[TowerTalk] Elevated vertical

[Ken]

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 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.