about efficiency of resonant slotted waveguide antenna array

Dear fellows,

I designed a 7-element resonant slotted waveguide antenna array (material is aluminium).
the array is designed to work at 10 GHz and it is based on WR-90 waveguide (22.86mm x 10.16mm).
the simulated efficiency is 98% in HFSS, but the measured efficiency is only 60%.

is it reasonable?
could anyone give me some references about efficiency of resonant slotted waveguide antenna array?

Thanks!

Conductivity of aluminium depends heavily on the type/grade you have. I am sure what you have isn't pure aluminium as it is too soft for regular machining. It may vary between 25% to 60% of conductivity of pure copper. But even this will not bring down the efficiency from 98 to 60%.

Other issue can be the surface irregularity. When it has optical shine, you can ignore that for 10 GHz use.

What is the accuracy of your test setup. From 98 to 60% is 2.13 dB (that's the gain of a dipole w.r.t. isotropical antenna, no mistake?).

In case of using a linear polarized measuring antenna and determining efficiency based on main beam gain, is the polarization of the antenna under test truly linear?

Dear WimRFP,

Thanks for your help.
What I used is the most common machining process.
Could you give me a reasonable range of the efficiency?