present from any plane. In those cases the important parameter to be considered is the total field strength, that is the vector sum of the signal from the horizontal and vertical polarization planes at any instant in time. o Labor saving SMT – also ensures higher quality yield rate o No antenna tooling cost for customer o Tunable antenna

11. MOUNTING

The antenna is designed for surface mount using standard reflow soldering. The antenna is packaged in tape and reel format. The antenna has passed high temperature reflow tests (260 degrees peak at 41 seconds).

Ideally the antenna should be mounted on the shorter edge of a rectangular board.

12. ENVIRONMENTAL CONSIDERATIONS

Close proximity to components or housing affects the electrical performance of all antennas. When placed on a non-conductive area of the board, in most cases ideally there should be clearance of 20mm in all directions from the board/housing for maximum efficiency. A reduction in the efficiency of the antenna efficiency and shift in tuned frequency will be observed if these clearances are not adhered to. Proximity effects will also have an adverse effect on the radiation pattern of the antenna. Device housings should never be metal or have metal materials

13. TUNING

PIFA antennas are less susceptible to detuning from the close environment than other antennas due to their design. However tuning optimization can be carried out by;

1. Appropriate choice of matching circuit

If the customer can give the housing and reference PCB to Taoglas then Taoglas can carry out the necessary S11 response measurements in both magnitude and Smith Chart format of the complete system. The necessary impedance matching circuit should be designed on the customer side if he/she has access to a software modeling tool which simulates the system. The matching circuit values obtained from this exercise can then be employed and adjusted via trial and error to obtain the optimal 50 Ohm match at the frequencies of interest. It should be noted that the impedance matching can improve the response of the antenna at certain frequencies (or bands of frequencies in the case of wideband matching circuits) but a reduction in the response at other frequencies may be observed.

2. Capacitor or Inductor

The PA-25 needs a lumped component at to obtain a 50 Ohm impedance match across the frequency bands. We recommend to start with a 2nH inductor here. Further optimization could be done as part of a matching circuit.

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3. Special Transmission Line

Please note in the circled area that the connection between the antenna outside the grounded area is not a CPW neither is it a traditional micro-strip with ground on the other side of the board as a non-standard impedance is required to make the distributed component act as a lumped component. Do not add ground on left side of this line to try to make it into a CPW or the antenna will be compromised.

4. Other Techniques for tuning

Lower Frequency

A. Lengthen distance of PIFA to ground B. Cutting a bridged meander line on side of antenna will lengthen wavelength and reduce frequency. C. Larger surface area on contact pad will lengthen effective wavelength and reduce frequency Note B and C techniques will improve the performance of the antenna at most more than a few percent and are not as important as no.1 rule, to increase clearance to metal components from antenna.

Higher Frequency Copyright 2009 All Rights Reserved. Taoglas Limited Unit 3, Enniscorthy Technology Centre, Mile House Road, Enniscorthy, Co. Wexford, Ireland. [email protected] www.taoglas.com the antenna solutions provider

A. New screen-print would be needed on manufacturing side. But this is a very expensive process so we designed the antenna so that in practice the antenna should not need to be tuned to higher frequency.

14. ISOLATION

Isolation is a measure of coupling between two different antennas. In general care should be taken to ensure isolation between all antennas in a device. The easiest and by far most effective method is to place them as far away from each other as possible.

For example in a CDMA diversity antenna plan the target is to get -10dB isolation between the main and the auxiliary antenna. Greater isolation can be achieved by using different polarizations on the two antennas. For example the main antenna has horizontal linear polarization and the auxiliary antenna has vertical linear polarization. In practice this is difficult for omni-directional internal cellular antennas as there is cross-polarization of the waves occurring. So the normal solution is to keep the distance from both antennas as far away as possible.

The two antennas cables should not cross over or come close to the other’s antenna.

Testing is carried by sending a signal in one antenna and measuring the power of the signal at the other antenna. There should be a 10dB or more difference between the transmit and the receive signal. The easiest method is to keep moving the two antennas farther from each other until the target isolation is achieved.

15. TRANSMISSION LINE

A co-plane waver guide (CPW) should be designed for 50 Ohm impedance for that board. Note however the section on special transmission line above.

16. MAJOR NO-NO’S

DON’T LEAVE COPPER GROUND-PLANE AROUND SIDE OF ANTENNA DON’T PLACE COMPONENTS ON SAME EDGE OF ANTENNA DON’T PLACE METAL COMPONENTS BESIDE ANTENNA Copyright 2009 All Rights Reserved. Taoglas Limited Unit 3, Enniscorthy Technology Centre, Mile House Road, Enniscorthy, Co. Wexford, Ireland. [email protected] www.taoglas.com the antenna solutions provider