Problem with waveguide-slot-array-antenna 23.6 GHz resonance vs 24.1 GHz simulations.

Problem with waveguide slot array antenna 23.6 GHz resonance vs 24.1 GHz simulation.

Hello! I have problem with slot array antenna design. It is single ridge air filled waveguide with 8 slots, with resonance at 24.100 GHz and relatively wide bandwidth (around 1GHz for S11=-6dB) and S11=-20 at resonant frequency.
I do not have VNA or any other good equipment to measure at 24GHz.
Currently my setup is:
1. VCO (23 to 25 GHz)
2. Two slot array antennas
3. Receiving mixer

Performance is very poor at 24.1GHz. By tuning VCO frequency i found resonance around 23.6GHz, and I estimate best S11 level around -5dB or worse.
I have few prototypes with different slot lengths (DL=+-0.5mm), but same distance between slots. It seems that resonance does not depend on slot length significantly:
DL=+0.5mm F=23.7GHz (for wider slot I expected lower resonance)
DL=0mm F=23.6GHz
DL=-0.5mm F=23.65GHz

Can anyone suggest how I could alter my design and do some more prototypes to tune slot array to 24.1 GHz without access to 24GHz VNA?
Currently I suspect that distance between patch must be reduced, and currently my array works as non-resonant slot array at 23.6GHz.

Re: Problem with waveguide slot array antenna 23.6 GHz resonance vs 24.1 GHz simulati

congratulations, you got pretty darned close!
Investigate parasitic reactances you might have left out of the simulation, and get the simulation to agree with the actual measured data, and you will be there.

Re: Problem with waveguide slot array antenna 23.6 GHz resonance vs 24.1 GHz simulati

What I want to see is very small losses at any resonance from 24 to 26GHz.
At first my idea was to build 3 prototypes with some frequency step, compare simulation to reality, and then do some tuning in model.
Problem is prototype resonances do not shift with slot length.

I can't understand differences between simulation and prototypes
Simulation:
L+DL [mm] - resonance around 23.2 GHz
L+0 [mm] - resonance around 24.100GHz
L-DL [mm] - resonance around 25 GHz

Prototypes:
L+DL [mm] - resonance around 23.6 GHz - strongest received doppler reflection from rotating fan. But i think that S11 is very poor, worse than -6dB.
L+0 [mm] - resonance around 23.6 GHz
L-DL [mm] - resonance around 23.6 GHz
Why resonant frequency almost the same? At first I suspected that slots does not radiate at all, and maybe 23.6 GHz is some connection leakage dip in S11 or waveguide resonance. But covering TX or RX antenna in radiation direction gives significant effect. Also I observe that beam is pretty narrow in horizontal direction (8x1 arrays are placed horizontally) and wide in vertical direction, just by pointing antennas toward rotating fan.

Maybe problem with guided wavelength, but from my (small) experience air filled guides match simulation very well in all aspects.

I am planning to make more prototypes and search for better resonances:

two-slot arrays, varying:
1. slot length
2. slot distance
3. slot inclination

3 values for each parameter, 3x3x3=27 prototypes.
or 3x3=9 prototypes, and tune slot length by drilling
also can tune waveguide short distance somehow.
Any ideas?