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Quarter Wave Length Whip Capacitance Formula

 
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Zura
Antenna-Theory.com Newbie


Joined: 28 Mar 2017
Posts: 1

PostPosted: Tue Mar 28, 2017 5:34 pm    Post subject: Quarter Wave Length Whip Capacitance Formula Reply with quote

Hi Guys,
I want to build a rubber ducky antenna for 8 MHz Freq.
wavelength is about λ= 37.5 meter (123 ft) and λ/4 = 9.4 meter (30 ft).
But that is too long,so decided to build a shorter one,Max 1 meter (3 ft).
as i know,at frequencies below resonance,the whip antenna will show capacitive reatance (Ca).
for compensation and make antenna to resonance,i'll add a inductance in series with it But...

How can I calculate the Ca ??


I found a formula on net but I don't know its correct for my design!

Ca= (17*L) / { (Ln[24*L/D]-1) } * { 1- ([F*L/234]^2) }

Ca = capacitance of antenna in pF
L = antenna height in feet
D = diameter of radiator in inches
F = operating frequency in MHz



Regards,Zura
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admin
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Joined: 03 Jan 2007
Posts: 210

PostPosted: Thu Mar 30, 2017 9:29 pm    Post subject: Reply with quote

Haven't seen that formula so can't vouch for it.

You can put an inductor in shunt or series actually to cancel out the reactance. If you use series your impedance will end up very low (1 ohm), if you do shunt your impedance will end very high (kOhms)

Your impedance of a very short monopole/dipole will be something Z = X - jC, where X is probably 1-2 Ohms.

I don't think your removing much mismatch loss with the matching honestly
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