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Radiation pattern for circular loop in HFSS

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ankitmore

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Hi all,

I have designed a loop antenna in HFSS using a lumped port. I was observing its radiation pattern in the far field region and noticed that HFSS gives a maximum in the plane perpendicular to the plan of the loop antenna and a null in the plane of the loop antenna. Which should infact be the opposite that is maximum in the plane of the loop antenna with a null in the plane perpendicular to the loop antenna!!

Any help will be great..thanks
 

Assuming that the loop's diameter < 0.1 lambda, there must be confusion between coordinate systems, as your expectation is correct.

When you simulate a straight half wave dipole (with same lumped port), does this give the expected pattern?
 

Yea, when I use a dipole antenna the radiation pattern is correct..i.e a torus around the dipole.

I also increased the size of the loop..i.e >0.1 lambda...but no luck

I also increased the size of the radiation box...and even changed the shape to a sphere instead of a rectangular box...but still no change in the pattern..

and for sanity sake I changed the plane of the loop...but still the radiation pattern was in the plane perpendicular to the plane of the loop....so i suppose that rules out any confusion between co-ordinates.
 

Did you check current distribution in the small loop (should be almost equal along the loop)?
 

I looked at the magnitude current distribution and it looks more of less constant through out the loop...however when I looked at the vector current distribution I noticed that there are two vectors pointing in a plane almost at 45deg to the plane of the loop, right where the opening in the loop is defined to place the excitation port. The magnitude of these vectors is fairly larger compared to that of the vectors around the loop. My guess is that this is what is causing the radiation pattern to appear perpendicular to the plane of the loop...

Now my question is 1) will that really happen on a physical loop antenna? (my guess is it should not happen) 2) how can i get hfss to give the correct results?? (mesh size? excitation gap in loop?)
 

When the lumped source has size << loop size, I would expect the correct pattern. Maybe some HFSS expert can comment on this one. Can you experiment with the width/length ratio of the lumped source?

Regarding a physical loop, the loop is tuned with a capacitor. As long as there is no large capacitive gap, the loop can be treated as a loop having constant electrical current distribution along the loop. This will result in the pattern you expect.

When the capacitive gap is large, the current segment opposite to the capacitive gap is no longer cancelled (as the displacement current does not radiate). In that case you will get a combined pattern of a Hertzian dipole and that of a loop.
 
I will try to experiment with the gap size...thanks for your help again
 

The loop is just in air. I do intend to print it later on a substrate but the simulation on the substrate din't make sense so I was simplifying things and first studying it in air.

Since i intend to print it on a substrate ...the loop is really a flat metal with a 10um width and the operating frequency is around 100GHz. i have found a few links myself trying to discuss this problem but have not come across a solution from anyone.

I do not have the file on this computer but will post it soon..
 

So, we found the problem! It is acting like a patch antenna rather than a loop. So if you just put it on a substrate you should get what you need. you are actually complicating the matter by simulating it without the substrate and the ground.
Let me know if this works.
 

I thought the same too...and was actually going to go one step back and simulate the loops with a wire diameter of 10um instead of a flat metal of 10um...will post my observations once I get to it.
 

As long as the cross section has size < 0.1 lambda, the shape of the cross section doesn't affect the radiation pattern from a small segment. So a loop made from strip will have same pattern as one made from round wire.

ankitmore: For the strip case, can you guess the current through the loop based on the current density and width? This should be equal to the current supplied by the port (as you did not add matching yet).

The two 45 degrees off-plane vectors look strange to me (as I assume you have a symmetrical structure). Does the current density plot also show the current density in the lumped feed? Does the reactance (S11 simulation) look like what your expect based on a hand calculation?
 

I might not be interpreting the structure properly. I thought that the microstrip line is wide and has a thickness of 10um. Now I understand that is has zero thickness and a width of 10um, which means my previous post was wrong. Can you give us more details and let us know about the simulation frequency?
 

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