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[aprssig] 2M 1/4 Wave antenna

Ray Wells vk2tv at exemail.com.au
Sun Jan 14 09:15:36 UTC 2007


Andrew Rich wrote:

> Searched for ages on google, looking for a 1/4 wave vhf calculation 
> for 145.825 MHz
>  
> I remember that the 45 degree radials had to be a ratio longer or 
> shorter than the vertical element.
>  
> Assuming speed of light at 300
>  
> lambda = 300 / freq
> lambda = 300 / 145.825 MHz
> lambda = 2.05 M
> 1/4 lambda = 2.05 M / 4
> 1/4 Wave antenna = 0.5143 M
> 1/4 Wave antenna = 51.43 cm
>  
> Cheers
>  
>
> -----------------------------------------
> Andrew Rich
> Amateur radio callsign VK4TEC
> email: vk4tec at tech-software.net <mailto:vk4tec at tech-software.net>
> web: http://www.tech-software.net
> Brisbane AUSTRALIA
>
>  
>
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>
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>  
>
That formula gives the free-space length. For a typical whip style 
element, diameter about 2mm, you need to reduce the length to about 95% 
of the free-space value.

The radials are typically 5% longer than the whip.

The reason for drooping the radials down at 45° is to raise the 
feedpoint impedance to 50 ohms. With horizontal radials Zo is 37.5 ohms, 
but even that value will still match quite well to 50 ohm coax.

At one stage many years ago, 37 ohm coax was used to feed whips on 
vehicles. This was in the days of valve PA's so matching the tx to the 
cable was as easy as an adjustment of the PA tuning and coupling.

Cheers ... Ray




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