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Folded Dipole Antenna

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frankieNrosie

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Hi,

I newbie in antenna area. Currently, I'm trying to design a folded dipole antenna. Thus, I need some ideas on how to design that antenna such the dimensions, length and others.

Please help me. Thanks.
 

sv1437

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See the figure attached. The length of the folded dipole as shown in the figure should be half of the free space wavelength at the design frequency. Generally, folded dipoles have around 300 ohm impedance across the terminals, and inorder to connect balanced to unbalanced line and also to convert the high input impedance of the folded dipole antenna (300 ohm) to around 75 or 50 ohms you need to use BALUN. The same is shown in figure.

37_1291433909.jpg
 

frankieNrosie

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sorry to say, but I'm still confuse. is it the total length of the driven element is λ? how about the gap between 2 wire in the middle of driven antenna?
 

sv1437

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Generally the length of the folded dipole should be 5% shorter than the half wavelength as shown in the figure (which I attached earlier). The are as such no requirements for the gap. When I made a folded dipole antenna, I adjusted the length and gap using the Network Analyzer. (Magnitude of S11 should atleast 10 dB at the design frequency)
 

frankieNrosie

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therefore, in order to calculate the driven element length of folded dipole antenna I'm just use the following equation:
length = 0.475×λ

is it right?
 

sv1437

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Yes, that's right. Are you making a Yagi antenna?
 

frankieNrosie

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yes, i'm wanna design yagi antenna using CST software..currently, finding the way on how to determine the dimension of the antenna..do you have any papers related to the design yagi antenna? btw, how about to determine the width of the conductor?
 

sv1437

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What I suggest is: Don't use CST or any software for making an Yagi Antenna. Take any book on Antennas (Balanis or Stutzman), they have the formulas for determining length of the dipoles and spacing between them. Just go ahead build it and adjust the gap/length for the folded dipole using the network analyzer. Don't worry about the thickness of the conductor at this point.
 

frankieNrosie

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it is requirement for me to design it using CST 1st..then, will make it as real after get approval..
 

kekosat300

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Hi,

I used the trial version of ANSOF to simulate the folded dipole and optimize the dimensions. The software allows you to put arced segments at the dipole ends, so I could model the curved shape of the dipole ends.
 

frankieNrosie

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how do you determine the dimension of antenna? any possible equations to determine it?
 

kekosat300

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Hi,

I couldn't find equations for this case, so I designed the folded dipole after some simulations, since I could modified the antenna dimensions easily.
 

frankieNrosie

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I'm still dont understand about balun. Somebody please explain to me about it?
 

rotmanlens

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HI Frankie

To answer your questions.

Beginning with yagi uda
1.you know its simply bunch of dipoles arranged on a boom with a certain distance between each other.main elements in the yagi uda antenna is driven element(main element) reflector and directors.

Driven element is the dipole connected directly to the supply

reflector and director elements are parasitic elements

normally the length of reflector is around 5% longer than the main element

where as director elements are smaller than then main element by a value of 3-4% ascending.

director elements make the beam shape more directive and decrease the beamwidth of the antenna. reflector decreases the backlobes.

next
Dipole
as you have seen most guys have already told you about the size of dipole it is supposed to be.5lambda and the radius should be atleast 100th of lambda to reduce the skin effect.but practically length of the dipole is around .47lambda.

i wish i could simulate the structure for you but i dont have acess to HFSS right now.but if u need anymore tips feel free to holler at me sajidmiya@gmail.com

Wishing you all the best
S.Mohammed.
 

davenn

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I'm still dont understand about balun. Somebody please explain to me about it?

BALUN = BALanced to UNbalanced

A folded dipole has a balanced feedpoint in this case an impedance of 300 Ohms
Coax Cable is an example of an unbalanced transmission line (say 50 Ohm or 75 Ohm impedance). So to match the coax to the antenna feedpoint you need a balun to match the transmission line to the antenna.
Now a 4 to 1 balun would provide a perfect match for 75 Ohm coax.
The mismatch using 50 Ohm cable isnt too bad, a VSWR of ~ 1.5:1 still quite acceptable.
Now if the folded dipole is used in a Yagi configuration as the driven element, then the impedance of the dipole feedpoint will drop. And depending on the exact design of the yagi it can easily drop as far as only a 200 Ohm impedance feedpoint. This makes it an ideal match for 50 Ohm coax and a 4:1 Balun.

There is one other type of balun that is often used ... Its not so much for matching feed point to coax but rather to stop RF currents from flowing on the braid of the coax, which will result in its radiation into the air and can cause problems, as well as less efficiency.
This style is a 1:1 balun and usually consists of a 4\gamma (1/4 wave) stubb or sleeve coming down the coax from the feedpoint

cheers
Dave
 

frankieNrosie

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I have saw many papers regarding to design the quasi-Yagi antenna. please refer figure below, is it BALUN in the red circle? if yes, why the BALUN is not symmetrical? I means, the length of right arm is not equal to left arm.
 

sv1437

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Hi frankieNrosie,
I will explain the figure which you attached. Intially, the input power is split into two arms using a T-junction power divider. For a dipole the currents in the arms should be 180 degrees out of phase. So here Balun performs that operation. The reason why Balun is not symmetrical is because the part that you circled in red, provides the 180 phase difference. The electrical length of L2+L1+L2 introduces 180 phase shift.

If there is no phase difference between the two outputs, the dipole will no longer function as an antenna.

In general, Balun not only does the impedance transformation, but also makes the outputs to have 180 phase difference between them.

-sv
 

frankieNrosie

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Hi frankieNrosie,
I will explain the figure which you attached. Intially, the input power is split into two arms using a T-junction power divider. For a dipole the currents in the arms should be 180 degrees out of phase. So here Balun performs that operation. The reason why Balun is not symmetrical is because the part that you circled in red, provides the 180 phase difference. The electrical length of L2+L1+L2 introduces 180 phase shift.

If there is no phase difference between the two outputs, the dipole will no longer function as an antenna.

In general, Balun not only does the impedance transformation, but also makes the outputs to have 180 phase difference between them.

-sv

Hi sv1437,
thanks for your explanation. what do you mean by L2+L1+L2 introduces 180 phase shift? i'm still not really understand. do you know any related equation to calculate the length of the arms? one more thing, is it possible to have symmetry arm (same length)?
 

sv1437

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L1 and L2 are specified in the figure that you attached. The total electrical length of that arm should be 180. In order to calculate the electrical length, you need to know the dielectric constant and height of the substrate.

Which simulation software are you using ? Generally, all the simulation softwares have in-built calculators, so you can make use of them.

What do you mean by symmetry arm ? As I mentioned before, in order to have 180 phase difference, the difference in electrical lengths for the two arms should be equal to 180.
 

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