Help with multiband dipole antenna

[ji3n]

Hi

i am designing a printed dipole antenna(full wavelength) with 1575Mhz frequency.
so after the calculations, i simulated the antenna with a whole length of 89mm and i got around 1575Mhz resonant frequency.

however when i simulated an antenna with the length of 212mm(whole length), i manage to get the frequency i wanted as well.
so when i look more thoroughly, the 1575Mhz is the 2nd resonant frequency.

does this means the 212mm antenna is a "two wavelength antenna"?

any help would be very appreciated. :smile:

 

[WimRFP]

One lambda in air is 190mm, so an electrically straight half wave dipole will be in the range of your 89mm (because of influence of dielectric and fringe capacitance at the dipole ends). Your resonance has low impedance when you feed the antenna about in the middle.

A full wave antenna (fed in the middle) will have high impedance at resonance, this can be in the kOhms range (depending in thickness or width of conductors).

Your 212 mm resonance length seems somewhat strange to me. Did you simulate with the source directly at the dipole, or at the input of the feeding network (balun?). This length looks more like 1.25 lambda. It is definitely not a "two wavelength antenna".

Though the impedance can be what you want, the radiation pattern may not be what you want (more like a clove or multi lobe pattern).

 

[ji3n]

Dear WinmRFP

the 212mm length is fed at the middle just like the 89mm one.


"One lambda in air is 190mm, so an electrically straight half wave dipole will be in the range of your 89mm" do you mean by 1 pole is 89mm, or 2 poles together are 89mm? :-?

i used this equation to calculate the length of the antenna.

143 / 1575 = 90.8mm
since it is a full wavelength, 90.8 x 2 = 181.6mm

however what i read from some websites, it says the dielectric constant will affect the length as well.
how can i calculate the length with the dielectric constant included?

Thank you

 

[WimRFP]

The full antenna length for a half wave dipole, is somewhat less then a half wavelength, Each pole is about 45 mm.

Your formula is to get the starting values for a so-called halve wave dipole in free air (as the length is about half of a full wave).

Dielectrics do affect the resonance frequency. The amount of length reduction depends on how much of the E-field path is through dielectric. More close: the smaller integral(E*ds)/(wire voltage), the less the influence. "s" is distance in dielectric. Same dielectric at some distance has less influence than same dielectric touching the antenna. A thin PCB will have less effect then a thick PCB. There are no closed form equations for this problem.

You will notice a huge change when there is a groundplane under your dipole as in that case virtually all field lines have to go through the dielectric. The dielectric has the effect of reducing the wave propagation speed in the dipole conductors, so the wavelength reduces.

If, due to the dielectric, the wavelength reduces from 212 to 100mm, your half wave antenna would have a length of about 45mm.

You may look into patch antenna design, there you will see that the actual patch size is mostly far below a half wave length for free space.