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pamela81
Joined: 12 Apr 2008
Posts: 12
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 25 Jun 2008 18:32 dipole in water |
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Hello everybody!
I'm a beginner in the use of HFSS and I need to simulate in open space some coils, so I began to run simulations with dipole, but in water the s-parameter obtained rescaling the length of the antenna to resonate at my desired frequency (433 MHz), is very low, and in particular under 1 dB with PML boundaries and under 3 dB with the condition of 'radiation'. What's the matter with the project?
The program seems to give best results if I don't shorten my dipole when I put it into water (infact I divide the lambda obtained from c/frequency by the square of the epsilon of the water).
Please somebody help me! Pamela
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jallem
Joined: 23 Jul 2005
Posts: 467
Helped: 63
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| 25 Jun 2008 18:57 Re: dipole in water |
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Pamela, the water has certain conductivity, did you take that in consideration?....
for other hand the model may not work if the
whole "antenna" suddenly is inmerse in water. I think it should
have at least non overlapping of structures of different materials,
most likely is not your case since otherwise HFSS will give you
a warning.
Post your model here would make it easier to advice you.
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pamela81
Joined: 12 Apr 2008
Posts: 12
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| 25 Jun 2008 21:19 Re: dipole in water |
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Hi jallem!
Thank you very much for the reply.I embedded the dipole in polyester before immersing it in water with positive conductivity.I'm posting three project because I don't know what is the best, since I'm trying to put PMLs in the best way, without knowing well how to do it: do you know why manually applying PMLs the program doesn't generate boundary conditions?
In the last two projects I put manually PMLs, in the first case covering them with pec. What's the right way to assign PMLs?
I would be very grateful if you could take a look to my projects.Bye bye!
Pamela
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jallem
Joined: 23 Jul 2005
Posts: 467
Helped: 63
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| 26 Jun 2008 3:27 Re: dipole in water |
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Somethings I do not understand:
1) The adaptive frequency is 433 MHz but the sweep is from 1 GHz to 10 GHz, Why?
2) Why use PML?
3) Why not to substract the dipole and the polyester case from the water?
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pamela81
Joined: 12 Apr 2008
Posts: 12
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| 26 Jun 2008 20:20 Re: dipole in water |
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Hi jallem, thank you very much for helping me.
In my original projects the frequency sweep was between 200 MHz and 800 MHz. I have been using PML because radiation didn't give me good response,so I thought it was a matter with boundaries.
I followed your suggestions and subtracted without cloning dipole and case from water and I assigned radiation to the outer box, but the validation check says:''Port face does not have a solved inside material on either side''. If I subtract after cloning the error becomes:''A radiation boundary face has solid objects with solve inside on both sides of it'', because, I suppose, the box' s face touching case becomes outer.
The last test was subtracting only dipole from case to use radiation without errors but the s-parameter was not so good, even if there was exactly one peak on 440 MHz with an amplitude of -0.30 dB. Have you any idea of how to improve this result? Thank you very much!!! I'm posting the last project.
Pamela
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jallem
Joined: 23 Jul 2005
Posts: 467
Helped: 63
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| 27 Jun 2008 3:03 Re: dipole in water |
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you realize the antenna is electrically small, Don't you?
It is only 18 mm for a frequency of 433 MHz. Is not
exactly sorrounded by water (er=78 ) but plastic which
er=3.2.
For other
hand the cube is small for that frequency also. You
tried to adjust it based on the frequency but I think
still small. Also still
have a sweep from 2 to 10 GHz. The following graph
is from your file with almost not modification just that I have
to redo a little bit since I do not have HFSS 11.
Remove the extension .pdf to see the file.
I would recommend to start with the dipole alone, not
plastic case, no water and then make it work. I do not
think it matter if the strctures are substrated. But definitely
matter the size of the dipole. Do not parametrize the rad
boundary unless you want to do an analysis at different
frequences.
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jallem
Joined: 23 Jul 2005
Posts: 467
Helped: 63
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| 27 Jun 2008 17:35 dipole in water |
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Hi Pamela.
Since I can not send private messages, for some
reason, I need to answer over here. Dont need for
thank you. Just let me know the final result if
work out for you. It always feel good to know the
finals results.
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pamela81
Joined: 12 Apr 2008
Posts: 12
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| 27 Jun 2008 21:06 dipole in water |
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Hello jallem, I'm embarrassed but I need to address you, that have been very kind, a last answer (I promise this is the last  ).
My intention was not that of making an electrically small dipole, i.e. a dipole long between lambda/50 and lambda/10, but a half wavelength dipole; so I computed lambda by the formula: lambda=c/(f√ε), were c was the speed of light, f my desired resonance frequency (433 MHz) and ε the dielectric constant of water, and then I made the dipole as long as lambda/2 i.e. 36 mm, as you can see from the project, instead by your suggestion and watching s-parameter I would lengthen dipole over lambda/2 to lower resonance frequency to my desired value. Where's the mistake?
Thank you very much!
Pamela
Added after 3 minutes:
A last thing: how to open the file with .pdf extension if the extension doesn't appear in the name nor in the properties of the file?
Thank you
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jallem
Joined: 23 Jul 2005
Posts: 467
Helped: 63
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| 27 Jun 2008 21:43 Re: dipole in water |
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Yea, your intention was to not make a electrically
small antenna, but because you use 1/√(78 ) to
calculate the length then the
antenna becomes small since is not sorrounded by
er=78 but instead for the plastic (er=3.2).
In other words, the antenna is small for the plastic
but not for the water.
That brings a question, if the antenna is resonant
at er=3.2 once is summerged in water the antenna
will still resonant?..I do not think so.
The antenna is just a transducer, trying to go
almost directly to er=78 using an "interface" of
er=3.2 will not work. If you use a plastic with
a er bigger the antenna will not be too big.
It is a nice project, probably the antenna efficiency
will depend on the radious of the plastic also.
That is way I adviced you to analyzed by step,
first the antenna alone, then add the plastic
and then add the water. I bet you will find a lot
of variables to play with.
The file that was attached has a name like this:
xxxxxxx.hfss.pdf , is the same HFSS file just you
have to strip out the pdf extension, to have
xxxxxxx.hfss, that is all. The reason is this forum
does not let you post hfss files, do not know why.!!
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