Balun for 2-18 Ghz. spiral antenna

[ayy2023]

balun spiral antenna

Hi all,
i am currently designing a spiral antenna operating at 2-18 ghz. with hfss v10.
i am about to finish simulations but i can not match it to 50 ohms so i need a balun.
and i don't know much about baluns. can you help me pls
all the helps are appreciated.

 

[vfone]

2-18 ghz antenna

**broken link removed**

 

[Azulykit]

cavity backed spiral antenna, balun

Two structures come to mind that maybe helpful. (I am guessing that you are probably working on a cavity backed spiral about 3 inches in diameter and about 2 inches deep.)

If you are looking for a balun I would also assume you are using a coax feed (you will need nearly 5 octaves of bandwidth).

One can attach the coax to a microstrip transmission line and then gradually taper the line to a pair of (broadwall coupled)traces. Think of a transition to twin lead. The longer the better as you will need around a half wavelength at 2 GHz. The symmetric end may then be fastened to the pair of spiral arms.

Another approach is to take a coaxial cable (like 0.141" semi rigid or even smaller) and cut the shield with a long diagonal cut. You will end up with a shield looking like the business end of a hypodermic needle. The sharp end of the taper and the coax center conductor now form a balanced line. These are fairly easy to fabricate if you have access to a friendly machine shop. There is the risk of slight beam squint with this one.

Since you have HFSS available you can easily experiment to refine the dimensions and get the match you need.

Corzine and Mosko's book might be helpful here.

 

[ayy2023]

spiral cavity backed antenna

thanks but i'm working on a planar equiangular spiral not a cavity backed spiral. and i don't understand why people work with a cavity backed spiral. what difference does the cavity back do and why people use it? i'd be thankful if you answer this too.
thanks again.

 

[melc]

antenna balun planar spiral

Factory manufactured baluns are either 100/50 either 200/50ohms.
I don't know at 18Ghz, but I can tell you for sure in 2.5-5Ghz range any balun will
decrease your overall antenna performance (at least the gain but some problem will be with SNR too).
Replacing baluns with passive components gets better results in the frequency range above, but is increasing also the circuit complexity on real world.

 

[Azulykit]

spiral 2-18 ghz ghz

One reason for the cavity is to redirect the power that would otherwise be radiated in the reverse direction and thus increase the gain. There are a wide number of schemes devised to accomplish this with varying degrees of success. The problem is to keep the reflection in phase over the entire band of operation. It is easy to say and hard to do.

 

[mfarhan1]

balun 2 to 18 ghz

i think spiral anteena itself is unbalanced structure and if you are feeding it with coax line which is also unbalanced transmission line you dont need balun, so there is some other problem not balancing problem,

 

[Azulykit]

antennas spiral 2-18 ghz

From the last post I see that I need to amplify my earlier comment. I was assuming that the question related to a two arm planar spiral antenna. This device could also be driven with a balanced line and would not need a balun.

Hopefully, that has not caused undue confusion.

 

[ayy2023]

18 ghz balun

thanks everybody
all comments are very useful
they helped me improve my knowledge

 

[ayy2023]

2-18ghz sprial antenna

i wanna ask something
if i feed my spiral antenna which i talked about earlier in this subject, with a 50 ohm real impedance and i try to use capasitance for imaginary impedance for my antenna's input impedance's imaginary part, does it achieve for uwb like 2-14 ghz? if doesnt what should i do???

 

[chamonz]

2-18 ghz balun

hi

do you know how to create balun or matching impedance
for my log-spiral antenna? i have an antenna with approximately 220 + j200 ohm. My frequency range is 1.8 - 6 GHz. In hfss design, i put lump port in center of spiral, between 2 spiral arms.
i want to match it with coaxial 50 ohm.i don't know too how to create
balun or matching impedance in hfss because i'm new in this software.
must i change the port with wave port?