[Antennas] resonance

[Peter Veach]

>
> 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?

Yes
Resonance is the absence of reactance. At this point all that is left is
radiation resistance, and losses.

>
> My question is does the impedance remain resisitive as the
> height above ground is varied and the impedance changes?

Yes, but the resonant freq will vary. If you lower the antenna, the resonant
freq will drop, raise it and it will go up, generally speaking.
The earth has a big 'pull' on antennas that are close to ground with respect
to wavelength.

Technically, an antenna is really only resonant at one specific freq, and
becomes more and more reactive as you deviate from that freq.

All that really matters is that your antenna has a good ratio of radiation
resistance to loss resistance in order for it to perform.
I wouldn't get too hung up on resonance. A lot of people out there will
preach than an antenna must be resonant in order to radiate, and this is
simply not the case. You must be resonant as a 'system' however. That is to
say, you have to get the power into the antenna, after that, the radiation
resistance does the work.

73

N9QZD