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Re: [TowerTalk] wire antenna question

Subject: Re: [TowerTalk] wire antenna question
Date: Fri, 14 Nov 2008 10:39:05 EST
List-post: <">>
Steve, when I plug your numbers into a transmission line program I get  these 
results:283.8 -j23.66 = 5.72 vswr (same as your example).
288.2 +j0 = 5.7 vswr which is just slightly lower than example #1 but  
illustrates better vswr when j=0.
I did not look into the 40 +j4 case but in general, real antennas show a  
reactance change much greater than a resistance change when they are in the  
range of one half wavelength. I would expect a reactance of maybe 20 ohms  
(positive) for an increase of 5 ohms in the resistive value. This would show a  
of 1.55 which is higher than the 40 +j0 value of 1.25.
Perhaps someone can model a dipole to confirm or refute this.
Gerald K5GW
In a message dated 11/14/2008 3:54:59 A.M. Central Standard Time, writes:

Be aware  that there are some "extreme" cases where minimum VSWR and 
Resonance do  not occur at **exactly** the same frequency.

For example, if I take a  34ft folded dipole, and am silly enough to feed 
it through a 50 Ohm line,  I get:

Minimum VSWR: 5.72 @ 13.62 MHz where  Z=283.8-j23.66
Resonance: Z=288.2+j0 @ 13.7 MHz where VSWR=5.76

It  just depends how fast the resistive component is changing relative to 
the  reactive component as you move away from resonance. Using the 
previous  40+j0 example, if away from resonance the impedance had become 
45-j4 the  SWR would have fallen from 1.2 to 1.1, not risen. This rapid 
shift of the  resistive component tends not to happen on simple antennas 
like dipoles,  but can occur in more complex arrays.

Steve  G3TXQ


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