[Antennas] resonance

[Chris BONDE]

Bottom post
>
> 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

I tend to agree.  There is an article by Herz I think that talks about conjugate 
matching.  Also, some good article on Cebiz (sp?) site.

It is the complete system from tuner to radiators that much be looked at.

The antenna will vary in 'resonace' depending upon length (physical), diameter (2 or 
more parallel wires will increase diameter), angle apart, heigth above ground, 
nearby objects (including feedwires, posts, trees etc), yes and weather (I have 
found).  

And the math of radiation resistance is an attempt to explain something that is really 
not there or is unmeasurable directly.

That is why there is so much to learn about the antenna to understand it and very 
little to use it.  A good tuner with a descent feeder and antenna will get you out.

Chris opr VE7HCB