[Antennas] resonance

[Robert Lay (W9DMK)]

Dear Frank,

You are correct about the behavior of the real component of antenna 
impedance as a function of height above ground (for resonant dipoles).

Just out of curiosity, I wonder why you are concerned about it.

To me, the more interesting phenomenon is the fact that depending on 
wavelength over diameter of conductor, the length of the dipole is generally 
not 1/2 wavelength at resonance. It's always shorter than a half 
avelength  - more so with thicker wire. In other words, with real thick 
wire, it would be real short - like 0.4 wavelength. Ain't that a hoot?

73 de W9DMK - Bob Lay, Dahlgren, VA
http://zaffora.f2o.org/W9DMK/W9dmk.html
w9dmk at crosslink.net

>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