Hello,
I have modelled a half wave dipole antenna in HFSS and set the length of the dipole so that it resonates at 5.28 GHz. When I sweep the antenna over frequency ranges from 0.5 GHz to 50GHz, I find that the antenna has in addition to the resonant peak, several harmonic peaks roughly at a distance of 10 GHz. These peaks are within 10 dB of the resonant peak. I need to transmit a signal with low bandwidth (Eg: 5 MHz) but I see that the signal being transmitted is very broadband. How can I adjust this? I tried varying the feed gap of the dipole and even the radius, but it only seems to make the problem worse. What can I do to rectify this?
My solution type is Driven Modal and I am using HFSS 13.0.
The half-wave dipole radiating at 5.28 GHz has a length of aprox. 28,4mm but it will also radiate at the odd harmonics of its fundamental radiating frequency. These are at 10.56 GHz, 21.12 GHz, 31.68 GHz...
To suppress the harmonics parasitic elements (stubs) have been used. One example can be found here:
http://www.arpnjournals.org/jeas/research_papers/rp_2016/jeas_0916_4991.pdf
The bandwidth of the signal you transmit is determined by the circuit that generates it, not necessarily by the antenna. The antenna bandwidth needs to be wide enough to include the bandwidth of the transmitted signal, but is not usually used to control the signal bandwidth. If you have to stop the transmission of signals outside a narrow bandwidth then use a filter before the antenna. Nearly every antenna will radiate at frequencies outside its intended band.
The problem is usually to make the antenna bandwidth wide enough so that it doesn't limit the signal bandwidth. If the required signal bandwidth is narrow that means the antenna design is easier, with more design types available and small size and efficiency easier to achieve.
Normally, we want to make wideband for an antenna but you want to decrease its BW. Strange. At 5.28GHz, it is very difficult to get BW = 5MHz using dipole (even not possible using triangular shaped antenna which is narrow band antenna). To use only 5MHz BW, you are requested to use filter.
In response to Daniel - I think you will find that the dipole is resonant at roughly 5.28, then 15.8, then 26.4 etc, i.e 1/2, 3/2, 5/2 etc. wavelengths.
A half-wave wire would have the resonances in between as well, e.g. 10.56, 21.12, which are one and two wavelengths, but the feed impedance would be wrong so the antenna will be badly matched for these if it is matched for 5.28 GHz.
can anyone plz tell me how to choose the size of radiation box for a geometry
Height of rad_box = lamda/4.....According to HFSS, lamda is the wavelength corresponding to solution freq. But, I prefer to define lamda by lowest frequency (not, solution freq) and Height of rad_box = lamda (not, lamda/4)
what are the steps that can be done to improve the s11 graph......
If you want to see freq vs S11/Zin, it is sufficient to use 10MHz as step size if working frequency is less than 8GHz. beyond that, it is better to use 25MHz and increase it as the frequency increases.......when, you want to plot gain at fo, then run your simulation from f1 to f2 where f1 = 0.9fo and f2 = 1.1fo and use 51 to 101 solution points else it will take long time to give final results
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