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I want to know about the directivity of thin dipole antenna of length 1.25λ for my assignment.Is there any formula related to its length?

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  • $\begingroup$ Is it a trick question? $\endgroup$
    – Phil Frost - W8II
    Commented Feb 8, 2017 at 18:42
  • $\begingroup$ @PhilFrost-W8II I doubt it; reads like homework. But yeah, $(1+\frac14) \lambda$ dipole... Anyway, we could change the question to read "given an $\alpha\lambda$-long dipole, can we derive characteristics like feedpoint impedance and gain based on $\alpha$?", and make it useful. $\endgroup$
    – Marcus Müller
    Commented Feb 9, 2017 at 15:35
  • $\begingroup$ If you take your title question and put it directly in Google you will find some examples where this has been taught in schools. $\endgroup$
    – SDsolar
    Commented Feb 24, 2017 at 4:00

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I would imagine that by now, you have completed your homework. But for the sake of closing the topic, the directivity of a 1.25 wavelength, center fed (a very important qualifier!) dipole is ~3.27. Therefore the maximum possible gain is 5.15 dBi.

You can apply the general formulas for field at any distance from a center fed dipole and solve for the maxima. As they are rather lengthy, if you need the specific E or H field formulas, just post back as a comment and I will supply them.

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