Discussion Forum

I wouldn't expect anything better for a monopole.

The gain could depend on the rest of your geometry. Is this a monopole over a large (or perhaps even infinite) ground plane, or something else? If you post your model file, then others here can take a look at it and may be able to offer suggestions, explanations, etc.

I wouldn't expect anything better for a monopole.

Thank you for your answer Edgar,

IEEE Standard for Definitions of Terms for Antennas says: "If an antenna is without dissipative loss, then in any given direction its gain is equal to its directivity."

The maximum directivity values of my monopole antenna are 1.82 dB at 915 MHz and 2.44 dB at 2450 MHz. Considiring the definition above, I think that the dielectric and conduction losses of my antenna are too much. But I couldn't find what I miss.

The gain could depend on the rest of your geometry. Is this a monopole over a large (or perhaps even infinite) ground plane, or something else? If you post your model file, then others here can take a look at it and may be able to offer suggestions, explanations, etc.

Thank you for your answer Robert,

I couldn't post my whole model for now but I have attached the images of the general geometry and the electric field distributions on the bottom surface of the FR4 substrate. I hope it can give some clue about the model.

Oh, nevermind... That doesn't look much like what I have in mind when I think of a "monopole over a ground plane." I tend to think of something more like this: https://www.electronics-notes.com/images/antenna-vertical-with-ground-plane-plate.svg . Your structure is quite complicated in comparison. And, if I understand your picture correctly, the ground plane doesn't even appear in the picture (right?!). The only ground I can see is just that ring at the feed and, maybe, part of the card's edge over there? And meanwhile, your antenna is dielectrically loaded by the FR4. Finally, since the actual (but not-shown) ground plane is likely to be very important to the performance (since this is a monopole, not a balanced dipole, and thus the image currents in the ground plane, wherever it is, play a key role in the antenna's operation), I'm just not sure what else to add, except to say that I can't guess what the gain, pattern, or S11 curve would be likely to show based on that geometry. (Now, I'm not saying there's anything wrong with your design, per se; I just don't have much intuition about it.) Best regards.

p.s. FYI, based on the apparent "ragged edges" in the transitions between colors in your plots, I suspect your mesh is too coarse.

And, if I understand your picture correctly, the ground plane doesn't even appear in the picture (right?!).

p.s. FYI, based on the apparent "ragged edges" in the transitions between colors in your plots, I suspect your mesh is too coarse.

The ground of the model is on the bottom side of the substrate. I highlighted the ground area with yellow on the second image file which I have attached to this reply.

Yes, you are right, unfortunately, I use extremely coarse meshes to speed up the calculations. I need it especially when I sweep a certain frequency range such as range(500, 10, 2700) MHz. I don't know how the other softwares such as HFSS can do it very fast.