Using a small RFID tag concealed on my bike,I would like a RFID reader
to  monitor the RFID tag on my bike.Please could someone give some
basic design for a passive RHID tag(small,40mm by 40mm).Also a basic
design for a RFID reader.The reader could emit pulses and signal a
alert if there is,or no response ,whatever.The passive RHID tag,would
have no battery.

Being an electronics engineer, I can create a package like this for
around $150, using an RFID tag development kit, a solid-state relay,
and a high-decibal buzzer.  The problem is that RFID has such a poor
range, your bike would have to be continuiously less than two feet
from the RFID reader...  You'd be better off with a two-foot bikelock
cable.

For an RFID reader that has a range of more like 15 feet, you'd need a
reader in the range of $1500.  These readers are typical to large
department stores, and usually also require full computer hardware
(like an additional $500 computer).

A laser trip might be a more reasonable solution.  Park your bike
across the laser beam, and the alarm goes off if the beam reaches the
sensor.

There are all sorts of other options, but they're highly dependent on
environment ... I might be able to help more if I know where the bike
is being stored.

Hi axil221-ga,

   I second purpleprogrammer-ga, on the advise regarding the use of
RFID.  I am assuming that your interest in RFID, is due to the absence
of a battery powered transmitter, and the ability to work in proximity
without physical contact.  You could do the same just with a custom
crytal receiver(used for RF-DC conversion), and a std. wireless encoder
(used in wireless keyfobs).  All you need to do is to tune to a strong
carrier of a local station(make sure it is 24hrs) and power the wireless
keychain tx(after sufficient voltage multiplication using low leakage schottky
diodes and caps, charging a li-ion button cell).  By making the tuning coil
large enough(replace a section of the bike bar with fiberglass?(use a portion
to enclose the receiver/tx)) you should have sufficient power to give you 
enough range.  All you need is the receiver module and a siren attached to it.
You could also use the inexpensive proximity alarms(used to protect laptops)
with a receiver/transmitter, modified accordingly.

Regards
ldavinci-ga