[TowerTalk] resonance/swr/impedance plots

[alsopb]

Jerry,

To further illustrate my point.  Min SWR is NOT ALWAYS the indicator
of true resonance.

Here are two tables of data generated by EZNEC for a dipole cut to
resonance at 13.8 Mhz.  One assumes a reference impedance of 50 ohms
for SWR calculation the other assumes 200 ohms.

Zsource               =50 ohms                                 =200
ohms
freq  13.5  z = 95.2  SWR =1.905  (complex =-33.7, real =67.7)  3.045
      13.6  z = 81.5  SWR =1.636  (complex =-22.0, real =69.3)	2.973
      13.7  z = 73.6  SWR =1.473  (complex =-10.3, real =70.7)	2.834
      13.8  z = 72.3  SWR =1.448  (complex =+1.46, real =72.3)	2.766
      13.9  z = 73.9  SWR =1.561  (complex = 13.1, real =73.9)  2.720
      14.0  z = 75.5  SWR =1.772  (complex = 24.9, real =75.5)  2.697
      14.1  z = 77.2  SWR =2.051  (complex =36.6. real = 77.2)  2.694
      14.2  z = 78.8  SWR =2.384  (complex =48.3, real =78.8)   2.710

The true resonant point is defined where the complex part is zero. 
That is slightly below 13.8Mhz.  The z=50 case seems to track this. 
However, if you look at the z=200 case, the resonance point appears
(from min SWR) to be about 14.1 Mhz.  It "shifted" 300 Khz.  The
antenna didn't change. 

Minimum SWR is only a good indicator of resonance if the zsource is
near the resonant impedance of the antenna.  What does that mean in
practice.  For a dipole, not much.  However, if your antenna is a beam
with an impedance near 20 ohms or a folded dipole with an impedance
near 300 ohms, the error in inferred resonance is substantial.  Being
off several hundred KHz is not my idea of being close enough.

A case in point is the published inverted L lengths.  They are too
long.  The antenna is resonant at a lower freq.  However, if you look
at the design you will see a series capacitor which brings "tunes the
antenna to resonance". It isn't tuning the antenna to resonance at
all.  It is just canceling the inductive reactance of the antenna to
bring the impedance down and hence provide a better SWR. 

73 de Brian/K3KO

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