Hold on - 69mm is a little too long!There needs to be a correction for the diameter of the antenna element.
This is called 'end factor correction' K.
65 mm will resonate at 1090MHz.If you want the explanation read on.
Using V = f x λ we get:
wavelength = Velocity / frequency
wavelength = 300,000,000 / 1090,000,000 = 0.275 metres or 275 mm.
¼ wavelength whip antenna will be 275mm / 4 = 68.8 mm, all correct so far. So what am I beefing about.
As I said 68.8 mm will not be resonant at 1090MHz, it will be 'overtuned', it needs an end factor correction to bring it into resonance.
End factor correction is dependent on the ratio of the half wavelength to the thickness of the antenna element.
In our case 137.6 mm to 1.5 mm or 92:1
By looking this ratio up in any good antenna theory book the antenna length should be 0.96 times its calculated size.
68.8 x 0.96 = 65.4 mm.
So there you have it
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