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Can someone please shed some light on the definition of resonance in
conjunction with an antenna.

I'll make an attempt.

Let's agree that "resonance occurs when +/- jX is zero".
It does not take knowledge of x-degree differential equation , modelling ,
size of wire , moon phase etc.
It is applicable to ANY AC circuit, antenna included.

True. The definition of resonance requires the capacitive reactance equals
the inductive reactance or: -jX = +jX. Since the two cancel eachother,
only real resistance remains at resonance. For an antenna in free space,
that resistance amounts to the sum of the radiation resistance plus loss
resistance in the conducting structure. In practice, losses in the field
return structure ("ground" or earth losses required in some installations
as in a 1/4-wavelength vertical) also contribute to losses.

Let's also agree that dipole , by definition resonant radiator, exhibits
APPROXIMATELY 50 Ohms impedance
at the center feed point. For a sake of simplification let's also agree
that variation of the center point impedance with - insert your favorite
parameter here - is not fundamental to the discussion .

A dipole is understood to be a resonant radiator. A doublet is a dipole
configuration with resonance structure outside the frequencies of interest,
but still exhibits resonance(s).

At typical amateur heights above soil surface, not DX antennas and
installations which are far above the typical amateur budgets, yes, 50-ohms
feed or radiation resistance of a dipole is in order.


Now for the punch line
By definition , ANY length of transmission line of characteristic impedance
will transfer impedance at the load - AKA terminal impedance EQUAL of
transmission line characvteristric impedance TO the input of such
transmission line.

Absolutely true. That's what the Smith Chart is all about. A coaxial
transmission line of known length and characteristic Zo acts as an
impedance transformer. In reality, why is the *antenna* terminal
impedance so important when it must be connected to a transceiver through
that coaxial transmission line (or any other transmission line)? What
really counts is the terminal impedance at the *station end* of the
transmission line! It's academically interesting for the design engineer
(like myself) to know the antenna terminal impedance just to verify the
results of putting the whole system through the Smith Chart exercise, but
from a strictly practical viewpoint what's really important is what the end
of the feedline presents to our station inside where its climatically
controlled. So, measure at the station end of the transmission line and
forget about the transformative properties of the transmission line.

Others: please don't flame me for that statement, but from a practical
standpoint, that's all that is important.

Place note - I am still using general terms, adding SPECIFICS - such as
velocity factor of RG58 coax is immaterial - for the sake of this
discussion.

Described ideal system ,for purpose of staying with basic, not woo-doo
electronics , with "real data of 50 Ohms substituted " parameters of
source impedance of 50 Ohms ,
transmission line of characteristic impedance of 50 Ohms
and load / antenna impedance of 50 Ohms
will PRIMARILY exhibit FUNDAMENTAL resonance @ ONE frequency.
IF these parameters are SAME and EQUAL on harmonics , resonance @
harmonics will be observed.

Yes, resonances at harmonically related frequencies to the fundamental
1/2-wavelengths will be present. However, they likely will not exhibit
the same resistance as the intended resonant frequency. Again, decouple
the concept of SWR from resonance!

Hope this helps just a little.....

Dave - W?LEV

--
*Dave - W?LEV*
*Just Let Darwin Work*