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I'm very new to Ham radio and would like to know in simple terms how to calculate antenna the length of an antenna required for a 477 MHz uhf radio and or half wave or 1/4 wave whatever is better. I would like to add an external aerial on the end of coax. Thanks

Cos Davidson
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    You might want to look at https://ham.stackexchange.com/questions/283/calculating-antenna-length-on-the-fcc-exam-vs-in-reality – user3486184 Mar 21 '22 at 07:25
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    The linked question really answers this completely, even though it is about the FCC exam question. All exams have their own approximations for wavelength you have to learn, but the takeaway is that it is an approximation. –  Mar 21 '22 at 13:50
  • Welcome to Ham.SE. Make sure you take the [tour] if you have not already. Note that one of the only rules around here is "one question at a time". So the question about deciding on a $\lambda/4$ vs. $\lambda/2$ should be a different question. –  Mar 21 '22 at 13:53
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    Thanks for your comments I'll check the tour out soon. Makes sense to ask one question at a time Thanks – Cos Davidson Mar 22 '22 at 05:44
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    https://ham.stackexchange.com/questions/283/calculating-antenna-length-on-the-fcc-exam-vs-in-reality has a different title but is a clear answer to this question. – David Hoelzer Mar 23 '22 at 18:37

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Antenna length is generally very simple.

You first have to calculate the wavelenght, which is given by the formula : L=C/F

With C = speed of light 3x10^8

F = 447x10^6

You have obtained the wavelenght, divide this by 4 to obtain a quarter wavelength antenna. There are lots of online calculator which will give the result to you even corrected for some factors. You should definitely use them.

https://www.66pacific.com/calculators/quarter-wave-vertical-antenna-calculator.aspx

Wireless Learning
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    Your equation is needlessly scaling MHz to hertz. Just use the rounded figures mentioned in the linked Question for meters or feet, adjusting for below 30MHz or so. This is the way it has been done for decades since one is going to trim the length to tune anyway. Also, that link will be dead in an internet minute, leaving this Answer less useful for others in the future. There isn't any reason to provide a calc link site, of which there are hundreds with a simple web search. –  Mar 21 '22 at 13:48
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    I like to use SI units, it might be due to the time I spent at school. Which also give the result in .... SI unit, I know the meter is fancy for some place in the world but that's how you do it.

    In a nutshell, thanks a lot for the help.

     – Wireless Learning Mar 21 '22 at 14:34
  • Just use $c = 300$ to get $\lambda$ in metres then, and scale hertz to MHz. That 300 has to be adjusted by 0.95 or so below 30MHz anyway. There is no need to use hertz in the equation. (My comment was not about using or not using SI units. It was about the convention that all hams use when approximating an electrical length for $\lambda$ [or some fraction thereof] using scaled values for $c$ and $f$.) –  Mar 21 '22 at 14:49
  • See https://ham.stackexchange.com/a/330/21417 –  Mar 21 '22 at 15:01
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    Speed of light does not very with frequency. – AG5CI Mar 23 '22 at 01:27
  • I think there is a typo @AG5CI – Wireless Learning Mar 23 '22 at 15:07
  • @AG5CI but the relative speed of the electrical fields exciting a conductor does change with frequency in practice, which means the electrical wavelength often needs to be adjusted for that. The easiest way to take this into consideration is to scale $c$ (usually down, usually by some small amount) when making practical calculations. –  Mar 23 '22 at 15:49
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You ask whether a half-wave ($\lambda/2$) or quarter-wave ($\lambda/4$) antenna is better.

It depends on where the antenna is mounted and what you are trying to achieve.

The most common use case in your band is repeater communication. You might also look into a $5/8\lambda$ antenna as well, which provides a little extra gain. There are tons of these available cheaply from various manufacturers, usually supporting dual-band operation.

AG5CI
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