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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

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    $\begingroup$ You might want to look at ham.stackexchange.com/questions/283/… $\endgroup$ Mar 21, 2022 at 7:25
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    $\begingroup$ 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. $\endgroup$
    – user21417
    Mar 21, 2022 at 13:50
  • $\begingroup$ 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. $\endgroup$
    – user21417
    Mar 21, 2022 at 13:53
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    $\begingroup$ Thanks for your comments I'll check the tour out soon. Makes sense to ask one question at a time Thanks $\endgroup$ Mar 22, 2022 at 5:44
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    $\begingroup$ ham.stackexchange.com/questions/283/… has a different title but is a clear answer to this question. $\endgroup$
    – David Hoelzer
    Mar 23, 2022 at 18:37

2 Answers 2

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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

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    $\begingroup$ 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. $\endgroup$
    – user21417
    Mar 21, 2022 at 13:48
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    $\begingroup$ 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. $\endgroup$ Mar 21, 2022 at 14:34
  • $\begingroup$ 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 $f$.) $\endgroup$
    – user21417
    Mar 21, 2022 at 14:49
  • $\begingroup$ See ham.stackexchange.com/a/330/21417 $\endgroup$
    – user21417
    Mar 21, 2022 at 15:01
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    $\begingroup$ Speed of light does not very with frequency. $\endgroup$
    – AG5CI
    Mar 23, 2022 at 1:27
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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.

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