Re: How to simulate an equiangular spiral antenna in HFSS

Sir,

this is what i did..i make the ports wth a phase delay of 90degree each in clockwise manner for the ports to generate only mode 1 excitation ..that is 0.-90,-180,
-270 degrees in clockwise manner beginning from x axis ..this antenna is expected to give good gain but the gain is negative in all frequency range , also gain rhcp
and cross polarisation is not high ..but axial ratio is quite reasonable ...So wht shud i do in order to make gain atleast 2 or 3 db ?

thanks

Well, my friend, that's where the antenna engineering takes place

As you probably know: Gain = Directivity - Loss, and if you check the directivity, everything is okay (DirRHCP > 7dBi), thus you have about 10dB loss (just an assumption, you can see the actual value by plotting the RadiationEfficiency (under the AntenneParameters in the FarField-Plots).

Most (practically all) of the loss is due to the dielectric imperfection (finite isolation -> dielectric loss). There are only two ways to reduce dielectric loss:
- use a better dielectric (but your Rogers is already pretty good)
- minimize the electric field areas and intensities.

The first you can check by right-clicking on the Rogers Ultralam material, choosing "Properties" and then cloning the material and setting the loss tangent to zero and simulating the whole model again.

The second is more tricky (but the way to go here, I think). To minimize the field intensity, the distances between the arms and between the arms and GND would have to increase -- which is probably not too easy to do. On the other hand: why are your arms so long? I redid your simulation with n=1 (just one turn) and got a good result (5dBi) right away (of course there is a trade-off here, which is probably the polarization ratio, here, I think... but I didn't investigate that any further).

Hi Roxxon,
I need some help regarding my 2 arms spiral antenna design in HFSS, I am not getting the return loss as I would have expected. The antenna is without the ground plane. So I think that I need to feed it with a balun. Can you guide me how to design that in HFSS? The design is attached

Well, I took a look at your model and... well, there were several things wrong there...
- The radiation boundary has to be all around your antenna
- The excitation by lumped ports is already balanced -- but the port should only be connected on the two sides where the integration line ends
- The port should also not lie on something already assigned PerfE to (although HFSS might do the correct thing and just ignore that assignment, I don't know)
- The radiation boundary was too big, I adjusted it to a distance of lambda0/3 everywhere

HI Roxxon, I am getting an error in the design which you sent me, the simulation crashes in 9th adaptive pass and says out of memory.so I set 8 adaptive passes only but with this configuration my gain is reduced to 2.5 db only and a return loss of only -12.5 dB , what was the gain and return loss you got with this antenna?. Secondly I want to know that how can I feed this antenna with a balun because I want to manufacture the layout of this antenna design. So in reality I think we can only design this antenna via balun as it is without a ground plane. Can you give me any explanation about that ? Can you make balun feeding in this design as I have no idea about that? I will be really thankful

Well then your fucked -- of course you can't just stop the adaptive passes somewhere, where it did not converge to a good solution yet
You can try to decrease the size of the radiation boundary a bit -- but at least lambda0/4 space between the radiating elements and the boundary as to preserved!
But what do you mean with an RL of "only" -12.5 dB -- that's generally okay... is that really not enough for you? And what about the "only" 2.5 dB gain -- what did you expect there? It is essentially a dipole antenna with additional loss due to the loss behavior of the dielectric...

As for your Balun problem: the lumped port is of course balanced in HFSS, so in the simulation you won't have any problem. As for the practice run, I'd probably use a lumped balun (e.g. from Murata, Johanson or whatever) and a coax stub. However, I assume you want to built a broadband antenna, so you will probably have to build several antennas with different baluns, since those are generally not broadband.

Hi Roxxon,
I was expecting a RL of atleast -15 dB and a gain of over 5 dB. I have already designed this antenna on Microwave Office with good results and just wanted to double check it by simulating it on HFSS as well . So I just wanted to know that what return loss and gain did you achieve when you simulated this design? Can you tell me about some papers of how to feed this design via balun because I got not idea about it and I have to design that as I need to get the layout of the design.

Hi,

I am trying to design & simulate a spiral antenna. In reality (when the antenna will be fabricated), the copper spiral (top layer) will be printed on a duroid substrate backed by a meshed ground plane (bottom layer). The spiral will be fed from the center using a SMA coax connector. I am trying to deign it in hfss but my results are very bad. I have attached the hfss file. There are 3 designs in it.

The first design, I just draw a rectangle from ground plane to center of spiral & assign it as lumped port. I get a good return loss & good results.

BUT in the next 2 designs, I tried to draw coax probe & excite using waveport. The s11 value is very bad & it converges very slowly.

If you have some time, can you please help me with it? I'll be really glad