The resistive sheet on the front of the absorber is ideally a frequency independent sheet with 377 ohm resistance for free-space impedance matching. Varying the resustive sheet will affect your absorprion null depth and bandwidth.

One solution is to use nichrome film on a dielectric sheet.  Resisitivities of  hundreds of ohms per square are available.  Resistivity is constant with frequency.  The films are thinner than the skin depth, so the skin depth effect is not significant.

For Salisbury screen design, please see Knott et al.'s Radar Cross Section.  Unfortunately, resistive sheets or R-cards are very dispersive.  More info on that can be found in the attachment. 

I think that you can also use a periodic structure of surface mounted resistors of 377 ohms (I have found 376ohms available from digikey).

Since you need a conductive backplane at a quarter wavelength distance (probably you can use a PCB with ground in one side and resistors in other side), this quarter wavelength distance will be a limitation in your frequency bandwidth. The metallic parts of the resistors and the pads used for soldering may have an effect in introducing undesired capacitances. You may optimize your design with a full-wave simulator such as HFSS using Floquet boundary condition as a first step.

I have not tested in depth this method (I just did some simulations, trying to increase the bandwidth with FSSs). Maybe someone can argue on it.

Best regards

Attached is a conference paper where we did something very similar to Halim's suggestion.  Perhaps it will be useful.

In reference to Halim's answer, you can switch the resistive 377ohm sheet on the front for an impedance sheet, formedby a periodic structure. The advantage of doing this is that you can, in some cases (based on periodic structure resonance), reducethe thickness of your dielectric spacer. As part of my dissertation, I fabricated some ultrathin absorbers this way that were at least 1/6th the 1/4 wavelength thickness.

Thanks Milo Hyde for the reference that you attached. Do you have also experience with the Design of multilayer absorber? and how can we choose the values of each resistive sheet?

I personally have never designed a multilayer absorber.  My interest is in characterizing them.  I highly recommend Knott et al.'s Radar Cross Section for Salisbury screen and Jaumann absorber design, in particular, Chaps. 8.4 and 8.5. 

Thanks Milo Hyde, but I dont have this Book. Can you attach it here?

Mostafa, for multilayer absorbers you have many more variables to account for. Not only the multiple resistive sheets, but also the dielectric properties and thicknesses of the extra spacers. I also recommend Knott's book for Salisbury screens and Jaumann absorbers. It is worth the purchase. Maybe your school library has it or can get a copy? Also, look up any papers on Jaumann absorbers, they should help. I recommend you write a short program for a multilayer absorber and use some basic optimization routines to determine the properties of the resistive sheets and dielectric spacers for your particular criteria. I'd suggest beginning with a 2-layer Jaumann first. Also, if you were not aware, the Jaumann absorber increases the bandwidth of the -20 dB absorption/reflectivity by introducing more nulls (based on number of layers and absorber properties).

Thanks Richard , I desire at first to design multi-layer Salisbury screens absorber using dispersive resistive sheet and qurter wavelength spacer with constant permittivity or free sapce , so I think in this case i only need to design the resistive sheets. I tried to drive the equations and write a Matlab code, but I dont get the expectable bandwidth with more nulls. Consequently , I think the error will be in the values of resistive sheets, do you think the same thing as me??

Right, so you're simplifying your problem to just deal with the resistive sheets.  Are you sure your code is correct?  It's always good to check your code versus some published results in the literature.  Once you can model someone else's design with your code all you need to do is write a quick optimization routine to calculate your resistive sheets for whatever frequencies and bandwidths you want.

Dear Richard, do you know how to determine mathematically the conductivity of the resistive sheet of salisbury screen?

Hi Mostafa, if you are looking to optimize your salisbury screen by calculating the necessary resistive sheets, you need to select some criteria to optimize for. For example, if you require -20 dB reflectivity, rewrite your expression for the reflectivity to solve for the resistive sheet term and substitute the desired reflectivity in. If your spacer is free-space this will simplify your equation a lot.