helical array in HFSS..doubts please solve

[ruchak]

hi i am designing a helical antenna array placed in cavity. (Each element is placed in a different cavity).
i chose the distances to be lamda/4.
here i encounter few problems.
1. first i simulated a single helix to see the s11 and other outputs. they are just not satisfactory.
2. i am confused as to which solution type is required?
3. How to design an impedance matching technique in hfss.
4. the behavior of a helix in a cavity is more like a resonator so i reffered few ieee papers. which gave me few parameters but i had to convert the element of antenna into strip. this helped in s11. but when i implemented the design in array form it wont give s11 less than -4.
am i making a mistake in excitation or design?
please help.

 

[kae_jolie]

any luck with finding an answer. I have a similar problem especially with impedance matching.

 

[drkirkby]

A normal mode helix has an impedance of about 140 Ohms, so obviously S11 will not be great in a 50 Ohm system.

One way I have seen to match a single helix to 50 Ohms is to make the bottom quarter-turn wider. Then it acts as a lower impedance transmission line of length lambda/4, and so makes a standard quarter wave matching transformer.

I've no idea what happens when its in a cavity.

When you say a cavity, do you mean a groundplane with sides? I know making sides on the groundplane can increase the gain, but don't happen to know what it does to the impedance.

Why don't you post a picture of your model? Posting the HFSS file would no doubt allow more people to help you.

I have some notes around from an HFSS antenna design course that covers an array of helical antennas. There's no cavity in the example, and no matching network, but if you want, I can probably dig them out.

Dave

 

[ruchak]

Actually the cavity adds to the capacitive reactance. so we need to implement the ways to cancell out the capacitive reactance part.
so i used a strip helix. of 1mm x 8mm cross section and 1mm thickness as i read it in a paper that increaseing the crosssection would reduce the impedance. i didnot implement any impedance matching techniques. still i am getting s11 around less than -10.
i also increased the diameter of helix and increased the strip size. it enhanced the matching to s11< -15.
still i had implemented methods like triangular strip, flat strip parallel to ground plane under the last quarter turn, and helical feed, etc. i can help with the design procedure.