[Antennas] resonance

[Chris BONDE]

Dave:
Thank you.  Your explanation was much better than mine.

Chris opr VE7HCB

At 12:28 PM 12/17/2005 -0500, you wrote:
>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).

The input impedance of an antenna does change as its height above
ground varies.  The reason for this is that some of the radiation from
the antenna reflects from the ground and returns to impinge on the
antenna.  Just like an electromagnetic wave arriving from some distant
station (or noise source), the reflected wave causes current to flow
along the antenna.  The current induced by the reflected wave is
superimposed on the current created by the transmitter (which is
carried to the antenna via the feed line).  This alters the standing
wave characteristics of the antenna, which in turn changes the input
impedance.  The end result of all this interaction is that the real
and imaginary parts of the impedance are affected.

The height of the antenna affects the amplitude and phase of the
reflected wave that arrives at the antenna after its round trip.  As
height increases, the distance the reflected wave must travel
increases, which in turn increases its phase shift and decreases its
amplitude relative to the current induced by the transmitter.  The
variation due to antenna height is most pronounced at low heights,
where "low" means a fraction of a wavelength.  If the height is
greater than a wavelength or so, the reflected wave is relatively weak
by the time it arrives back at the antenna, and its affect on the
input impedance is minimal.   A resonant dipole more than a wavelength
above ground will likely have an input resistance between 60 and 80
Ohms.  The input impedance also varies with the type of ground, 
because a "good" ground reflects waves differently than a "poor"
ground.

Thus, an antenna that was resonant (i.e., had zero reactance) at one
height might be nonresonant (i.e., have a non-zero reactance) at
another height.  Also, if the antenna is tuned to resonance at various
heights, the real part of the resulting impedance is usually different
for different heights.

Note that other nearby objects can affect input impedance as well for
the same reason that the ground affects it.  Namely, reflected waves
from those objects impinge on the antenna, changing the standing wave
characteristics of the antenna current.  Small, distant objects
therefore have less effect than large, nearby objects.

73,
Dave NB4J


--
David F. Kelley, Ph.D.
Assistant Professor
Electrical Engineering Dept.
Bucknell University
Lewisburg, PA 17837
(570) 577-1313
dkelley at bucknell.edu

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