[TenTec] OT SWR VS Power Loss

[Jim Brown K9YC]

On Mon, 7 Jul 2008 14:52:10 -0700, Alfred Lorona wrote:

>Zero reactance in a wire antenna feedpoint is a necessary but not sufficient 
>condition for it to be truly resonant. Citing the Antenna Fundamentals, chap 
>2, page 2-1 of my 16th addition of The ARRL Antenna Book, a resonant wire is 
>defined as 'The shortest length of wire that resonates at a given frequency 
>is one just long enough to permit an electrical charge to travel from one 
>end to the other and back in the time of one RF cycle.'


>Clearly, a short wire with a finite pure resistive radiation impedance is 
>too short physically for the required condition to exist as stated in the 
>Handbook definition..

>On the other hand, elsewhere in the book, a zero reactance 'resonant' 
>condition does not mention the required physical length of the wire. If this 
>is all that is required, ANY wire can be called resonant and there is 
>nothing to distinguish such wires from a halfwave wire. Somehow this doesn't 
>sit right with me. What do you think? Do you think that this discrepency 
>begs for a minor clarifying rewrite?

The text is fine -- you missed an important part of the sentence -- "in the 
time of one RF cycle." That establishes the length of the antenna as long 
enough to be resonant. It will only be "very short" if the frequency is very 
high! When the antenna is resonant by the definition of the text it will be 
resistive. 

It is also possible to resonate a non-resonant antenna by tuning out its 
reactance. Here's a simple example. A quarter wave vertical typically has less 
than 30 ohms radiation resistance at resonance, but if we make it longer, we 
can find a length where the impedance at the feedpoint is 50 +jxxx ohms. We can 
then add a capacitor in series to tune out the inductance (+jxxx), and the 
antenna now looks like 50 ohms and matches the coax. Now, we have a resonant 
circuit that includes the antenna and the capacitor. 

73,

Jim K9YC