Effect of antenna length

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Hielke Hendrikse

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May 6, 2021, 12:14:09 AM5/6/21
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Hello all,


My memory is somewhat vague on this subject, but if I remember my classes correctly an antenna of 1/4th of the wavelength would be optimal for RF transmitters such as the Flex unit. 

My units came shipped with an ~82mm antenna, which corresponds nicely with 1/4th the wavelength of 913MHz. However, as I operate on the European 868.3MHz, I was wondering if it would be worthwhile changing to an 86mm antenna. 

Does anybody have experience with the effect of better matching the antenna length with the frequency used? Would the effect of such a small change in length be too insignificant to notice, or could it make a real difference? 

Thanks in advance!

Gadjet Nut

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May 6, 2021, 7:45:26 AM5/6/21
to Hielke Hendrikse, PrivateEyePi
82mm is the right 1/4 wave antenna length for 868.3MHz. 
915MHz : 78mm


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Hielke Hendrikse

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May 10, 2021, 8:40:51 AM5/10/21
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Thank you for your reply. 

Are you sure your calculations are correct? I'd say: 
Wavelength in meters = speed of light/frequency

So for 915MHz: 
299792458/915000000=0.32764m 
0.32764/4=0.082m =82mm

And for 868.3:
299792458/868300000=0.34526m 
0.34526/4=0.086m =86mm

Op donderdag 6 mei 2021 om 13:45:26 UTC+2 schreef gadj...@gmail.com:

Gadjet Nut

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May 10, 2021, 9:29:09 AM5/10/21
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Just google it :-)

You are using a pure speed of light calculation. Depending on the antennae in use you need to apply a Velocity Factor. We use an insulated copper wire whip antennae that has a velocity factor of 95%. 

Here is how it is calculated: 

So for 915MHz: 
(299792458*0.95)/915000000=0.31126m 
0.31126/4=0.078m =78mm

And for 868.3:
(299792458*0.95)/868300000=0.328001m 
0.328001/4=0.082m =82mm

Hielke Hendrikse

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May 11, 2021, 5:15:28 AM5/11/21
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Ahh, that explains it. Excuse me for my ignorance and thanks a lot for the clarification! :-) 



Op maandag 10 mei 2021 om 15:29:09 UTC+2 schreef gadj...@gmail.com:
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