0

I am reading about antenna design and have a few questions which I could not get clarified.

I read about "Stubs". I got to know that Stubs are used for impedance matching of an Antenna either by leaving it as an open circuit in one end or as an short circuit.

Question - How to find the input impedance of an Antenna of a particular length? Is there any method and how to calculate the stub length? Is there any formula?

Would really help me if someone could provide an example for calculating the input impedance of the antenna and the stub length for impedance matching

1 Answers1

1

Question - How to find the input impedance of an Antenna of a particular length?

This is for a monopole but there are equivalent diagrams to be found for other antennas such as dipoles etc..

enter image description here

You've seen this before in a question I answered here.

Is there any method and how to calculate the stub length? Is there any formula?

This is the formula that folk use: -

enter image description here

  • \$V_P\$ is velocity of propagation and is around 0.7c for your average coax cable.

Picture taken from my answer here.

Andy aka
  • 434,556
  • 28
  • 351
  • 777
  • Thank you for your answer and the formula. If you could provide an example with numbers & calculation relating to the signal frequency, wavelength and finally to match the impedance,your stub length calculation, it would be really helpful for my understanding –  May 16 '20 at 15:45
  • I fully recommend that you use something like excel and try it out for yourself. This can be compared with many instances of calculations like this to be found on-line. – Andy aka May 16 '20 at 15:48
  • There is [this spreadsheet](https://www.qsl.net/aa3rl/tlcalc1.html) you can download. – Andy aka May 16 '20 at 15:54
  • Thank you for the answer. But just I realized that, suppose, I want to design a custom made antenna say, for transmitting a wavelength of a signal, how to find the impedance of the antenna then? –  May 16 '20 at 17:06
  • You need to match the antenna's design with a standard antenna type and hope that you can find the impedance on-line somewhere. Deriving the impedance of a bespoke antenna is not that easy. – Andy aka May 16 '20 at 17:19
  • Are we done with this question @Newbie or do you need clarification? – Andy aka May 17 '20 at 12:33
  • I am still unclear. I will not be able to understand until someone helps me to provide some numerical examples and calculations –  May 17 '20 at 12:35
  • I left you a link for a spreadsheet and surely you can make your own spreadsheet anyway? – Andy aka May 17 '20 at 12:36
  • The thing is that, I am a beginner in this field. I am trying to understand by reading articles, tutorials, lectures and other stuff available on the internet. So, most of the doubts I have is not cleared by searching, but only by asking questions in this forum. So, unless I see an example, and analyze it for sometime and corroborate it with the theory I have read, I will not be able to understand –  May 17 '20 at 12:39
  • Make your own example and add it as an addendum to your question - you need to do this work and tailor it to specific types of situations that might mean more to you than if I did it. You have to help yourself more - any random numerical example I might produce is going to raise more questions - think about a scenario for a certain stub length at a certain frequency and do the math yourself. You can easily double check your work when the wavelength equals 0.25 because a short becomes an open circuit and vice versa. – Andy aka May 17 '20 at 12:42