[Antennas] resonance

Sat Dec 17 11:22:44 EST 2005

Resonance is actually a function of the antennas electrical length. The fact
that the exact center of a half wave dipole is purely resistive at resonance
is a happy coincidence. It is possible (common actually) to have a resonant
antenna who’s feed point isn’t purely resistive or even close to the 75 ohm
value of a dipole. For instance if you were to keep the length of the dipole
exactly the same and tap it at a point other than the exact center the feed
point wouldn’t be purely resistive or anywhere near 75 ohms but the antenna
would remain resonant.
Think of a tuning fork with it’s ‘natural frequency’. At particular lengths,
antenna structures exhibit a ‘natural frequency’ or resonance at radio
frequency.
Steve .. AI7W
-----Original Message-----
From: antennas-bounces at mailman.qth.net
Subject: [Antennas] resonance
I may not understand all I think I know about resonance.
Take the case of a dipole antenna.
Resonance is where the impedance becomes resistive when the
length is cut to the desired frequency. If the dipole is
flat-topped at 1/4 wavelength above electrical ground, that
resistive impedance is 75 ohms.
That is what I know. Is that all there is to it?
What happens when the dipole is not 1/4 wavelength above
electrical ground. A chart in the handbook indicates that
under those conditions the impedance can vary significantly
from 75 ohms.
My question is does the impedance remain resisitive as the
height above ground is varied and the impedance changes?
Regards,
Frank Kamp
K5DKZ