Balun to feed spiral antenna

I designed an Archimedes Spiral antenna which operates from 300 MHz to 3GHz, but the impedance is around 100 ohms. Since the antenna is balanced, I also need a balun, but I can't find any 100:50 balun available in this range. Can anyone help me? How can I possibly feed this antenna?

Either a Marchand balun or a Bawer and Wolfe balun are popular choices for spiral antennas.Try Googling for them.

Hi mateusbatera

read this paper

Plasma

Look what I found: http://www.token.com.tw/pdf/smd-coils/smd-coils-1.pdf

The part number TCB4F-617DB-1673, whose chart can be found in page 3, seems to behave well in the range between 10 MHz and 3000 MHz. The insertion loss varies between 1 and 4 dB, which is acceptable. The Impedance ratio is 200:50 ohms, which is OK too, because I can easily increase the antenna impedance to 200 ohms.

My main concern here is that they do not provide any return loss chart (nor Smith Chart). Maybe they did not inform because the balun doesn't perform so well in return loss. I mean, when the stuff is too good, I tend to suspect that something must be wrong...

hi mateusbatera
How did you get 100ohms Archimedean spiral antenna. i want to design 100 ohms two archimedean spiral antenna. can i know the details of spiral design
inner radius, outer radius and no of turns, width of arms and gap size.

i would be grateful, if you give some details

thank you

The best aproach to reduce the antenna impedance is material loading. I simulated a spiral antenna printed in a Rogers RT6010 substrate, for it has the largest dielectric constant among the materials available in CST. The impedance was very close to the 100 ohms goal, however, a single 20"X20" RT6010 board costs more than $1,200.00! Although this is a good solution, it's too expensive...

In case you need, here are the parameter values:

Inner radius: 3mm
Number of turns: 6.5
Growth factor: 3.5
Outer radius: 14 ~ 15 cm
Arm width: 2mm
Substrate: 2.5mm thick RT6010

hi
thank you for your reply.
as you told, i am also using RT6010 Roger as the substrate in CST. in this case
i set the lower frequency as 2GHz and upper frequency as 20GHz. so these spiral parameters don't affect the above frequency or do i have chage the inner and outer radius according to this frequency. if so, it doesn't change the impedance (100 ohms).
plz suggest me, how can i alter the parameter for above frequency range.

thanks again.

Regards
Kaandee

---------- Post added at 02:27 ---------- Previous post was at 02:04 ----------

hi
i have used the following equation to find out the inner and outer radius
r1= C/ 2* pi* f1 (f1= 20GHz) and r1 inner radius,
r2= C / 2*pi* f2 (f2= 2GHz) and r2 outer radius.
is this equations are ok to find our inner and outer raadius . And also, i am using 0.635mm RT6010 substrate. it will affect the impedance?

thank you

First, let`s talk about the substrate thickness: simulations have shown that, the higher the thickness, the lower the impedance. If your impedance is higher than 100 ohms, try to thicken the substrate. However, avoid make it too thick, or else, the impedance will oscilate with frequency variation. That`s what I`ve noticed using CST.

I`m not sure about the equations, but there is indeed a relation between the highest frequency and the inner radius (the same happens with the lowest frequency and the outer radius). Since your lowest frequency is 2 GHz, you don`t need to make the antenna as large as I did, but in the other hand, the inner radius must be much smaller. But remember that you must regard the fabrication process limitations. I mean, you cannot fabricate an antenna with a gap as small as you want. Besides, the inner radius has some influence in the impedance.

I think I have a paper about an spiral antenna covering the range you need, but I`m not home now, so I can`t send you until monday. I hope I was able to help. And I will be glad to send you any useful paper as soon as I get home.

hi mateusbatera
thank you for your reply. I am looking for your paper.
as you told in previous mail, substrate height has an effect in impedance. unfortunately, i have the TR 6010 rogers with a height 0.635mm.
I think, according to inner and outer radius, i can set the lower and upper frequencies.
I have a question...No of turns and growth rate have any effect in the impedance?
In your previous details....
what about the gap width? did you assume the gap width and arms width as same?
Where you are using Growth in CST?
in CST....
it ask you ti enter
Height
Inner radius
Width
Number of turns
Gap
Segments/turns ??? (What is this)

in Angle offsets
at start of spiral ??? (What is this)
at End of spiral ?? (What is this)

i am looking forward to hear from you
thank you

Regards
kaandee

---------- Post added at 04:42 ---------- Previous post was at 04:39 ----------

In CST...when we design an Archimedean spiral antenna....
What about height...it is antenna height or substrate height...

---------- Post added at 05:48 ---------- Previous post was at 04:42 ----------

in my design, i need to keep inner radius as much as small because balun which i am using to feed spiral antenna has CPS . and it has (1.43 mm ) gap .
so select the inner radius as 0.7mm. it affects the impedance much?

thank you

To be honest I'm not really an expert in this subject, so I can't answer your questions precisely. So far I have noticed that the substrate thickness, dielectric constant, inner radius an arm width are the parameters which influences the antenna impedance. I'm not sure exactly how, so you will have to tweak those parameters and see what happens.

And about the CST, I'm not using the spiral macro. I'm using the "Analytical Curve" or something alike. Then I enter the equation of the archimedean spiral "(ro + a*t)*cos(t)" for x(t), "(ro + a*t)*sin(t)" for y(t) and "0" for z(t), where "ro" is the inner radius and "a" is the growth rate. "t start" must be 0 and "t end" must be 2*pi for one turn, 4*pi for 2 turns and so on.

You can find more information in the docs attached.

thanks friend..
i will look that docs