[Antennas] resonance

Sat Dec 17 15:37:12 EST 2005

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