design for rf efficiency - cambridge wireless · friis’ equation 𝑷𝒓 𝑷𝒕 = 𝒕𝜼𝒕...

BSC Associates Ltdwww.bscassociates.co.uk1

BSC Associates Ltdwww.bscassociates.co.uk

Increased range and longer battery life:

Design for RF efficiency

Brian Collins, BSC Associates Ltd

Friis’ Equation

𝑷𝒓𝑷𝒕

= 𝑮𝒕𝜼𝒕𝑮𝒓𝜼𝒓𝒄

𝟒𝝅𝑭𝑫

𝟐

This is the best you can do

Lower frequencies preferred for longer range (D)

In most practical cases propagation is worse than inverse-squared (3rd power or greater)

Only small antenna gains are possible to allow omnidirectional antennas

Antenna gain and efficiency are both important




How do we measure RF performance?

Efficiency: Radiated power/offered powerTRP: Total radiated powerTIS: Total isotropic sensitivityEIRP: Effective isotropic radiated power

Intermodulation productsNoise and spurious emissions



How do we measure RF performance?

Calibrated anechoic chamber

Sample the radiated field over a sphere containing the DUT

Plot as radiation patterns

Integrate to obtain TRP

Measure sensitivity over sphere to compute TIS

Photo: Cambridge Consultants



A design method…

• Take your well-performing digital platform

• Add a modem (GSM/GPRS/3G, 4G, LORA, Sigfox, NB-IoT, or whatever flavour you prefer...)

• Add an antenna that fits in any space you have left...

— and now you have a modern wirelessly-connected IoT device!



NO!

This almost always leads to a sub-standard device

• limited range and poor battery life

• failure to meet mandatory performance standards

• failure to meet EMC specifications

• … and redesign from the ground up

• late market entry – wastes money



Small antennas

• A standard half-wave dipole in the 868MHz ISM band is ~173 mm long — 63 mm in the 2400 MHz ISM band – bigger than my whole device!

• But we want small neat devices. We can buy much smaller antennas than that!

• So, what’s the trade-off?


8

Small antennas

• The Chu-Harrington limit says that the Q of a linearly polarised antenna is given by:

As antennas are made smaller, the bandwidth shrinks and radiation resistance becomes smaller comparedto the loss resistances that are present. Result is reduced radiation efficiency.

Reduces range, and shortens battery life by excessive ARQ repetition.

where k = 2π/λand a is the radius of the sphere containingthe antenna

BSC Associates Ltdwww.bscassociates.co.uk9 BSC Associates Ltdwww.bscassociates.co.uk

Result

Size (2πr) /λ(r is the radius of the sphere enclosing the antenna)

Efficiency

Fundamental limits in antennasR C Hansen, Proc IEEE, Vol 69 No 1, 1981

Small antennas: • limited efficiency• limited bandwidth• critical tuning

We can trade these off, but we can’thave them all!

So to do more, we have to cheat!

100

40

40

20

10

6

4

2

1

Q

0.1 0.3 0.5 0.7 0.9 1.1 1.3 1.5

5%10%

50%

100%

Narrow band

Wide band



How about these?

Performance Comparison of Fundamental Small-Antenna Designs Best & Hanna, EEE Ant and Prop Mag, Vol. 52, No.1, February 2010

4-arm folded spherical helix Matched spherical resonator

BSC Associates Ltdwww.bscassociates.co.uk11BSC Associates Ltdwww.bscassociates.co.uk

Practical devices

Power supply Processor, memory, clocks,interfaces, sensors…

Modem module

Antenna

Most of the antennas we use are UN-balanced

They have a single terminal and a ground connection

Balanced antennas are almost always too big!!


Practical devices

• It’s the current on the ground-plane that radiates!• The only way to get small size, useful bandwidth & efficiency.

E-field 15mm from the face of the PCB, about λ/2 long (110mm at 900MHz, 40mm at 2.4GHz).

Platform PCB

Current

“Antenna”


Practical devices 2

Same example, double the frequency

Higher frequency, more complex modes

Image: https://openi.nlm.nih.gov/detailedresult.php?img=PMC3136495_pone.0022063.g001&req=4

Then they went and put it down an ‘ole!

Dipole with 2 dBi gain in a concrete footway box embedded in damp soil

8 dBi directivity in a vertical direction, 5 dB loss

14BSC Associates Ltdwww.bscassociates.co.uk

Source: Geoff Hilton, private communication


The lesson

The thing we buy as an antenna doesn’t do much of the radiating. It excites currents on the platform…

… it’s the currents on the platform that do the radiating.


The lessonThe PCB design must facilitate radiation. It must:

• provide a low-loss RF surface path from end to end

• avoid coupling surface RF currents into the internal circuits (this causes loss, not radiation)

• Internal noise currents must not emerge on to the surface or they will couple via the antenna to the receiver.


This is not a new ideaNewman (1979):

“…in use, antennas are often mounted on support structure such as a ship, a tank, a man, or an airplane.

The basic idea here is to think of the small antenna not as the primary radiator, but rather as a probe to excite currents on the support structure.

Since the support structure is often not electrically small, it can be an effective radiator.

Thus the radiation resistance and efficiency of a small antenna can be increased by properly locating it on its support structure…”


Antenna position

The position of an “antenna” on a platform hugely influences its performance

Most small antennas are in the form of monopoles and PIFAs — they dominantly excite electric field and are best placed at the ends of a small platform

Notches and loops are dominantly current drivers and are best when placed near or on the long side of the platform, away from the corners.


Antenna performance

Many antenna vendors over-claim on performance

Any antenna will work to some extent, but real examples show that range can be doubled, or energy consumption reduced by 50-75%, by competent implementation.


Antenna specifications

The specification for any small antenna must be read in conjunction with information about the platform on which it was measured

Most manufacturers don’t provide this information

Without it, it is unlikely that you will achieve the advertised performance, even if you do your design right.


Antenna specifications 2

Many off-board antennas fed with coax cable are not properly balanced and their performance depends critically on the length and routing of the cable

Many work just well enough to avoid complaints

Any small antenna needs to be measured and matched in situ on your device.


IoT devices

The ‘Thing” that we’re trying to connect to the network is often RF-unfriendly:

• its noisy (like an electricity meter)

• There’s no obvious location for RF circuitry and an antenna (beer keg)

• It’s very small

• It’s in a closed metal box

Each of these is likely to need a different and inventive solution


Design guidelines

For an on-board antenna,

• Place the antenna appropriately

• Design the PCB with groundplanes on both external faces

• Tie the groundplanes together with vias all round the board edges

• Mount oblong components aligned with the radiating current


Design guidelines

For an on-board antenna,

• Flood ground everywhere you can, remembering the direction of the radiating current

• Where large holes in the groundplane are inevitable, consider using screening cans to maintain continuity for radiating currents

• Provide for a matching network and the components recommended by the RF chip vendor.


A bonus

If we follow this kind of design method

• We maximise RF efficiency of the device

• We screen much of the noise and spurious that may otherwise be radiated

• We provide a clean environment for the received signals, so sensitivity is enhanced.

• We optimise energy efficiency


IoT applications are very challenging

o Adding a communications device onto a platform that has a completely unrelated primary function

o Devices go in poor radio locations (cellar, lift room, meter closet, footway box)

o Network coverage may be marginal

o There may be a coverage SLA with financial penalties

o We need all the performance we can get

Design right, design once!

design for rf efficiency - cambridge wireless · friis’ equation 𝑷𝒓 𝑷𝒕 = 𝒕𝜼𝒕...

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