Problem with dielectric in parabolic antenna

Parabolic antenna Dipole

Hello all,
I designed a parabolic antenna for 2.4 GHz, but I have the following problem: when I probe the dipole for match the parabolic this is work very fine, but when I probed the same dipole when a dilectric it performance down 3-6 dB. Dielectric is used for protection.
Dielectrics, that I probed, are Epoxi resine, and other that I not remember the name. Later I probed that a polycarbonate dielectric and the performance is very good.
I know that the first dielectrics have a dielectric constant between 3 and 5, and that the polycarbonate is minus that 3.
Constant dielectric is the problem?
I don´t use the polycarbonate dielectric for the cost of the matrix. And I find one dielectric material that it manipulable and cheap.

Dienal

Re: Parabolic antenna Dipole

Hi dienal

What is the loss factor (tan d) of your dielectric at 2.4GHz?

Bye

Re: Parabolic antenna Dipole

I don't know specific. The range of the loss factor is 0.0055 to 0.0005. The people that I bought this product they didn't know specific value.

Dienal

Re: Parabolic antenna Dipole

Hi Dineal,

Your problem probably is that the dielectric coating/covering on the surface of your dipole shifts its original resonant frequency.

It would be good if you could change either the frequency up or down, or the length of your dielectric-covered dipole to see which way the resultant resonant frequency changed. Does the matching (return loss/VSWR) also change with the coated case? But this latter maybe changes too little to notice it.

My understanding is that dielectric coating makes the the original resonant frequency lower (the EM waves can propagate only slower in them than in free space.

rgds, unkarc

Re: Parabolic antenna Dipole

Hi dienal

These loss-factors shouldn't be a problem. So I agree with unkarc that a change in the resonant frequency could be the problem.

Bye

Parabolic antenna Dipole

Does the dielecric protect the dipole only or protect entirely the dish?
If the dielectric is "near" the dipole 2 effect may occur:
1) the equivalent er (air "mixed" with the dielectic)change a lot.
2) the tand is so high that the signal dissipate inside it.
If the dielectric protect the dish as a radome,it may be so thick that became a lens and modify the beam

Solutions:
1) the radome should be farer from feeder
2) use lower er and/or lower tand material.
3) use thinner material.

A very cheaper solution is the use of "polistyrene" foam sheet.

The best polistyrene foam sheet is the BASF 3035, a green coloured, resistent material, used on housing.
It has been tested in microwave region and it'is very "transparent".
Anyway the most common, white coloured, polistyrene foam used on packaging is good enough bau less resistant to wheater.