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 How to calculate an antenna length by its frequency

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Daryn 2E0DKD

Posts : 526
Join date : 2011-12-07
Location : South Shields, Tyne and Wear, UK

PostSubject: How to calculate an antenna length by its frequency   Fri Feb 24, 2012 7:02 am

Calculating a dipole and vertical antenna length's

The official formula for a half wave dipole is 468/f where f is in megahertz (MHz) or
468÷ frequency (MHz)
ie to work out the length of a half wave dipole for 10metre band which is 28mhz to 30 mhz you would do the sum like this pick the inbetween frequency first, so between 28mhz and 30mhz the inbetween frequency is 29mhz. Now for the sum
468÷ 29= 16foot.14 inches near enough so what I would do is make it slightly longer maybe
2 or 3 inches longer and cut it back to get the SWR (standing wave ratio) as good as I can get.

For a 1/4 wave vertical the above numbers would be half. Therefore, the height of a simple 1/4 wave vertical can be calculated by 234÷f where f is in Megahertz (MHz).
For a full wave you would double the half wave calculation therefore the height of a simple full wave can be calculated by 936÷f where f is in Megahertz (MHz).
Since ground (including radials) act as the other half of the antenna a vertical is really a half wave antenna with a vertical polarization.

Some tweaking might be in order due to nearby objects. End effect may also change resonance requiring slight adjustment in length.

It is best to cut a dipole/antenna a few inches longer than the calculated length then cut it back for resonance. Starting longer is a LOT better then starting at the calculated length or shorter!
Proximity to ground, trees, homes, even the feedline greatly influence resonance so make sure you check it.

Dont forget when making a dipole you need two length's the same size, so if your antenna length calculation comes to say 18inches then you need two of these one length for the bottom and one length for the top.

So lets simplify it for you.
¼ wave antenna is calculated as follows 234÷ frequency in (MHz) double 234 for a half wave
½ wave antenna is calculated as follows 468÷ frequency in (MHz) double 468 for a full wave
Full wave antenna is calculated as follows 936÷ frequency in (MHz)

And dont forget to gave yourself extra length so you can trim it down as coax, homes, trees etc can effect the SWR.

These are the most common antenna wave lengths used in amateur radio.

Hope this helps to explain a few things for you, this is not everything there is to know it is just the basics to help get you going and help with your exam. Please feel free to copy and print it off.

Daryn 2E0DKD

_________________ ...........This is my pride and joy, my little travel site and forum which has lots of photos and reviews take a look. .......... My personal radio site still under construction
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