An NRL Arch is a microwave measurement system that can measure the free space radar reflection coefficient 

of flat radar absorbing materials (RAM).   NRL stands for Naval Research Laboratory, where the first such measurement

system was built. 

    The system uses two microwave "horn" antennas.  One antenna is connected to a microwave transmitter, the other

to a microwave receiver.  The microwave energy is launched from the transmit horn, travels to and reflects off of

the target, and travels to and is collected by the receive horn.  Pyramidal absorber is placed behind the target to eliminate

microwave energy that travels around the sample.  

    Here is the NRL arch I designed and built while on summer research.   This design differs from a traditional

arch in that horns are located below the target and point upwards.  In a typical arch, the antennas are suspended from

atop the arch, and the absorber is placed on the floor with the target resting on the absorber.  

   This design required "invisible" foam target support table to be constructed for the sample to rest on.

The advantages to this arch design are:

      1)  No reference phase adjustment needed due to sample thickness.  The zero degree phase plane is always

           at the top of the foam table.

    2) Spatial separation between the pyramidal foam absorber and the target allow for Fourier Time Domain Gating

        for increased signal to noise ratio.

   3)  The pyramidal foam absorber hung from the ceiling, instead of the laying on the floor, keeps it safe from kicks

        and other possible damage, as well as free from debris that could possibly fall and collect on it.