[TowerTalk] wire antenna question

[Blair S Balden]

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,
73,
Blair NP2F 



----- Original Message -----
From: Terry Conboy <n6ry at arrl.net>
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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