HFSS - Half-wave dipole has an input impedance of 84 + j 51 Ohms or even under 1 Ohm.

I've tried to simulate a half-wave dipole in HFSS, using a perfect electrical conductor (pec). The dipole is 500 mm long in total, with a small gap between the two arms to allow a lumped port to be put there. So this should be a half-wave at 300 MHz.

Using a wire radius of 1 mm, with a 1 mm gap between the terminals, the input impedance is 84 + j 51 Ohms. See 3rd and 5th colums below.

The impedance of an infinitely thin half-wave dipole is known to be 73 + j 42 Ohms, but since this is a finite thicknesss, I would expect the thickness to decrease the impedance somewhat, so 84 + j 51 Ohms makes no sence whatever to me.

If I change the wire radius to 0.1 mm, which should more closely approximate the theory for an infinitely thin dipole, and narrow the feed gap to 0.1 mm too, so the results go totally bizare - an input impedance of 0.000003 + j 0.05 Ohms. (See 2nd and 4th columns)

Can anyone see what"s wrong with the model attached? The AirBox is sufficiently large to allow analysis down to 100 MHz, so I'm a bit puzzled why the results are so bizzare.

I find it quite worrying using HFSS. I often get results, and really have no idea if they are sensible or not. It is very easy to get incorrect results, and I personally find it hard to know when my model will introduce problems.

I used the Antenna Design kit (free download for HFSS uses) and synthesised a dipole with that. It chose a radius of 7.5 mm for a 300 MHz dipole, but in practice people will make them much thiner than that. But with my model, that gives an impedance of 106 + j 43. So whilst the imaginary part is about right, the real part is way off.

Of course, I could look at the design produced by the antenna design kit, and no doubt use that to get a better result for a half-wave dipole. But that just proves I can't get decent results myself, which is worrying.

Deborah

Hello Deborah

Had a chance to see your design,Everything looked absolutely unquestionable except the solution type that was chosen.When i changed the solution type to driven modal i was able to get an impedance of 84+j49 ohm's at 300MHZ and a gain of 2.6GHz.

Now i am sure you will ask me why Driven Modal and not terminal.Well i will let you refer to the explanation presented by HFSS which is as follows

Driven modal
– S-matrix solution expressed in terms of incident and reflected powers of waveguide modes
– Always used by wave solver
– Integration lines set phase between ports and modal voltage integration path (Zpv and Zvi)
– Use for modal-based S-parameters of passive, high-frequency structures such as microstrips, waveguides, and
transmission lines
• Driven terminal
– S-matrix solution expressed in terms of linear combination of nodal voltages and currents for wave port
– Equivalent “modes-to-nodes” transformation performed from modal solution
– Use for terminal-based S-parameters of multi-conductor transmission line ports (with several quasi-TEM modes, etc.)

I understand your frustration,its just these simulation softwares come with such a diversified terminology and concepts its very difficult for a beginner like me and others to understand and encapsulate the information properly.

Hope that i could shed some light.

Regards
Sajid Mohammed.

Hi there,
I assume you mean a gain of 2.6 dBi, not GHz.

I just simulated this in MMANA-GAL (method of moments code based on NEC2), and get a gain of 2.17 dBi, which seems more realistic than 2.6 dBi. The input impedance with a 1 mm radius wire is 85.89 + j 47.15 according to MMANA-GAL. I must admit, I thought the non-zero thickness wire reduced the impedance over the 73 + j 42 for an infinitely thin wire, but it seems I was mistaken.

Thank you for that. I just checked and the Ansof Antenna Design kit uses a driven modal for the solution type.

Yes, thanks a lot. It was helpful.

It seems there is a place for someone to write a book on HFSS. One can find books on Word, Autocad, Photoshop and most other programs, but nothing at all on HFSS.

Definetly Agreed to your idea of writing a book. then we have to see the spectra of fields that HFSS involves compared to autocad,photoshop etc. Use of HFSS is vast and writing a book according to me a huge challenge.

But i am sure there are plenty of learned people out there who like challenges(:P).

Thanks a lot

Regards
Sajid Mohammed.

I guess the book would have to concentrate on specific ares.

* An introduction to HFSS
* Antenna Design with HFSS

would be the two most useful books to me. I'm sure someone could make a decent amount of money from such a venture, as they would be entering a market with precisely zero competition. There must be several hundred books written about Mathematica, and that is a pretty specialised program, though not as specialised as HFSS I would admit. But people write books about more speciialised topics than HFSS. Looking on here, and searching with Google, it seems I'm not alone in finding HFSS difficult to use properly, so I think there would be a decent sized market for a good book.


Deborah

Hey guys I found a dipole model from Clemson Modeling a Center-driven Dipole with HFSS When the results were produced the impedance I got was about right, but the imaginary part was lower then expected, and as I increased the number of passes the imaginary part of the impedance "converged" to 30 ohms. This model was from the website above, but the low pass criterion are saved in the model attached.

Also Deborah, wouldn't you expect the real part of the resistance to be higher since that means there is more loss due to conductor and dielectrics?

I have not looked at the other replies by other posters, so, this may have been addressed; it appears your dipole is actually cut for 285 MHz, where the reactance component is nearly zero your Re (real) shows 69 some Ohms ... it looks good, but you need to adjust then 'length' to play mother nature's Physics game according to her rules. (Diameter of the dipole plays a part too; affects overall capacitance and where resonance occurs.)

Experience in real life (with a VNA on the bench) underscores the need for dipoles to be cut a little shorter than the first-pass

. . . Dipole_length = 1/2 * ( speed-of-light / freq )

formula would lead one to believe.


Reiterating: Dipole resonance is where the im (imaginary) term goes to zero: the rest is Rrad (nominally speaking).


Jim