Hello,
As there is no active component in your circuit, there is no change on instability (parasitic oscillation), for every combination of source and load, the power gain of your two-antenna setup will be less then 0 dB. Therefore, you will not encounter |S11| or |S22| greater then one.
The source and load impedance are defined in the S-parameter setup (I assume default 50 Ohms). For a differential feed, the default values can be different. Perhaps other people can give useful info on this (if this is of interest).
Hello,
The ratio of |S21| and |S12| has to to do with the stability of electronic components/circuits, just use |S21|^2 to find the power transfer between the antennas.
The maximum power transfer (after matching both load and source) would be: |S21|^2/{ (1-|S11|^2)*(1-|S22|^2) }. Assuming |S21| << 1.
From the above formula, you can see that when |S11| and |S22| are well below 1 (that means low relfection), you can use |S21|^2 (or |S12|^2 ) as a good approximation for the maximum power transfer.
Hello,
input/output reflection is frequently expressed in dB. It can have a form of 20*log(|S11|). This should give a negative or zero number for passive circuits.
so |S11| = 0.333, should give -9.5 dB. With this definition, you shouldn't get positive dB numbers. You may change your display to smith chart, and check S11 on the graph, it should always be in the passive region (so |S11| < 1)
You may also encounter the Return Loss (the loss of the reflected signal). This is defined as RL = -20log(|S11|) (note the minus sign). For a passive network, RL >= 0. In cable and antenna related stuff you will see this definition frequently.
Note that for many two ports, the actual input reflection (so (reflected voltage)/(reverse voltage) may not equal S11, as actual input reflection coefficient may depend on the reflection coefficient of the load.
So generally spoken: RL = -20*log(|input reflection coefficient|) .
|S21|^2/(1-|S22|^2) should give a positive number less then one for your antenna case.
When expressed in dB's: 10*log{|s21|^2/(1-|s22|^2)} should give a negative number. Note the "10" as the formula itself is a power ratio already.
Regarding the meshing frequency, this depends on how fast versus distance the current density may change. As a wild guess, you may start with a mesh size of about lambda/20, based on the wavelength in the substrate. At a certain point, reducing the mesh size will not result in changed simulation results (but very long run times).
If you have some interpolation scheme active, make sure you have sufficient frequency points. Interpolation schemes may give weird results.
Do you have a plot/graph of what you are simulating?
Hello,
Point 1. I am not fully clear of what you mean, do you have a drawing or screen plot?
Point 2. For a real source impedance, you have conjugate match when S11 is (almost) zero (with your impedance as reference for the S-parameters). How close it must be to zero depends on your application. Most antennas for telecom applications have VSWR<2 that equals |S11| < 0.333.
For non-real sources, I think you can't get it directly from the simulation as the S-parameters are defined for real impedances only. If your complex source impedance is a + bj, the required load impedance for conjugated match (equals maximum power transfer) = a - bj .
juz a question came in mind , if i use 4 omni antennas in one building, will they cause interference due to radiation? all 4 antanna working on orthogonal channels.
Hello,
What you are doing is completely off what I had in my mind; you are in near field (reactive field) communication. What is the distance between the layers and frequency?
I assume there is ground present (because of the four ports). You may consult the manual/help file for details of the feed schemes as this has large influence on usefulness of the results. .
Ports with respect to ground that use de-embedding, you can't put on top of each other (maybe some momentum expert can confirm this?).
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Other thing is when you have conductors very close to each other, current may concentrate at certain regions. The meshing has to be fine enough to enable simulation of steep current density changes versus distance.
Regarding source impedance, I don't know the circuit that will actually drive these antennas. For good functioning, your electronic circuits need to see certain impedance. For the TX case this is to assure certain power transfer, circuit stability, efficiency, etc. For RX it is mostly a Noise Figure or stability issue.
---------- Post added at 23:16 ---------- Previous post was at 23:14 ----------
With interference, do you mean that the 4 channels will interfere each other (mutually), or that the combined radiated field of the 4 channels will interfere in some other system (EMC issue)?
Can you further describe "orthogonal" (is this in frequency, time or spreading sequence, etc)?
When you limit yourself to metallic antennas, antennas and propagation are linear, obey superposition, but can be strongly time variant.
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