to measure rf radiation

How about a radio receiver tuned to the desired frequency, and fitted with a signal strength meter?

RF can be effectively measured by the Tesla reading of the magnetic field using a "magnetometer", then converting this reading to rf.

I don't know what Airport PAR means, but I will answer generically. You need a radio receiver with an indication of measured rf power or potential, and an antenna that covers the signal frequency band. The antenna will have a transducer factor that relates the electric field impinging upon it to the rf potential it presents to a matched load at its antenna terminal. That transducer factor is either the antenna gain or its effective height, depending on whether you want electromagnetic field power density (Poynting vector) or electric field strength. If the signal frequency is below 30 MHz, you will need either an electrically short whip, or a shielded loop. The loop is preferable - it is more accurate. Between 30 - 1000 MHz the simplest antenna is a tunable dipole. The dipole is one-half wavelength long - that means five meters end-to-end at 30 MHz, and that means it may not fit in your room and it also means that at lower frequencies it is difficult to use in vertical polarization. The fix for that problem is a broadband dipole known as a biconical antenna. Its dipole elements are in the shape of cones with apex at the antenna feed point. The standard biconical is 137 cm tip-to-tip, so it is easy to use in either horizontal o vertical polarization. It covers 20 - 200 MHz, sometimes to 300 MHz. Above 200/300 MHz there are a variety of antennas available, but below 1 GHz for a single frequency you can't beat the tunable dipole. Above 1 GHz you may see some log-periodics or horns in use. There are broadband antennas covering 1-18 GHz, and there are octave band horns as well. If you want pics or sources for these antennas, message back and I can provide all. The radio receiver nowadays will likely be a spectrum analyzer. I can provide pix and sources for those as well. If you don't have this stuff and don't want to buy for one use, you can rent it all from test equipment rental houses. Can provide that info as well, for USA.

For measuring through horn pick up in 18 Ghz and giving to spectrum analyser - how will I value the Power level with Placed distance ?

Also , using directional coupler to get portion of signal for measurement and this approach - Would they be Equalled in any method ? - please .

Moreover , Sir - To indicate radiation hazard level to individuals in open space and closed testing room ; What instrument should be available ? - please .

Eagerly expecting responses .