1JAG. Jan 2005

HF Antenna Configuration

2JAG. Jan 2005

HF Antenna Configuration

Background– HF tags were developed with particular applications in mind

■ Express parcels and packages

■ Airline baggage.

– These applications had certain performance criteria■ A reading zone 1m × 1m × 1m (approx 39” cubed)

■ Conveyor belts moving up to 3m/s (600 ft/sec)

■ Multiple tags in the field (around 5)

– These criteria should be kept in mind when exploring more ambitious applications (greater reading distances/ more tags in the field)

3JAG. Jan 2005

HF Antenna Configuration

What if I want more than 1m range?

– As a general rule, the larger the antenna loop, the longer the reading range.

■ A 1m × 1m Antenna can achieve up to 1m range

■ If the antenna gets much larger, the range drops again

■ These large antennas may exceed legal power limits.

■ Large antennas have a low field density and perform poorly when there are large numbers of tags in the field.

– It is better to use opposing pairs of antennas■ They can be driven from a single reader using a splitter

■ They can be optimised for tags parallel or at right angles to the antenna

■ They can be ‘rotated’ to read tags in more than one orientation

4JAG. Jan 2005

HF Antenna Configuration

What if I want to read lots of tags in the field?

– As a general rule, the smaller the antenna loop, the better the ability to read closely packed tags

■ When tags are close together they►De-tune each other►Absorb more of the available RF energy

■ Smaller antennas can ‘illuminate’ tags that are closer together

– If pairs of opposing antennas are to be used■ Antennas on splitters develop a stronger field (Both driven)■ Antennas that are ‘rotated’ have a weaker field (only one

antenna is driven.■ ‘Rotating field’ antennas should be closer together than those

on a splitter■ Another method is the alternately drive each antenna with a

MUX

5JAG. Jan 2005

Magnetic Field Strength

2.001.901.801.701.601.501.401.301.201.101.000.900.800.700.600.500.400.300.200.100.00

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

H(x)r = 0.5 mH(x)r = 0.4 mH(x)r = 0.3 mH(x)r = 0.2 m

Metres

H(x

) [A

/m]

– Larger size antennas have lower field strengths close to the antenna, but read at longer distances.

– Small antennas have high field strengths close to the antenna but the field quickly decays with distance.

HF Antenna Configuration

6JAG. Jan 2005

Magnetic Field Strength

– The activation field strength for Tag-it HF-I inlays is 93 mA/m

0.5 0.6 0.7 0.8 0.9 1.0

Metres

0.00

0.10

0.20

0.30

0.40

H(x

) [A

/m]

H(x)r = 0.5 m

H(x)r = 0.4 mH(x)r = 0.3 m

H(x)r = 0.2 m

152 dBµV/m

(100 mA/m)93 mA/m

HF Antenna Configuration

7JAG. Jan 2005

HF Antenna Configuration

Reading close packed tags (1)– The Antenna below was designed to create a strong field to read boxes of optical lenses. – It uses parallel loops to ensure each tag is near to an antenna– This is an straight-forward application – the tags are all in the same orientation

Box of Lenses

8JAG. Jan 2005

HF Antenna Configuration

– The 72 tags are only a few mm apart and can be in any vertical orientation

– The two crossed antennas are multiplexed

Reading close packed tags (2)– The Tube reader below uses small antennas to maximize the field strength

9JAG. Jan 2005

Antenna Field Lines– With a pair of antennas on a splitter, the field patterns depend

on the phase.

HF Antenna Configuration

In-Phase Antennas

Out-of-Phase Antennas

10JAG. Jan 2005

Using Splitters (1)

– If antennas need to be used facing each other, I.e. across a conveyor system, they can be used with a 0º two way splitter to more than double the range.

– Although a splitter adds losses, when antennas are used opposing each other, with careful tuning good performance results.

– It is important though, to ensure that the antennas are in phase in we need to optimize for Tags passing parallel to the antennas.

HF Antenna Configuration

SPLITTER

11JAG. Jan 2005

Using Splitters (2)

– If antennas need to be used facing each other, I.e. across a conveyor system, but the tag orientation is not optimal, use a 90º two-way splitter to ensure that the antennas are exactly 90º Out-of-Phase.

– The same effect can be achieved using a ¼ wavelength of extra feeder cable to one antenna

HF Antenna Configuration

SPLITTER

12JAG. Jan 2005

HF Antenna Configuration

What if the tags can be in any orientation?– If the tags can pass through a system in any orientation, then multiple

antennas have to be used.

– We can create a ‘Rotating field’ effect by having a ‘Basic’ antenna that serves both transmit and receive functions (Tx/Rx) and opposing it, a ‘Complementary’ antenna that is receive only (Rx)

– By careful adjustment of the Complementary antenna we can get the RF field to rapidly oscillate between In-Phase and Out-of-Phase thus reading Tags in both orientations

Rx only

Tx and Rx

13JAG. Jan 2005

Complementary Antennas

– A pair of rotating antennas will detects all tags in vertical orientations and some tags in high and low horizontal positions as they pass through.

– Unfortunately, there is ALWAYS a reading hole at the centre for horizontal tags.

HF Antenna Configuration

14JAG. Jan 2005

Removing the Reading Hole– The reading hole can be removed with a second

set of antennas

– The second set are connected to the same reader using two splitters

HF Antenna Configuration

Complementary (RX)

Basic (TX/RX)

– The reader power output will need to be increased to compensate for losses in the splitters

– The overlap removes the hole

15JAG. Jan 2005

Removing the reading Hole (2)– The reading hole can also be removed with an extra

horizontal antenna either above or below.

– This antenna is on a splitter with the Basic (RX/TX) antenna

– The antenna MUST be outside the zone of the Basic/ Complementary pair

Complementary (RX)

Basic (TX/RX)

HF Antenna Configuration

16JAG. Jan 2005

HF Antenna Configuration

What if I need the portal to be more compact?

– One approach is to use three antennas and read each one in turn using a multiplexer. This was the basis of the British Airways tunnel reader and other portals in commercial use

17JAG. Jan 2005

HF Antenna Configuration

Three antennas cover the three tag orientations

18JAG. Jan 2005

HF Antenna Configuration

Three antennas in position

19JAG. Jan 2005

HF Antenna Configuration

London Heathrow System

20JAG. Jan 2005

HF Antenna Configuration

London Heathrow System Antennas

21JAG. Jan 2005

HF Antenna Configuration

If we were to build another!