very basic antenna...

what frequency, mite help :) ?

900 Mhz......

Low Cost DiY 900Mhz Omni Dipole Antenna - RC-CAM Forum

what are the specification of monpole antenna to operate at this frequency..?

300 / freq (900MHz) = wavelength (.33m (33cm)) / 4 = 1/4 wave monopole (.083m (8.3cm)) x.95 V.F. (velocity factor) = 7.91cm total length of the monopole

you could use a bit of brass rod, stiff copper wire mount it above a groundplane for better matching feed 50 Ohm coax to the base of the monopole ... braid of the coax to the groundplane and the centre conductor to the bottom of the monopole

the velocity factor is for the monopole wire. .95 is a pretty standard "rule of thumb" for a wire, brass or an aluminium rod.

Dave

my antenna is working at 3 GHZ through this lenght...:(
what do u mean by velocity factor?
and what r the radius and groud plane measurements?

3GHz ? you stated 900MHz earlier which is why I gave you the calculations for that freq

regardless use the same formula and use 3000MHz (3GHz) instead of 900MHz
At 3GHz it would be more likely that you would use a probe into a waveguide but a standalone dipole or monopole can still be done.

Velocity Factor... briefly... its how the electrical wavelength is determined. a radio wave travels at the speed of light in air/freespace (300,000 metres / sec). it travels slower in a medium such as a copper wire, a waveguide, aluminium tubing of an antenna etc.
examples .... many standard coax cables have a velocity factor of ~ 0.66 some hi quality ones are up ~ 0.80 - 0.89
a copper wire/ aluminium rod in air for use say in a dipole antenna has a VF of ~ 0.95

So your free space wavelength must be multiplied by that VF to determine the electrical wavelength, as I did in the above calculations.

Dave