[TowerTalk] Elevated vertical

[TexasRF at aol.com]

Hi Ken, we have a differing definition of what constitutes resonance: You  
are talking about the radiating element only and I am talking about the  
radiating element plus everything else that constitutes an antenna system;  
matching devices, coax etc.
 
Your example of .28 wavelengths does indeed move the R component to 50 ohms 
 but the +X that happens makes the vswr about 4 to 1 until the series 
capacitor  is added. The series capacitor cancels the +x leaving only 50R. In my 
view and  definition, that is a resonant system. 
 
I "think" we may be saying the same thing but from differing  perspectives.
 
73,
Gerald K5GW
 
 
 
 
 
 
In a message dated 12/23/2011 4:47:16 P.M. Central Standard Time,  
wa8jxm at gmail.com writes:

Gerald,  


I disagree.   If a quarter wave vertical or ground plane is  lengthened to 
about .28 wavelength, the radiation resistance raises to about  50 ohms and 
a 1:1 SWR.  It is not resonant, but it has a lower SWR than a  "resonant" 
antenna.  The reactance can be tuned out with a series  capacitor.  It is not 
at resonance but it does have a low SWR.  Moving down in frequency, to where 
the antenna is resonant,  the  SWR will go up.     (Reference ARRL Antenna 
Book, 10th edition, page  125).


This is, of course, the same technique used with 160m inverted L antennas  
to improve the feedpoint match:  lengthen the antenna beyond the quarter  
wave resonant point.


IMO, people confuse resonance and low SWR.   An antenna does not  have to 
be resonant (or anywhere near it) to work effectively.  A prime  example is 
the "Double Extended Zepp" which is no where near resonant.  


When people say they tuned their antenna until it was "resonant with a  1:1 
SWR" I believe they are mistaking matching the feedline impedance (low  
SWR) with being resonant, which it is probably not.


I am confused by your suggested wording "does not UNnecessarily indicate  
resonance".  Perhaps you meant "does not necessarily...."  but even  then, if 
we don't know whether or not it does, then it is a meaningless  statement.  
 While resonance and low SWR MIGHT happen, there is no  defined 
relationship.  I'll stand by my statements that "low SWR does not  indicate resonance" 
and "resonance does not ensure a low SWR".


A 1:1 SWR indicates that the feedline impedance matches the antenna  
impedance, nothing more.  It does not indicate resonance.  Does  a dipole change 
it's resonance when you change the feedline from 50 ohm to 75  ohm to 300 ohm 
or 600 ohm?   Of course not.   But the SWR sure  changes!




73, 
Ken WA8JXM





 
On Dec 23, 2011, at 8:54 AM, _TexasRF at aol.com_ (mailto:TexasRF at aol.com)  
wrote:


Hi Ken, the statement "low swr does not indicate resonance" is a bit  
narrow. Perhaps "does not unnecessarily indicate resonance" would be more  
accurate.
 
If the feedline has a high loss, the swr would be low even with a  highly 
non-resonant load connected
 
If the feedline loss is low, then minimum swr would coincide with  
resonance. Using your example of 30 ohms, at resonance the resulting 50 ohm  vswr 
would be 1.67. Moving off frequency until there is 10 ohms +x results  in a 
vswr of 1.77 (ignoring any change in the 30 ohm value). Moving off  frequency 
until there is 10 ohms -x also results in vswr= 1.77 (also  ignoring any 
change to the 30 ohm value).
 
Minimum vswr then indicates resonance in that system. As you say,  
resonance does not necessarily provide a low vswr. Resonance does provide  minimum 
vswr at the frequency in  use.




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