Computation of RF Surface Resistance Using the Anomalous Skin Effect Theory (Correspondence)

Excitation in the outer loop is adjusted to obtain a minimum detected signal at the centered probe. This can be done quite accurately and defines antisymmetric excitation for the antennas (az = —aJ. Antenna #2 is then shorted at the ground plane resulting in (bl/aJ= – 1, and the phase bridge is balanced to produce a null at the detector. This balance occurs when b,= –uZ = al and establishes a phase reference between the incident wave in arm #1 and the reflected wave in arm #2 as sampled by the directional couplers in these arms. Since the directivity of the directional coupler in arm #2 is greater than 40 dB, this setting can be made to an accuracy of about A 0.6 degrees. Antenna #2 is then used as a parasitic element with a matched load by removing its excitation. This is accomplished by using the shorting switch in arm #2 and inserting maximum attenuation with the attenuator of arm #2. It is important to use a ferrite isolator before the shorting switch and to use a magic tee to divide the power between arms #1 and #2, because this isolates arm #1 (the calibrated phase reference) from the large reflrxtions in arm #2 when the switch is thrown. The null in the phase bridge, which had previously been defined by the reflected wave bj = al in arm #2, has now moved to a new position defined by the wave bj =S,,al. The shift in null position is a function of the phase angle of S12 and can be measured by varying the precision phase shifter to bring the null back to the detector position. When the coupling is very small (S, Z<<l), the attenuator in the bridge circuit must be adjusted to make the null easily observable. The adjustment of this attenuator is the most important source of error in the system, but it is not a serious limitation because it is easy to obtain commercial attenuators of the rotary vane type which have less than A 3 degrees of phase shift variation for up to 50 dB of attenuation. When the coupling is greater than –20 dB, even this minor source of error can be avoided by amplitude insensitive phase bridge techniques as discussed by Bnrton[sl or Scharfman. 161The variable attenuator was used throughout these measurements because the coupling was typically –20 dB to –40 dB. \p~o~~