[Antennas] resonance

[fkamp at comcast.net]

David Robbins K1TTT wrote:
> 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...

Yes, I have seen that happen, but does the impedance remain 
purely resistive as its value changes with height variation? 
  And is a purely resistive impedance the only measure of 
resonance.  (Please no cracks about dummy loads being 
resonant at ALL frequencies).

> 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.

Have you actually experienced this?  I mean acutally seen 
this happen on an antenna you installed?  I dont understand 
why a height change would have an effect on frequency AND 
impedance AND resonance.  The chart in the handbook shows a 
definate variation in impedance with height change but no 
change in frequency.

Regards,
Frank Kamp

> 
> 
> 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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> 
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