Hi,
I’m trying to determine the resonance length of a single radiating slot on a simple rectangular waveguide. Based on my understanding, the resonance frequency of an element can be identified when the imaginary part of the input impedance (Z11) becomes zero, or when the S11 parameter shows a deep negative value, indicating resonance.
In my setup, I used a air-filled WR-90 waveguide and placed a rectangular radiation slot on the broad wall (top side), with a width of 1.5 mm and an offset of about 2 mm from the central axis. At a frequency of 9.5 GHz, I varied the slot length from 14 mm to 16 mm in steps of 0.05 mm.
1)I did not observe resonance for any slot length, as the imaginary part of Z11Z11 remained above 650 ohms.
2)Additionally, there was no significant dip in the S11 parameter that would indicate resonance.
However, in various articles, it is mentioned that resonance should occur around a slot length of 15 to 15.5 mm at frequencies near 9 or 9.5 GHz.
Could you help me understand what I might be doing wrong?
Maybe you should try it with a bigger offset. It's not coupling much power so it might not show on the parameters you are looking at. It looks like you are looking at the reflection from a reasonably well matched waveguide. A small amount of power going into the slot probably won't show up much on that. Alternatively, try it with the slot centre a quarter guide wavelength from a short. You could also put in a far-field monitor so you can see the radiated power.
This looks like it should be helpful http://www.w1ghz.org/antbook/ch7_part1.pdf
Thank you, Malcolm White. I appreciate your response and the article you provided, which I recently read. In another article, I noticed that when determining the resonance length, a short circuit is sometimes used. Is this because it couples more power to the slot? Doesn't the short circuit alter the resonance length? If a slot is placed near a quarter wavelength (λ/4), the resonance length appears to be insensitive to the slot's length during high impedance conditions. However, it is quite sensitive at half wavelength (λ/2) from the short circuit
If you use a short, most of the power goes back as S11 and it is easier to see if some of it is missing, rather than using a good load and the answer being two values close to s11=0.
At a short, magnetic field is across the guide and the currents on the end wall are up and down, and on the top and bottom of the guide are mainly in the direction of propagation and don't couple to a longitudinal slot, likewise half a wavelength from this. A quarter wavelength from the short, at the open, the magnetic fields are longitudinal and the currents run up the sidewalls and sideways across the top and bottom of the guide, and couple more strongly with the slot, if it isn't on the centre-line where the currents meet and stop. I would be surprised if the slot resonance is insensitive to the slot length at this position.
I'm not sure. However, the slots may be fairly low Q, in the radiating mode, and so have wide bandwidth. If they are at a s/c position, you might be seeing reflections from the two ends of the slot, for longitudinal currents in the guide wall. These currents don't produce radiation from the slot. Slot radiation is due to dD/dt across the centre of the slot (which is like a capacitor) which results in a magnetic dipole in the same direction as dD/dT (the displacement current). The electric currents around the walls of the slot are clockwise on one half and anticlockwise on the other, meeting head on at the centre where the displacement current continues them across the gap. The wall currents at a s/c have hardly any component of this pattern in them.
This antenna has slots 1/4 wave from the short.
https://oa.upm.es/54832/1/INVE_MEM_2018_297606.pdf
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