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Re: [TowerTalk] wire antenna question

Subject: Re: [TowerTalk] wire antenna question
Date: Thu, 13 Nov 2008 12:33:45 EST
List-post: <">>
Blair, that is not quite right. Using a 50 ohm swr indicator, the lowest  swr 
is shown when the reactance is zero.
Using a transmission line program, 40 ohm load 40+j0; swr=1.25.  40+j10=1.37; 
So, when you tweak your dipole for the lowest vswr at a given frequency,  you 
are in essence resonating the antenna at that frequency.
Gerald K5GW
In a message dated 11/13/2008 11:00:20 A.M. Central Standard Time, writes:

Thanks  to everyone who responded.  I feel much better about what I've been  
doing.  Terry, yours made such perfect sense, I thought I should have  known 
it already!  Because the dipole's resonant Z is usually higher than  50 ohms, 
getting minimum SWR would mean I'd cancelled out all the  reactance.  If the 
resonant Z were less than 50 ohms, I could get a lower  SWR by introducing a 
reactive component.  But, I suppose it would be rare  to have a dipole with a 
resonant Z below 50 ohms.

Thanks again to  all,
Blair NP2F 

----- Original Message  -----
From: Terry Conboy <>
Date: Sunday, November 9,  2008 10:54 pm
Subject: Re: [TowerTalk] wire antenna question

> At  07:07 AM 2008-11-09, Blair S Balden NP2F wrote:
> >When I put up  dipoles, whether they be "straight" or inverted 
> >V's,  I've  always started out with the wire a little longer than 
> the 
>  >formula length of 468/f.  Then I've trimmed for as close to a 1:1  
> >match as I can get to my 50-ohm coax, as indicated by my SWR  
> >meter.  I can usually get a 1:1 SWR.
> >
>  >These antennas usually work very well, but I realize that a 
>  resonant 
> >dipole will not have exactly 50 ohms resistance.   So, I must be 
> >cutting these antenna somewhat off resonance.   I also realize 
> that a 
> >mismatch at the antenna (SWR of  greater than 1) results in loss.
> >
> >What I'm not sure  about is whether it would be better to get the 
> >antenna resonant  at the desired frequency and accept the 
> mismatch, 
> >or  whether I should continue to go for the best SWR (minimizing 
>  >losses due to the mismatch).
> >
> >BTW, my rig does not  have an internal antenna tuner, and I am 
> >currently without an  external tuner for it.  So, I'm running it 
> >without a tuner  (if this matters).  Also, I have not been using 
> any 
>  >kind of matching device at the feedpoint, just direct connection 
>  >from the coax connectors to the wires.
> >
> >If someone  on here can give me some insight as to whether I 
> should 
>  >try to go for resonance instead of low SWR, please respond.  I  
> would 
> >truly appreciate any help with the  question.
> In your case, resonance and 1:1 SWR are exactly the  same 
> thing.  Resonance just means that the antenna impedance is  purely 
> resistive.  Resonance isn't necessarily the point of  highest gain: 
> many antennas actually have higher gain when they are  much longer 
> than the resonant length (e.g. double-extended  zepp).
> Indeed, a thin-wire straight (nearly) half-wave dipole  in free 
> space, 
> or at certain heights, will have a resistive  component of the feed 
> impedance of about 72 ohms.  However, at  your mounting height and 
> with your surrounding conductors, the  resistive part of the 
> antenna 
> impedance is 50  ohms.   This is actually pretty common, as you 
> have  
> discovered.  (Because of the bending, V-shaped antennas  typically 
> have a feed impedance under 72 ohms, even in free  space.)
> Even if your antenna did have a feed impedance of 72  ohms at 
> resonance, which would give a 1.44:1 SWR, you can only worsen  the 
> SWR 
> by changing the length of the wire, which mostly  just adds or 
> subtracts reactance.
> With a 50 ohm  line, the ONLY way you can ever get a 1:1 SWR is if 
> the 
> load  Z is 50 +/- j0 ohms.  (A Smith Chart would never lie.)
>  In your situation, there really isn't anything else you can do 
> with  
> trimming to get higher efficiency.  You already have  "nirvana".  
> Life 
> is good.  Be happy.  As they  say in Seattle, "keep clam".
> 73, Terry N6RY
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