[Antennas] resonance

Sat Dec 17 11:14:15 EST 2005

Yes, if you are tuned to resonance the impedance at the antenna will be
purely resistive. And yes that impedance value can change as you vary the
height of an antenna... to further confuse things, the change in height can
also change the frequency where the antenna is resonant. So if you have an
antenna 1/4 wave up tuned to resonance at 7000khz with an impedance of 75
ohms and move it higher the resonant point may move to 7150khz and the
impedance at 7150 may be 70 ohms.
David Robbins K1TTT
e-mail: mailto:k1ttt at arrl.net
web: http://www.k1ttt.net
AR-Cluster node: 145.69MHz or telnet://dxc.k1ttt.net

> -----Original Message-----
> From: antennas-bounces at mailman.qth.net [mailto:antennas-
> bounces at mailman.qth.net] On Behalf Of fkamp at comcast.net
> Sent: Saturday, December 17, 2005 15:56
> To: antennas 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
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