how can find RF direction

Have you thought about making the cards double sided (ie- the face of the card is also displayed on the back). Then you just need to use optical card recognition techniques (ie- look at them from across the table).

If this is for game monitoring, cameras over the player's shoulders or from underneath the table (if it is transparent) may be a better option. Possibly a special non-visible die on the cards alloying passive card verification.

If this is for cheating I think the players would notice the increased card thickness and weight if you implant them with even passive RFID circuitry.

Please elaborate on the application.

Purpose seems to be For Cheating. Crash your TANK Jack of all trades instead of giving solution

Actually, the best application would be for magic tricks. Possibilities abound and the extra thickness would not be too much of an issue since the magician would be the one handling the cards, at least most of the time.

The real issue would be how to "read" the RFID signal. Have an accomplice with a mike/RF transmittor and a small ear piece to get their input?

Interesting idea, but for the time being, I think most magic shops have much more conventional and inexpensive solutions.

And you can still visit the casinos.

If this is for cheating I think the players would notice the increased card thickness and weight if you implant them with even passive RFID circuitry.

WHAT extra thickness? See http://www.idtechex.com/printedelectronicsworld/articles/hitachi_powder_chip_japan_00000823.asp for chips that are 0.05 x .05 x .005mm in size. In other words, corner-to-corner dimensions are smaller than the thickness of ordinary paper; laid flat, you could stack 20 of the chips and not equal the thickness of a playing card. I'm betting that this implies no observable extra weight, either.

Yes, but how do you put them in the playing card without special equipment. About the only easy ways to do it (without destroying the card) are to use two cards placed back to back, that's double the thickness of a standard card right there. Possibly using a needle or similar to push it in. Either way you will likely get noticeable card distortion unless you are very careful (expect to go thru a lot of cards getting it right).

Remember, a high-tech solution is not very helpful if the person cannot get a hold of the technology or implement it ("ok have chips, how do i connect to serial port?").

Perhaps he/she could print own artwork for the back and hide the smallest obtainable RFID unit either under it or right out in plain sight on the back of the card with the artwork built around it to make it look like it belongs.

That just leaves the reader and card discriminator technology. That is going to be a little tricker as it would need to be directional and also ignore the deck of cards in the middle of the table.

I figured that (especially for magic tricks) the cards would be printed onto special cardstock. The RFID chips could be sprinkled into the wet slurry used to manufacture it, and the embedded numbers read from each card after printing. This adds a randomness to the face vs. number relationship, further hiding the scheme. A simple look-up table would convert RFID codes into face value or any other desired relationship needed to make the trick work.

I suppose 0.05 mm X 0.05mm object will not be easily detectable by human naked eye, without special attention, so no need to do any special efforts to hide it. It will get hidden in any artwork (original too)

0.05mm X0.05mm labels are tiny suficiant. but its antana is not. However I hope to embed it inside. Thanks for the comment.

I am not an engineer and not sure if I fully understand your question. However, to identify signal direction, you would simply need to use multiple readers and triangulate the position of each card. The strength of each signal would indicate it's location.

Another idea would be to use a rotating antenna for reading. A wheel of some sort and software to analyze signal strength based on antenna position. This again, could indicate the direction of the signal.

Once you know how to do it, don't forget to send a copy to the Las Vegas/Reno casino's fraud commitee... They'll love it.

Wangito.