Investigating the properties of several RFID tags

2014-03-10 17.19.46

There are two types of tag in the above picture, passive(the thin paper like tags) and semi-passive (chunky plastic tags). The passive tags have no internal power supply and rely on energy from the RFID readers antenna to power its internal circuitry and transmit back a modulated signal. The semi-passive tags have a small battery built that is used to power the internal circuitry of the tag, these tags still communicate using power taken from the readers antenna.

See below pictures for the tags held up to a light source. The antennas are more clearly visible.

Holding the airport tag up to the light

Holding the airport tag up to the light

Another tag held up to the light

Another tag held up to the light

This is the RFID reader being used for the experiments. This device transmits a sinusoidal wave in the Ulta High Frequency Band (865-868 MHz), the antennas in the tags are tuned to this frequency meaning they are optimised to convert this frequency of radio wave energy in to electrical energy. The tags then transmit back a modulated sine wave that the reader can decode.

RFID Reader

RFID Reader

RFID reader rear view

RFID reader rear view

The two black cables on the right are for transmit and receive. The device is powered via the Ethernet cable on the left. In the picture below is the antenna used. It has two coils in it, one for transmit and one for receive.

Antenna on stand

Antenna on stand

Antenna rear view

Antenna rear view

Rear view of the antenna. There are two ports for the transmit and receive coils and a mounting bracket, a very simple piece of equipment.

The idea behind this experiment is to find how the readable range of the tags is affected by distance from the reader and reader transmit power. The power will be varied from 30dBm (1 Watts) down to 0dBm(1mW) in decrements of 10dBm. For each power level the distance between the reader and the tags will be varied from 2 metres to 30 metres. This is done by sticking all of the tags to a wooden door (as shown in picture below), placing the reader and the antenna on a trolley and moving the trolley backwards in steps of 1 metre. The reader will be set to medium speed and medium sensitivity for the experiment.

RFID tags on door

RFID tags on door

RFID reader with antenna on a trolley

RFID reader with antenna on a trolley

Reference picture for the names of tags:
This is for the graphs below so each tag can be matched to its data series.

labelled_tags

labelled_tags

There was another tag used that is not in any other pictures. It is a bicycle product bought from a local shop with an RFID tag behind the sticker. It is referred to as bike_product_tag.

bicycle_tag

bicycle_tag

30 dBm (1 Watt) results:

Due to the relatively high power, all of the tags were easily read so the graph was split into passive and semi-passive because all of the results on the one graph makes it hard to see individual results.

Passive tags:

30dbm_passives

30dbm_passives

Semi-passive tags:

30dbm_semi_passives

30dbm_semi_passives

The confidex and security tag are both missing from the results. Neither of the tags could be read, its likely that they are not working properly.

20 dBm (100mW) results:

20dBm_graph

20dBm_graph

Notice how all of the passive tags can no longer be read. Even from 2 metres away, 100mw was not enough to power the tags.

10 dBm (10mW) results:

10dbm_graph

10dbm_graph

0 dBm (1mW) results:

0dBm_graph

0dBm_graph

Conclusions:

To get a usable range using the passive tags, a transmit power of 1 watt would be best. Some shorter range tests with just passive tags would be useful in getting a better understanding of the usable range. It is possible that less than 1 watt would work with passive tags in different conditions.

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