Simple flat dipole in air, impedance is NOT 73 ohm. WHY?

Hello,

This is the simplest design one can ever do in HFSS 12.

I wanted to test a simple flat dipole of length 33.5mm and 62.3mm
These are resonating antenna and are half the wavelength.

The dipole is in air, NO substrate.

The impedance of a half wavelength resonating dipole is ALWAYS 73 ohms plus whatever is the reactance.

However, when I run this design of dipole length = 33.5mm, from Z parameter first resonance occur at 4.29 GHz and impedance is 92 ohm at that resonance

Again when rerun the the same design but now the length of dipole is changed to 62.3mm, the Z parameter gives the first resonance at 2.43 GHz and impedance of 163 ohm at that resonance.

I cannot understand why the impedance values will be so high for these two lengths of the same dipole. Any help will be appreciated. This is urgent so any reply will be highly appreciated.

I am attaching the HFSS simulation file and Z parameter graph.

To change the length of the dipole: Go to Project on top>project variables>l1=l2=16.75mm for 33.5mm dipole or l1=l2=31.15mm for 62.3mm dipole

Thanks in advance.

I cant open your file. Anyway

1) what is the airbox size
2) how did you model the feed

I actually had small air box. I enlarged it, like more than a quarter of a wavelength. Now the impedance is 67 ohm and first resonance at 4.06GHz for 33.5ohm; 69 ohm and 2.19 GHz for 62.3 ohm. But still it is not 73 ohm and the resonance occurring does not correspond to half the wavelength.

A resonant dipole impedance is about 67ohm with length about 95%+ of halfwave length.

73+j42.2ohm is at half wave length including the conductor diameter (thickness) and input gap.

Hello,

The impedance of 73 + j42.2 Ohm is for an infinitesimally thin half-wave dipole, that is, a dipole with zero thickness (zero radius for a cylindrical antenna). Then, this value is obtained theoretically by a limit process in which the dipole cross-section tends to zero. The values around 90-95 Ohm in the input resistance is then due to the finite thickness of the dipole. It can be demonstrated that the input resistance is more affected by the "finite thickness effect" than the input reactance.

kekosat300 said:

Hello,

The impedance of 73 + j42.2 Ohm is for an infinitesimally thin half-wave dipole, that is, a dipole with zero thickness (zero radius for a cylindrical antenna). Then, this value is obtained theoretically by a limit process in which the dipole cross-section tends to zero. The values around 90-95 Ohm in the input resistance is then due to the finite thickness of the dipole. It can be demonstrated that the input resistance is more affected by the "finite thickness effect" than the input reactance.

Click to expand...

Shouldn't it be the opposite? The induced EMF approximations (see Balanis, Jasik, King, or probably a lot of other antenna texts) all give expressions for input impedance that only have dipole radius terms in the reactive side of the equation.

kekosat300 said:

Hello,

The impedance of 73 + j42.2 Ohm is for an infinitesimally thin half-wave dipole, that is, a dipole with zero thickness (zero radius for a cylindrical antenna). Then, this value is obtained theoretically by a limit process in which the dipole cross-section tends to zero. The values around 90-95 Ohm in the input resistance is then due to the finite thickness of the dipole. It can be demonstrated that the input resistance is more affected by the "finite thickness effect" than the input reactance.

Click to expand...

That's wrong. The real part of the input impedance falls with increasing diameter of the wire, so for a finite diameter the real part is likly to be closer to 70 Ohms - certainly not 95 Ohms.

Dave