Increasing the range of an RFID door access control

[Gefreiter Krueger]

I got a cheap door access controller from Hong Kong. The keyfob has to be placed within 1.5 inches of the keypad for it to be picked up. Is it possible to make it sensitive enough so that approaching the doorway with the keyfob in your pocket would unlock the door? (Much like some cars do). It picks up the fob using a coil of wire round the edge of the keypad (which is 10cm across). The coil looks like about 100 winds of thin wire of the same sort used in CRTs. If I made a huge version of this coil right round the doorway, would the range be increased? Or would there not be enough power in the chip to transmit over that much area?

[Bob Minchin]

Not a lot you can do without a lot of test equipment and knowledge. These things are specified for a range of 100mm maximum. There is a different standard to cover the 100cm units that are possibly used in cars.

1) you need extra power so that sufficient RF power arrives at the tag to power it up. 2)Tinkering with the coil will de-tune it and will transmit far less power and reduce the RF sensitivity.

[Gefreiter Krueger]

Is amplifying the signal an easy thing to achieve? If not, where can I buy one with the same range as a car one?

Bloody advert said 10 metres. I bought it ages ago, the ones on sale now say

10cm (I assume it was a typo). It's not 10cm though, it's more like 3.

[Bob Minchin]

I'm afraid not, The problem is that the amplifier much switch very rapidly and the receiver must recover very quickly from the overload. This is akin to having your head inside a drum that is being hit hard and then needing to hear a pin drop a few microseconds later!

What you have is a proximity system which must have no MORE than 100mm range for security reasons such as payment system (contactless smart cards, oyster cards etc) These are described by ISO 14443

The up to 100cm system are 'vicinity' systems and covered by ISO 15693 and should be what you desire. Note that others will be able to read your tag at up to 100cm range and may then clone the information to gain access to your door. A balance of security and convenience needs to be considered.

[Gefreiter Krueger]

Presumably it's not that easy to read and copy it, otherwise all these keyless cars would be getting stolen. Perhaps some kind of encrypted code is sent both ways?

I shall play around with slightly different aerials and see what I can do. Even doubling the distance would be good, at present you have to almost touch the fob on the panel, and have it orientated correctly aswell.

Is the signal stronger INSIDE the loop of the aerial perhaps? If I brought the loop out of the casing and placed it round the frame of one of the door's window panels, the fob could easily be inserted into the coil by touching the pane of glass with it.

[Bob Minchin]

You might get a little more inside the coil just, but it is vital not to de-tune the antenna from about 13MHz The codes are rolling codes with challenge and response but there are ways of stopping it rolling and the oyster card codes have been cracked but fraud does not yield enough monetary advantage for the crims who ride in cars not trains Contactless payment cards are under attack by the bad guys. More profitable but still low value transactions. Read up on the work being undertaken by Markus Kuhn and his department at Cambridge University. Higher security car keys have just lead to increasing occurrences of car-jacking where the crims nick the car keys and the car from the driver or break into the house to get the keys to the cars in the drive. These system are secure enough until broken to the extent that the banks and insurance companies will no longer pay up.