[Antennas] 468/Freq in Mhz

[Alex Eban]

I will try to answer that.
Any wire element has a characteristic impedance. That means it has
inductance and capacitance. 
An antenna element is no different. Now, the capacitance is related to the
surface area of the element.  For a quarter wavelength element this
inductance and capacitance resonate at the operating frequency. So, if you
increase the diameter of the element, you actually move the resonant
frequency lower than if the wire were a thin one. Therefore you have to
reduce the inductance by shortening the element: the inductance is related
to the wire length.
Paul Lee-I don't recall the callsign right now- has a table in his book,
correlating the length to width ratio of the element to the percentage of
shortening required for a given frequency.
Alex	4Z6KS


-----Original Message-----
From: antennas-bounces at mailman.qth.net
[mailto:antennas-bounces at mailman.qth.net] On Behalf Of Andy
Sent: Monday, August 24, 2009 8:06 AM
To: antennas at mailman.qth.net
Subject: Re: [Antennas] 468/Freq in Mhz

492/F is the half wavelength in free space, and an infinitesimally thin bare
wire would resonate at that length.

468/F is an empirical formula that works pretty well for most common wire
diameters and no (or very thin?) insulation.  If the non-zero wire diameter
is mostly what is responsible for the reduction from 492 to 468, then there
probably is a difference between thin wire and tubing.

But 468/F is just a starting point, and should never be taken as exact!

I do not know the precise mechanism that causes the difference.  I don't
think I have seen a formula that starts with 492/F and then adds terms that
depend on wire size (and other factors), to get from there to ~468/F.
Antenna length also depends on things like proximity to the surroundings.
Each of the sections of an antenna have capacitance to the rest of the
antenna and to remote objects, and inductance along the length of the
antenna wire, and these things all add up to make the thing resonant at some
frequency.

Andy

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