[Antennas] resonance

[Dr. William J. Schmidt, II]

Yes, correct.  Good discussion...

Sincerely,

Dr. William J. Schmidt, II  K9HZ
Trustee of the North American QRO - Central Division Club - K9ZC

Email: bill at wjschmidt.com
WebPage: www.wjschmidt.com

"It's not what you take with you... but what you leave behind that counts. 
Live each day as if it were your last."


----- Original Message ----- 
From: "David Kelley" <dkelley at bucknell.edu>
To: <antennas at mailman.qth.net>
Sent: Saturday, December 17, 2005 2:37 PM
Subject: Re: [Antennas] resonance


> 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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