Dipole antenna input impedance

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randyorton

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Hello all

I am trying to design a dipole antenna for 150MHz by using a material as a copper but i am getting input impedance as 83-84 ohms. in books i have seen that it should come to 73 ohms but i am not getting it so is it correct or any thing is wrong with it?
 

zz0468

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Hello all

I am trying to design a dipole antenna for 150MHz by using a material as a copper but i am getting input impedance as 83-84 ohms. in books i have seen that it should come to 73 ohms but i am not getting it so is it correct or any thing is wrong with it?
"using a material as a copper"? I'm not sure what that means.

The impedance will be affected by wire size and composition, and objects located within the near field. There could also be instrument errors in your measurements. What are you measuring with, and how have you calibrated it?
 

randyorton

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Thanks for your quick reply,


Actually I was designing it in HFSS, which is a simulation software for antenna designing. Now i am getting it as a 77 ohms which is more practical. I was just wondering that in reality what is the input impedance of a dipole. theoretically its 73 ohs but in reality what is the range?that is my basic question.
 

zz0468

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Thanks for your quick reply,


Actually I was designing it in HFSS, which is a simulation software for antenna designing. Now i am getting it as a 77 ohms which is more practical. I was just wondering that in reality what is the input impedance of a dipole. theoretically its 73 ohs but in reality what is the range?that is my basic question.
Like I said before, it's dependent on whatever real world conditions it's built in. It could be as low as 30-40 ohms, if it's close to the ground or other objects. It could be higher depending on what you've used to build the dipole.

73 ohms is a theoretical value, requiring that you ignore wire size and ohmic losses in the wire. If you're getting 77 ohms in a software program, it's likely that it's asked you what wire gauge you're using and factored that in.
 

nanZor

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.. it's dependent on whatever real world conditions it's built in. It could be as low as 30-40 ohms, if it's close to the ground or other objects. It could be higher depending on what you've used to build the dipole.
It's the real-world that always gets me after modeling. :) A quick visualization aid to help the op might be to envision the perfect horizontal or vertical dipole in free space as having 75 ohms impedance.

Now start bending the elements out of a straight line. For example lets change a perfectly vertical dipole to one where half of it is now horizontal. ie, a 1/4 vertical, with a 1/4 wave horizontal "radial" if you will. 36 ohms impedance.

Split the difference, where the 1/4 wave horizontal section is angled down 45 degrees. 50 ohms. One of the reasons why ground plane verticals usually have radials sloping down at 45 degrees or so, rather than being perfectly horizontal. I've seen guys take this to the extreme trying to match up to 75 ohm coax, and have radials angled down at 70 degrees or so ... approaching pure vertical dipole or a sleeve dipole at this point...

When getting out of free space, the environment and building materials, typically very thin or very thick has a major impact as well as the feedpoint itself getting closer to ground changing these values in the real world... :)
 
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